GEUS Bulletin 9, Scientific results from Lopra-1, pp. 23-40

Borehole seismic studies of a volcanic succession from
the Lopra-1/1A borehole in the Faroe Islands, northern
North Atlantic

Philip Christie, Ian Gollifer and David Cowper

Extruded basalt flows overlying sedimentary sequences present a challenge to hydrocarbon explora-
tion

using

reflection

seismic

techniques.

The

Lopra-1/1A

re-entry well on the Faroese island of Suðuroy

allowed us to study the seismic characteristics of a thick sequence of basalt flows from well logs and
borehole seismic recordings. Data acquired during the deepening operation in 1996 are presented here.

The re-entry well found that the seismic event at 2340 m, prognosed from the pre-drill Vertical

Seismic Profile (VSP) as a decrease in impedance, was not base basalt and the deepened well remained
within the lower series basalts. Nonetheless, compressional and shear sonic logs and a density log were
recorded over the full open hole interval. These allowed a firm tie to be made with the reflected
wavefield from a new VSP. The sonic logs show a compressional to shear wavespeed ratio of 1.84
which is almost constant with depth. Sonic compressional wavespeeds are 3% higher than seismic
velocities, suggesting dispersion in the basalt flows. Azimuthal anisotropy was weakly indicated by the
shear sonic log but its orientation is consistent with the directions of mapped master joints in the
vicinity of the well.

The VSP downgoing compressional wavelet shows good persistence, retaining a dominant period

of 28 ms at 3510 m depth. Average vertical velocity is 5248 m/s, higher than previously reported.
Attenuation can largely be modelled by geometrical spreading and scattering loss, consistent with
other studies. Within the piled flows, the effective Q from scattering is about 35. Elastic layered
medium modelling shows some hope that a mode-converted shear wave may be observed at moderate
offsets. Like its predecessor, the 1996 VSP indicates a decrease in impedance below the final depth of
the well. However, it is unlikely to be basement or sediment and is probably an event within the
volcanic sequence.

Keywords : Faroe Islands, Lopra-1/1A borehole, basalt, vertical seismic profile, seismic attenuation

_______________________________________________________________________________________________________

P.C., Schlumberger Cambridge Research, High Cross, Madingley Road, Cambridge CB3 0El, UK. Formerly: on second-
ment to BP, Farburn Industrial Estate, Dyce, Aberdeen AB21 7PB, UK. E-mail: pafc1@slb.com
I.G., Fugro-Jason UK Ltd., Unit B Kettock Lodge, Campus 2, Aberdeen Science & Technology Park, Balgownie Road,
Bridge of Don, Aberdeen AB22 8GU, UK. (Formerly: Schlumberger GeoQuest, c/o BP, Farburn Industrial Estate, Dyce,
Aberdeen AB21 7PB, UK. )
D.C., BP Egypt, 14 Road 252, Digla, Ma'adi, Cairo, Egypt. (Formerly: BP, Farburn Industrial Estate, Dyce, Aberdeen
AB21 7PB, UK.)

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

The North Atlantic igneous province, of which the Faroe
Islands are a part has been estimated to comprise 10 mil-

lion

intruded

and

extruded

basaltic

igneous

rocks

(White & McKenzie 1989). They were emplaced by the
processes of rifting and sea-floor spreading which resulted

in the opening of this northern part of the Atlantic Ocean.
The

basalts, which were extruded in a relatively short peri-

od of time in the early Palaeogene, cover pre-existing sedi-

mentary rocks which may well be prospective hydrocar-
bon traps. However, the difficulty of using reflection seis-
mic imaging to probe beneath basalts has been recognised
for some time and motivates studies into the characteris-
tics of basalt flows which are relevant for seismic wave
propagation. Such studies rely on boreholes which have
penetrated significant amounts of basaltic material and in

which good

quality

geophysical logs have been recorded.

The

Lopra-1/1A research well, on the Faroe Islands, is one

such borehole which not only has good quality logs but
also has a Vertical Seismic Profile (VSP).

The Lopra-1/1A well-site is located near to the coastline

on an isthmus on the southern Faroese island of Suðuroy.

The exposed basalt sequence on the Faroe Islands has
been divided into a lower, a middle and an upper series,
each about 1 km in thickness (Rasmussen & Noe-Nygaard
1970, 1990). The 3 km of exposed lava flows in the Faroe
Islands are tholeiitic flood basalts whose extrusion was con-
temporaneous with the opening of the Norwegian-Green-

land Sea in the Palaeogene. The exposed and drilled lower

series lava sequence is dated to about 56-59 Ma (Larsen
et al. 1999) and is characterised by flows with an average
thickness

thought

have originated from fissure

eruption sites with a NW-SE trend (Rasmussen & Noe-
Nygaard

1970;

Kiørboe

Petersen 1995). The basalt flows

dip

to the north-east and the area around the Lopra-1/1A

well-site has mapped sets of near-vertical master joints

trending NW-SE and NE-SW. The Lopra-1/1A location was

selected for probing the deep basalt layers and their sub-

strata because the ground surface is about 750 m below

the top of the lower series.

The

Lopra-1

research borehole was originally drilled to

a total depth (TD) of 2178 m in 1981. It was logged and
suspended with most of the drilled interval left uncased.
The well was surveyed by the Geological Survey of Den-
mark

with

a zero-offset VSP and a walkaway VSP, acquired

by Prakla-Seismos, in 1988. A refraction profile was ac-
quired by the Faroe Islands Natural History Museum and
the University of Bergen in 1989. The results and inter-
pretation are summarised by Kiørboe & Petersen (1995).
In

1996, the well was re-entered and deepened in a number

stages by the Lopra Deepening Consortium. The original

well was extended to 3158 m KB (measured depth rela-

tive to the Kelly Bushing) using a larger rig with KB 16.2 m

above mean sea level, retaining the original name Lopra-

For technical reasons, a side-track, Lopra-1A, was drilled

from

3091

3565

KB.

The

well

was logged

several times during drilling, and logs were run in both
the side-track and the original well. In this paper, we deal

with data from a composite of the log runs in both the 1
and 1A wells, and therefore we use the name Lopra-1/1A
re-entry well or the Lopra-1/1A deepening for the combined

extensions drilled in 1996.

One goal of the Lopra project was the seismic charac-

terisation of piled basalt flows. In the event that signifi-
cant siliciclastic sediments were encountered beneath the
basalt sequence, the consortium partners had agreed on a
programme of multiple-azimuth, walkaway VSPs to charac-

terise the properties of compressional and shear wave trans-
mission and reflection at a basalt-sediment contact as func-
tions of both the vertical polar angle and the horizontal
azimuthal angle. Layered systems of high velocity con-
trasts, such as basalt flows, are expected to exhibit trans-
verse isotropy with an axis of symmetry perpendicular to
the layering, at wavelengths long compared to the layer
thickness. Such anisotropy has its fastest velocity in the
direction parallel to the layering. Kiørboe & Petersen (1995)

had reported velocities higher in the vertical direction than

the horizontal and offered an explanation in vertical frac-
tures around basalt columns, possibly in combination with
the nearly vertical master joints. Such vertical fractures, if
aligned, would be expected to result in an azimuthal varia-
tion of seismic velocity which should be fastest in the direc-
tion parallel to the fractures.

In fact, neither sediment nor basement was encoun-

tered in the well and so the borehole seismic programme
was confined to a short-offset VSP and check shot survey
designed to measure the short offset reflectivity at the well

and to identify depths of intermediate reflectors penetrat-

ed by the bit. In particular, Kiørboe & Petersen (1995)

had reported a reflection on the VSP at an interpreted
depth of 2340 m which was thought to result from a de-
crease in impedance. Such a reversal in impedance might
have corresponded to the base of basalt/top of sediment
but

turned

out

not

to be the case. Further objectives of the

borehole seismic analysis were to calibrate the sonic log,
thereby providing a detailed velocity-depth model, and to
estimate the seismic attenuation of the basalt sequence.

The VSP was complemented by the acquisition over

the full interval of the well of compressional (P) and shear
(S) wave sonic logs, acquired in four component mode to
estimate azimuthal anisotropy parameters (Esmersoy et al.

1994), and a density log. The log data enabled a good
well-tie to be made and allowed a modelling study to sup-

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

port estimates of anisotropy parameters and attenuation.

This

paper presents a summary of the data acquisition,

compares the results with those reported in previous stud-

ies and discusses their interpretation in the context of the

seismic characteristics of piled basalt flows. It draws upon

previously unpublished data and reports analysed and com-

piled by the authors for BP, their Atlantic Margin partner
Shell, and their associates in the Lopra Deepening Con-
sortium.

Data acquisition

The earlier VSP surveys (Kiørboe & Petersen 1995) had
found evidence for a strong seismic reflector at 2340 m,
just below the 1981 TD, interpreted as a reversal in impe-
dance. This event, and other deeper events seen on the
VSPs, were targets for the deepening of the well in 1996.
Further motivation was given by the discovery of meth-
ane and nitrogen at a pressure of 20 bar when the well was

re-opened in 1983 (Kiørboe & Petersen 1995). As men-
tioned above, the contingency plan for the 1996 deepen-
ing project called for offset VSPs at multiple azimuths in
the event of finding 200 m or more of siliciclastic sedi-
ments.

Exploring a range of offsets was intended to evalu-

ate the angular dependence of P- to S-mode conversions

in transmission within the basalt sequence, and their pos-
sible conversion back to P at the top of any basalt-sedi-
ment contact encountered.

This would test the applicabil-

ity

the

Faroese basalts of PS-mode converted imaging,

as later reported by Emsley et al. (1998).

Kiørboe & Petersen (1995) had reported that the ver-

tical P-velocity was about 10% faster than the horizontal
P-velocity in the upper 800 m of the basalt sequence and
appealed to fractures to explain this difference. It was
hoped that the new VSP would be able to explore the
nature of the vertical and azimuthal anisotropy through-
out the interval of the deepened well. The location of the
Lopra-1/1A

well on an isthmus would have facilitated the

use of a marine mobile source but because of the estab-
lishment of a fish hatchery in the fjord, the 1996 VSP could

not make use of a marine airgun and so twin Vibroseis
units

were shipped to the Faroe Islands for land walkaway

VSPs in different azimuths.

In the event, Lopra-1A TD remained in the lower ba-

salt series and a short-offset VSP was acquired by Schlum-

berger on 29 October 1996 from 3510 m KB to 1320 m
KB with additional checkshots up to 200 m, using the
two

Vibroseis sources in tandem. The Vibroseis units swept

from 10 Hz to 130 Hz over 16 seconds and 2-4 sweeps
were recorded at each level using a sample interval of 2 ms.

In addition, checkshots were recorded using an airgun
source in a water filled pit to calibrate the Vibroseis tran-
sit times. The downhole tool used gimballed triaxial geo-
phones mounted in a sensor package decoupled from the
body of the tool. The deepened well was surveyed at 20 m
intervals for VSP waveform processing. The cased hole
section was surveyed at similar intervals to a point above
the cement top where it became clear that the casing was
unsupported and no longer well coupled to the formation.

Some logs recorded after drilling the original well were

uncertain in their calibration. The sonic log was a cement
bond tool with a single source-receiver pair and lacked
shear sonic information (Nielsen et al. 1984). Since a goal
for Lopra-1/1A was to characterise seismic propagation
characteristics for basalt flows, a set of new logs were ac-
quired

over

the original open hole section, prior to setting

7-inch casing and drilling on with a 6.5-inch bit. All the
drilled intervals were logged with density, P- and S-sonic
from

a dipole shear tool in four component mode allowing

estimates of azimuthal anisotropy, and a formation micro-

scanner. The logs are not subject to petrophysical interpre-

tation in this paper, but were used for geophysical analysis.

Results

VSP

The raw VSP traces were correlated, edited and vertically
summed to produce a stacked trace at each level. The
stacked vertical geophone data are displayed in Fig. 1a in
one-way time, static corrected to mean sea level. From
the top of the VSP down to approximately 1840 m KB,
the waveforms following the first arriving compressional
wave are affected by borehole reverberations caused by
unsupported casing in a hard rock environment with a
non-attenuative fluid in the hole. The amplitude data in
this interval are treated with caution, though the arrival
times appear to be representative of basalt velocity.

Below the cement top, the stacks show good waveform

consistency from level to level. Clearly visible are:

A. down-going multiples (parallel to, but later than, the

first arrivals);

B. up-going primary reflections, both within and below

the drilled interval. These are characterised by an almost

linear moveout of equal slope but opposite sign to that
of the first arrivals;

C. suggestions of down-going shear energy, with a linear

moveout greater than that of the first arrivals;

D. a weak tube-wave (visible only above the cement top).

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

The vertical one-way time from mean sea level to the

deepest VSP level at 3510 m KB (equivalent to 3478 m
True Vertical Depth Sub-Sea: TVDSS) is 663 ms, corre-
sponding to a compressional velocity of 5248 m/s over
the interval. This is higher than values for basalt velocity
reported elsewhere in the literature (e.g. Planke & Cam-
bray 1998) and higher than the 4.35 km/s average velo-
city estimated from the first logs run in Lopra-1 (Nielsen
et al. 1984). This is partly due to the high velocity doler-
ites and partly due to the relatively thick flows in Lopra-
1/1A, giving a higher thickness ratio of fast, flow-centre
material to slower, flow-boundary material. As discussed
later, the average compressional wavespeed from the son-
ic log is slightly higher, supporting the VSP observation.

The trace scaling in Fig. 1a is constant for all levels,

revealing the total amplitude loss with depth in the first
arrivals. Although the VSP interval, from 3510 m KB to
1320 m KB, comprises 2174.5 m TVD of stacked basalt
flows, the amplitude loss from geometrical spreading, scat-
tering and attenuation still leaves a good level of signal
above the noise floor in the deepest section.

Fig. 1b shows the Y-component stacks (Y is the hori-

zontal component tangential to the borehole wall) on
which the shear energy is most evident. The strongest ar-
rivals are down-going direct shear arrivals, generated by
the vertically polarised vibrator acting on the rigid sur-
face. The average shear velocity across the logged interval
is about 2900 m/s, resulting in an average V_p/ V_s ratio of

1.8, which is in good agreement with the value of 1.84 +
0.01 (one standard deviation) estimated below from the
sonic log regression of V_s upon V_p.

Also visible on the Y-component stacks are down-go-

ing mode-converted shear events, E, generated by imped-
ance contrasts crossed by the drill-bit. These events are
parallel to and earlier than the direct shear arrival, C, but
originate at the time and depth of the direct compression-
al wave's encounter with the mode-converting impedance
contrast. These arrivals have a frequency content similar
to the associated P-wave and higher than the frequency of
the direct shear wave due to the lower cumulative attenu-
ation.

Finally, some weak up-going reflected shear events can

be seen. NOFurther processing of the shear arrivals has
been performed.

The VSP P-wave data were processed through a work-

flow comprising trace editing, vertical stacking at each
depth level, correction to mean sea level, spherical diver-
gence correction, up/down wavefield separation and de-
convolution to zero-phase wavelets with bandwidths of
10-70 Hz and 10-40 Hz. The former maximises the res-
olution at the cost of more high frequency noise, while
the latter minimises residual noise at the expense of band-
width. Correlations of the 10-70 Hz up-going wavefield
are presented and discussed in the well-tie section.

Geophysical log data

In this section we examine the 1996 log data and their
correlations to infer the seismic properties of the basalt
and to compare with the properties of similar basalts pen-
etrated in Hole 917A of the Ocean Drilling Programme,
as reported by Planke & Cambray (1998).

The P- and S-sonic and density logs, spliced from the

Fig. 1. a : Lopra-1/1A VSP: stacked
vertical component geophone data. b :
Lopra-1/1A VSP: stacked tangential
component geophone data. Labelled
phases are described in the text.

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

KB, the core measurements are either the same as the log
values or a little faster, consistent with dispersion of the high-

er frequency core measurement compared to the sonic log.

Discussion

Layer-induced anisotropy

Backus upscaling applied to a stack of isotropic layers pre-
dicts a transversely isotropic model having a symmetry
axis perpendicular to the layers. The upscaled elastic para-
meters can be recast in terms of Thomsen's (1986) weak
anisotropic parameters. Figure 3 shows the modelled val-
ues of epsilon, and delta, after upscaling the logs to

3 m. The parameters are given by the relations:

and ρ, V_p and V_s are the Backus-upscaled log values of

density, compressional wavespeed and shear wavespeed.
Epsilon is the measure of axial compressional anisotropy,
while delta controls the off-axis behaviour of the phase
velocity near to the vertical. Both enter the following rela-
tion from Thomsen (1986) governing the compressional
phase velocity behaviour as a function of angle to the

(vertical) axis of symmetry:

several logging runs made in the 1 and 1A wells, are shown
in Fig. 2 on a measured depth scale relative to KB. The
well is close to vertical, so no true vertical depth correc-
tions have been made for this part of the analysis. The
logged section comprises 3371 m, with the dipole shear
tool logged in four-component mode together with the
density tool. The logs are of good quality and allow the
identification of many subaerially emplaced flows over
much of the section. Figure 2 shows the P- and S-velocity
and density logs after re-sampling the data to 3 m using a
Backus average (Backus 1962; Folstad & Schoenberg

1992). There are wide variations in P- and S-velocities, in
an asymmetric, quasi-periodic manner, though the mean
velocities are very consistent over the well. The boundaries
of each flow are characterised by a shift to lower velocities

and density, caused by the formation of vesicles at the top
and base of the flow (Planke & Cambray 1998), and possib-
ly in places by weathering, alteration and rubble. The main
exceptions to this character are the high velocity dolerite
intrusions, encountered at about 600 m KB and 770 m
KB, and the zone of almost constant velocity and density
between about 2600 m KB and 2900 m KB, correspond-
ing to a thick, hyaloclastite sequence.

The dots near the compressional velocity log (Vp in

Fig. 2) mark depths and values of ultrasonic measurements
of P-wave velocity made by GEUS on a number of core
samples. The measurements were taken on 25 mm core
samples, pressure-saturated with distilled water and using
piezo-electric transducers of centre frequency 1 MHz
(2380 m core) and 2.5 MHz (all other cores) at room tem-
perature and pressure. Although the scale of the display
makes a visual match difficult, it can be seen that there is
generally a good correlation between the core measure-
ments of P-wavespeed and the log data. With only two or
three exceptions, at 2218 m KB, 2455 m KB and 2972 m

Fig. 2. P-velocity, S-velocity and density
logs, upscaled to 3 m sample interval using
a Backus average (Backus 1962) and dis-
played in measured depth relative to KB.
Core points are plotted from data supplied
by GEUS for comparison.

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

In Fig. 3, the anisotropic parameters are greatest where
the

logs show the most variance over the 3 m averaging

interval.

They

indicate

only

the

layering

component

the

anisotropy since we have no data on the intrinsic anisotro-

py of the basalt. However, delta is small with a mean of zero
while epsilon is positive with most values less than 0.05 and

almost all values less than 0.1. We found a similar range
of epsilon and delta when the averaging interval was in-
creased to 10 m. This means that the modelled effect of
layering is to produce horizontal compressional wavespeeds

around 5% faster than vertical wavespeeds, a prediction
which contradicts the observation by Kiørboe & Petersen
(1995) of vertical compressional wavespeeds being about
10% higher than horizontal compressional wavespeeds in

the upper 800 m of the basalt beds. Kiørboe & Petersen
(1995) appeal to vertical fractures associated with colum-
nar basalts and master joints to explain their observation,
a point which we discuss below.

Azimuthal anisotropy direction

Aligned vertical fractures would be expected to result in
azimuthal anisotropy. To test for this in Lopra-1/1A, the
dipole shear sonic tool was logged in four-component
mode whereby both of the two orthogonal dipole receiver

arrays recorded signals from each of the two orthogonal
dipole sources, energised sequentially. The data were proc-
essed for the presence of fast and slow shear waves corre-
sponding to shear propagation along the borehole with
polarisations parallel and perpendicular to the assumed
fractures. One output of the azimuthal processing is a log
of the azimuth of the fast shear wave. The direction of the

fast shear wave is found by determining the shear wave-
form rotation which minimises the cross-energy, for ex-
ample the energy recorded by the Y-polarised receiver from
the X-polarised source (Esmersoy et al. 1994), then pick-
ing the faster of the two shear estimates. The difference in

cross-energy between its maximum and minimum excur-
sions as a function of rotation angle is a measure of relia-
bility of the anisotropy estimate. In Figs 4a and 4b we
plot the fast shear direction results in different depth in-
tervals, where the vector from the centre of the plot to a
given depth point has an azimuth corresponding to that
of the fast shear polarisation and a magnitude correspond-
ing to the cross-energy difference normalised by the total
energy. In order to display only the most reliable estimates

of the fast shear direction, the estimates were windowed
according to cut-off values of calliper reading, the esti-
mated error in azimuth, the anisotropy estimate and the
normalised cross-energy difference (cut-off values indicat-

ed on the figure). The plots are radially symmetric through
the origin because of the 180° ambiguity in determining
azimuth. In Fig. 4a, from 511-540 m KB, we see that
there is a well-defined fast shear direction at N31°E, broad-

consistent with the NE-SW strike of one of the two

mapped master joint sets (Kiørboe & Petersen 1995, fig. 1).

Below 540 m KB, the fast shear direction rotates through
100°

N131°E,

although this direction is less well defined.

In Fig. 4b, there is a fairly consistent fast shear direction
of about N144°E in the interval 2991-3502 m KB, but
with a larger scatter in azimuth. The cross-energy cut-off
value in Fig. 4b has been reduced, compared to fig. 4a, to

capture more estimates. The estimates of the fast shear
direction deeper than 540 m KB are consistent with the
NW-SE strike of the second mapped joint set in Kiørboe
& Petersen's (1995) fig. 1.

From the earlier VSP and refraction profile data, Kiør-

Fig. 3. Log of Thomsen's (1986) weak
anisotropic parameters epsilon and delta,
modelled from the Backus upscaling of
the 15 cm logs to 3 m, assuming isotropic
individual layers. The compressional sonic
velocity is shown for correlation, also
upscaled to 3 m.

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

boe & Petersen (1995) reported vertical V_p values about

10% higher than horizontal V_p in the upper 800 m of the

section around the well and suggested that a combination

of vertical, columnar fractures and NW-SE master joints
crossing the ray-paths in the refraction profile may be re-
sponsible. The present analysis shows little azimuthal an-
isotropy on the dipole shear log and, while the fast shear
direction appears to be N131°E below 540 m KB, there
is a well-established, fast shear direction of N31°E from
511-540 m KB. If the shear azimuthal anisotropy results

from fractures or stress, then the compressional wavespeed
anisotropy should follow the same directions. Formation
microscanner data from the Lopra Deepening Project show

the presence of sub-horizontal conductive features, sug-
gestive of horizontal fractures, although there is a ques-
tion as to whether these are natural or drilling induced.
The only conventional core (2380 m KB) shows cement-
ed, horizontal fractures (L. Kiørboe, personal communi-
cation 1996) but few vertical fractures were observed, con-
sistent with the weak azimuthal anisotropy observed in

Fig. 4. a : Fast shear direction estimated
from the dipole shear log over the depth
interval 511-1010 m KB. The log was
recorded in four component mode and
rotated to minimise the cross-energy.
Two orthogonal directions are evident.
The better-defined direction is over a
fairly short interval from 511-540 m
KB, consistent with one of the mapped
master joints, and parallel to the offset
seismic surveys described in Kiørboe &
Petersen (1995). b : Fast shear direction
estimated over the interval 2991-
3530 m KB. The fast direction is less
well defined than in the shallower
interval but is consistent with mapped
joints.

User depth 511-1010 m
Data extent 174.346-2121.4 m

Cutoff values

Max. calliper (inch): 11
Min. ITT-based anisotropy: 1
Max. azimuth error (deg): 10
Min. cross-energy difference: 20

450

540

630

720

810

900

990

1080

Major tectonic axes

South-Nor

th (%)

-40

-80

-40

West-East (%)

-80

Depth

(

)

3600

3500

3400

3300

3200

3100

3000

2900

-10

-20

User depth 2991-3530.19 m
Data extent 2991-3502.91 m

Cutoff values

Max. calliper (inch): 11

Min. ITT-based anisotropy: 1

Max. azimuth error (deg): 10

Min. cross-energy difference: 1

Major tectonic axes

South-Nor

th (

-10

West-East

(%)

-20

-30

Depth (

)

-10

-20

-30

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

the dipole shear log. In addition, the layer-induced ani-
sotropy modelled by upscaling the log data suggests that
horizontal compressional wavespeeds should exceed ver-
tical wavespeeds by around 5%. Can we resolve the ap-
parent contradiction? Possible explanations include: (1)
the high-wavespeed, dolerite intrusions seen on the logs
down to 850 m KB affect the vertical velocity locally; (2)
the layering- and fracture-induced anisotropy revealed by
the well logs is not representative of the volume sampled
by the VSP and earlier refraction profile; (3) the unre-
versed refraction profile sees low apparent velocities be-
cause of the basalt flows that dip 3-7° to the north-east;
and (4) there is a mis-correlation of events between the
refraction profile and the earlier VSP. Kiørboe & Petersen
(1995) reported that a 700 m refraction profile shot to-
wards the WNW from the 1988 VSP source point through
the well yielded an apparent V_p of 4.8 km/s, the same as

that observed over the offset range 2.3-14 km on the long
refraction profile. The azimuth differences of these two
profiles weakens the case that structural dip or aligned
master joints affect the velocities. That the fractures in
the core sample are cemented also discounts the possibil-
ity that open, vertical, hexagonal fractures are prevalent,
although we cannot disprove this possibility because of
the small volumes sampled by core and borehole logs. Our
preferred explanation is that the geometry of the dolerite
intrusions increases locally the estimate of the vertical ve-
locities at the well. Further support for this hypothesis
comes from the well-tie, described below, where there is
evidence that the VSP sees a higher velocity than the son-
ic tool in the shallow section, possibly due to the dolerite
intrusions providing a VSP ray-path away from the bore-
hole that is faster than that seen by the sonic tool along
the borehole. Kiørboe & Petersen (1995) also found that
the velocities derived from the 1988 VSP were too fast to
simulate the arrival times on the longer offset data.

Log-derived seismic characteristics and
well-tie

In this section we develop further the seismic properties
of the stacked basalt flows in Lopra-1/1A as modelled from
the log data. Figure 5 displays the V_p / V_s ratio and the

normal incidence, two-way geometrical spreading, com-
puted using Newman's (1973) relation:

for the 3 m Backus-averaged data since there is no expec-
ted dependency on sample interval. In this relation,

D _two-way is the two-way loss, relative to the amplitude at 1 m

from the source, of a seismic wave propagating down
through a 1D stack of i layers, each of thickness d _i and

interval velocity V _i, and back to the surface again. V ₁ is

the velocity of the first layer and in this case is taken to be
2792 m/s. Because the thickness of the superficial layer
between the source and the top of the sonic log is
177.67 m, the two-way loss at the top of the sonic log is:

two-way

20log(2

177.67)

51.01 dB re 1 m

Figure 5 shows the modelled geometrical spreading in the
basalt flows, compared to the spherical loss in a uniform
medium for reference. The extra two-way geometrical
spreading due to the velocity variations is around 5.5 dB
for a reflector at the total depth of the well, resulting in a
spreading loss of 82.5 dB re 1 m.

The V_p / V_s ratio from the 3 m averaged logs (Fig. 5) is

almost constant over the well at 1.85, with a standard
deviation of 0.04. We also cross-plotted the log values of
V_p and V_s, upscaled to 1 m to reduce fluctuations, using

a Backus average. Figure 6 shows the cross-plot of V_p to

V_s

with a histogram of each variable indicating the number

of data points falling into velocity bins of 100 m/s for V_p

two-way

= 2

Fig. 5. Plot of logged V_P/V_S ratio and

short-offset geometrical spreading com-
puted using Newman's (1973) relation for
both the logged velocities and a constant
velocity medium. 3 m averaged logs.

-85

-90

-75

-65

-55

ratio

Measured depth KB (m)

Losses (dB r

e 1

)

3600

3000

2000

1000

2.8

2.6

2.4

2.2

2.0

1.8

1.6

1.4

-80

-70

-60

-50

Geometrical spreading

Spherical spreading

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

and 55 m/s for V

. The bin-widths are scaled approximately

by the V

/V_S

ratio, for which a value of 1.84 was obtained

by linear regression with a standard deviation of 0.01. The
similarity of the histograms supports the linearity of V_P

and V_S over such a large range of values. This linearity is

consistent with the observation of Planke & Cambray
(1998) who studied subaerially emplaced lava piles sam-
pled in Hole 917A of the Ocean Drilling Programme.
The absence of a trend, with the possible exception of the
hyaloclastite interval from 2600-2850 m KB, suggests that
there is no significant compaction effect with depth.

We computed two-way, plane-wave, transmission loss-

es for the 15.24 cm logs as well as for logs Backus-aver-
aged to 30 cm, 50 cm, 1 m, 3 m, 5 m and 10 m (actually,
the nearest multiple of 15.24 cm to these values). The
transmission losses are the 1D, ray-theoretical loss from a
unit amplitude plane-wave source pulse which is partially
transmitted and reflected from each interface in its two-
way path from the surface to a given reflector and back to
the surface again. It is computed from the well-known
relation

TL =

(1 - R

)

where R _i is the plane-wave, normal-incidence, reflection

coefficient at the i -th boundary

and where ρ _i and V _Pi are the density and compressional

wavespeed of the i -th layer. In this simple model, there is
no intrinsic loss. Instead, the loss is due to progressive
scattering and consequent removal of energy from the first-
arriving, primary pulse. The results are displayed in Fig. 7

along with the V

log for correlation. Losses over the full

interval vary non-linearly from 15 dB (10 m log) to 73 dB
(50 cm log), because of the non-linear dependence on the

reflection coefficient. The highest loss is seen at the 50 cm
sampling. As suggested by O'Doherty & Anstey (1971),
an impedance gradient represented by a single large re-
flection coefficient has more effect on the transmission
losses than several smaller coefficients, so very fine samp-
ling (15 cm) can produce a smaller loss than the upscaled
logs, as seen here. However, blocking intervals larger than
bed thicknesses progressively fail to represent beds at all
and so will predict less loss. While there is the expected
strong dependency of transmission losses with sample
interval, all the curves show broadly similar features, with
the lower contrast, hyaloclastite interval below 2600 m
KB exhibiting reduced loss compared to the section of
high contrast flow units, which resembles the cyclic ex-
ample in O'Doherty & Anstey (1971) and which is expec-
ted to have a high scattering loss.

The combined effects of modelled normal-incidence

spreading and plane wave transmission losses range from
97.5 dB to 155.5 dB re 1 m, where the wide margin re-
sults from the uncertain effects of the transmission losses.
We shall return to this point in the analysis of the VSP
amplitudes. However, we emphasise that the loss estimates
here are those due to near-normal propagation in a 1D
medium. As offset increases, geometrical losses increase
rapidly due to refraction effects at high velocity contrast
boundaries. Similarly, transmission coefficients vary
strongly with incidence angle at high contrast bounda-
ries. The cone of forward propagation of the compres-
sional wave is limited by critical angle effects and offers
opportunities for mode conversions to be observed over a
wider range of incidence angles and offsets.

The VSP first arrival travel times were picked to derive

Fig. 6. V_P-V_S cross-plot of 1 m Backus-

averaged logs with histograms and linear
regression. The histograms show the
frequency distributions of V_P and V_S data

gathered in bins of 100 m/s and 55 m/s
respectively, to maintain the same
number of bins for each variable. The
regression yields a V_P/V_S ratio of 1.84

with a standard deviation of 0.01. The
similarity of the histograms supports the
choice of a linear regression.

i + l

pi + l

i + l

pi + l

(m/s)

(

/s)

Frequency

eque

Vp histogram

histogram

= 0.5437 Vp - 16.647

= 0.9512

100

200

100

3500

2500

1500

3500

4500

5500

6500

7500

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

a seismic two-way time to true vertical depth curve. The
time-depth curve from the VSP was used to 'calibrate'
the TVD-corrected sonic log, provide a fine-scale, seismic
time-depth interpolation and to generate a synthetic seis-
mogram. A drift curve was computed by integrating the
compressional slowness log between the VSP time-depth
pairs and taking the difference between the VSP and son-
ic interval times. We observed positive drift (VSP interval
time greater than sonic) over most of the logged interval.
This implies a lack of environmental effects on the sonic
(which usually increase the sonic slowness) leaving the
normal dispersion effect between seismic and sonic fre-
quencies as the dominant mechanism. Negative drift (sonic
interval time greater than VSP) was seen in the shallow
part of the well, above 800 m KB. This would be normal
in a sedimentary sequence, indicating a washed-out hole
or altered, unconsolidated formations. However, in the
basalt environment, formation alteration is unlikely; even
though the well shallower than 800 m KB is often out of
gauge, it is not different from the rest of the borehole
drilled in 1981 which displays positive drift. Possible ex-
planations of the negative drift are unreliable VSP picks
due to shallow, unsupported casing interference (the com-
pressional wavespeed of the formation is similar to that of
the casing extensional mode), or possible refraction along
the high velocity (6.5 km/s) dolerite intrusions. If the VSP
wave front refracts along the intrusions such that the bore-
hole does not form the VSP raypath, then the VSP inter-
val time will be less than the sonic interval time. In any
case, the picks above 800 m have been interpreted in such
a way as to avoid undue, and possibly unrealistic, correc-
tion to the sonic.

The corrected compressional sonic log was used to con-

vert the shear sonic and the density logs from a common
depth scale to a common two-way time scale which is
assured to match that of the VSP. Note that this implicit-
ly changes the V_P / V_S ratio, since the VSP was not used to

correct the shear sonic slowness values. Deeper than 800 m,

the

compressional slowness drift is about 15 ms in 2200 m,

or 6 µs/m. However, the average interval slowness is about
190 µs/m, so the drift correction is reasonably significant
at 3.4% in slowness. Care was taken in choosing the cor-
rection points to avoid introducing false reflection events.

The time-based logs drove a 1D model of equal time-

thickness layers. The resulting primaries-only, reflection
coefficient sequence without transmission losses was con-
volved with a 40 Hz, zero-phase, Ricker wavelet to create
a synthetic seismogram. The synthetic was spliced into
the VSP up-going wavefield along the two-way time-depth

curve to facilitate event correlation (the right-hand panel
of Fig. 8). The VSP has been waveshaped to a zero phase
wavelet of bandwidth 10-70 Hz and both the synthetic
and VSP traces are displayed in reverse SEG polarity, so
that an increase in acoustic impedance with depth is dis-
played as a black peak. Figure 8 shows the correlation from

a Two-Way Time (TWT) of 650 ms, where several events
can be seen to tie in time and in character, resulting in an
unambiguous correlation. In an igneous province, a tie of
this quality is relatively unusual and suggests that the lat-
eral variability of the basalt flows is mild, at least over the
extent of a VSP Fresnel zone which is several tens to hun-
dreds of metres, depending on the elevation of the VSP
tool above the reflector. Residual ringing from the unce-
mented casing is visible in the up-going VSP wavefield on

the right edge of the section.

Correlations from the well-tie

Using Fig. 8 we discuss the correlation to the lithological
summary taken from the Lopra-1/1A End of Well Report
(EWR), subsequently modified by R. Waagstein (person-
al communication 2001). The displayed interval from
650-1430 ms TWT shows V_P, V_S, density and Poisson's

Fig. 7. Modelled plane-wave, normal-
incidence, two-way transmission losses as
a function of log sampling, together with
V_P log for correlation. Losses are non-

linear with sampling but show generally
similar behaviour with high contrast
layering resulting in the greatest scattering
loss.

10 m

5 m

3 m

15 cm

1 m

30 cm

50 cm

Losses (dB)

-80

-60

-40

-20

Measured depth KB (m)

3600

3000

2000

1000

8000

6000

4000

(

/s)

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

Ratio logs. At the left edge of the display is a non-linear
scale in True Vertical Depth extending from 1714 m to
TD at 3533 m TVDSS, which corresponds to 3565 m
KB. Horizontal lines mark particular correlations between
the well logs, the lithology summary, the synthetic seis-
mogram and the VSP. These correlations are summarised
in Table 1 and further comments relating to the pre-drill
targets are made below.

From 882-912 ms, the VSP shows a package of strong

reflections, with a white trough at 894 ms, seen on the
pre-drill VSP at 920 ms and identified as one of the target
horizons for the well. This event was prognosed by Kiør-
boe & Petersen (1995) at 2.34 km, 162 m below the ori-
ginal TD of Lopra-1 and interpreted as a decrease in im-
pedance. In fact, the reflection package comprises at least
two near-tuning events. A sharp increase in impedance at
882 ms (2361 m KB, 2345 m TVDSS) corresponds to
the top of a massive basalt flow that gives rise to a black
peak on the VSP. The trough associated with the sidelobe
to this wavelet reinforces a broad, weak trough caused by
the decrease in impedance at the base of the flow around
894 ms (2401 m KB, 2385 m TVDSS), which reflects the

highly amygdaloidal top of an underlying flow. The re-
flection package is seen more strongly on the VSP than
on the synthetic seismogram, possibly due to lateral het-
erogeneity near the well.

Another package of strong events was drilled above TD

from 1274-1340 ms. During drilling, it was hoped that
these reflections, seen at 1350 ms on the pre-deepening
VSP (Kiørboe & Petersen 1995), might mark either sil-
iciclastic sediment or basement. However, the sharp drop
in impedance at 3427 m KB (3397 m TVDSS) at the base
of a thick, massive basalt bed results in the trough at
1294 ms seen on both the VSP and the synthetic, which
correlates with another hyaloclastite sequence with a thin
tuff at the top (R. Waagstein, personal communication
2001). Another drop in impedance was drilled at 3512 m
KB (3480 m TVDSS), which also corresponds to a strong
white trough at 1328 ms on both the synthetic and the
VSP. It probably corresponds to the event at 1350 ms seen
on the pre-drill VSP. It displays moveout to earlier times
with decreasing geophone depth, indicating dip of the beds.

Just below TD, at 1412 ms on the VSP, is a persistent,

large-amplitude, symmetric, white trough, which may be

Fig. 8. Correlation of the VSP up-going wavefield with the log-derived, reflection coefficient sequence convolved with a 40 Hz zero-phase
Ricker wavelet. The VSP wavefield has been waveshaped to zero phase over the bandwidth 10-70 Hz. The VSP and synthetic are
correlated with the time-based logs and lithostratigraphy (R. Waagstein, personal communication 2001) as listed in Table 1.

Lithology
summary

Basalt flows

Hyaloclastites

Massive flow

Hyaloclastites

+ basalt beds

Hyaloclastites

without

basalt beds

Hyaloclastites

with

basalt beds

Massive flow

Hyaloclastites

with minor
basalt beds

1743
1797

1848

1900

1957

2007

2065

2122
2177

2234

2289

2342

1402

2454

2502

2550

2600

2649

2698

2748

2796

2844

2894

2995

2945

3048

3098
3151

3204

3256
3307

3360

3461

3413

3514

TVD

Depth

(
m

)

3000 5000

7000

P-wave (m/s)

2000

3000

S-wave (m/s)

1.5

2.0

2.5 3.0 0

Density (g/cm

)

0.30

0.10

0.50

Poisson

(
m

off

700

800

900

1000

1100

1200

1300

700

800

900

1000

1100

1200

1300

1400

Synthetic + VSP

3 7 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96 102 105

Total depth = 3565m MD KB

= 3533 TVD SS

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

Table 1. VSP eve

VSP eve

Top of VSP

Top of packa

g
e of stro

reflectio

sharp i

crease i

peda

Decrease i

peda

Base of packa

g
e of stro

reflectio

Moderate reflectio

Top of low reflectio

terval

Base of low reflectio

terval

Top of packa

g
e of stro

reflectio

I
n

crease i

peda

Botto

of VSP

Decrease i

pede

ce, dippi

bed

Botto

of packa

g
e of stro

reflectio

TD (total depth of Lopra-1/1A)

ifica

t decrease i

peda

FMS lo

g
i

terpretatio

(R. Waa

g
stei

, perso

al co

icatio

2001)

Kiørboe & Peterse

(1995)

Basalt flows

Hyaloclastites

Top

assive basalt flow

(2361-2401

)

y
g
daloidal basalt flow

(2401-2417

)

Hyaloclastites with basalt beds

Hyaloclastites with

o basalt beds

Hyaloclastites with basalt beds

Massive basalt bed (3396-3427

)

top hyaloclastites (0.9

tuff at top)

Base

assive basalt bed (3504-3512

);

hyaloclastites with

i
n

or basalt

beds (3512-?

)

496

865

882

894

912

980

1088

1274

1282

1294

1326

1328

1340

1347

1412

1320

2319

2361

2491

2450

2616

2882

3396

3427

3510

3512

3732

TWT

illisec.

920

1350

Depth

below KB

etres

2398

2443

2612

2880

3426

3512

lower basalt series

pillow lava series

pillow debris series +

basaltic tuff 1 +

tuffaceous sa

d 1

basaltic tuff 2 +

basaltic sa

d 1 + tuffs 1-2

tuffs 3-9

basal part of tuff 9

tuff 10

pre-basaltic tuff series B

(tuffs 11-12)

sk Olie- o

g
Gasproduktio

sk Operatørselskab (1997)

VSP before deepe

i
ng

Litholo

g
ical u

TWT

illisec.

Depth

below KB

etres

TVDSS

etres

Depth

below KB

etres

2340

Lithostrati

g
raphic u

its

Lopra-1/1A VSP

Sharp decrease i

Base

assive basalt bed;

3565

1304

2303

2345

2385

2434

2600

2864

3366

3397

3478

3480

3533

3700

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

interpreted as a significant drop in impedance. This is
about 86 ms, or 222 m, below the deepest VSP level at
3510 m KB (3478 m TVDSS) and therefore 167 m be-
low the final TD of 3565 m KB (3533 m TVDSS), as-
suming an extrapolation velocity of 5150 m/s. The event
is as tantalising as that seen at 920 ms on the pre-deepen-
ing VSP, which was drilled some 162 m below the origi-
nal TD of Lopra-1. However, similar, though weaker, 'soft
kicks' seen on the seismic data have turned out to be due
to contrasts within the volcanic pile and the reflection at
1412 ms TWT is unlikely to be basement, so it is proba-
ble that more volcanic sequence lies below the present TD.
In contrast with the event at 1328 ms, this strong arrival
below TD shows little moveout and suggests low dip.

VSP loss estimates and amplitude modelling

As noted above, ray-theoretical estimates of transmission
losses are strongly dependent on the log sampling, so we
derivedloss estimates from the VSP downwavebefore turn-
ing tOFull waveform modelling to simulate the observed
propagation effects. The contoured power spectrum by
depth is shown in Fig. 9, after windowing the first 150 ms
of data from the 2 ms sampled downgoing wavefield. Two
points are apparent: (1) there is a low-frequency roll-off
to the data caused by the start frequency of the vibrator
sweep at 10 Hz, (2) there is a smooth decline in frequen-
cy content with depth.

We estimated the root mean square (RMS) amplitudes

within a window of 150 ms about the VSP first breaks,
corrected the amplitudes for geometrical spreading using
the spreading loss curve displayed in Fig. 5 and plotted
the results in Fig. 10 together with the ray-theoretical
transmission losses at 3 m, 5 m and 10 m sampling. Since
the amplitude of the VSP top level is arbitrary, we matched
the slopes of the transmission loss curves by eye. The VSP
amplitude decay curve, after spreading correction, shows
a character similar to the modelled transmission losses,
although, given our earlier comments on the unreliability
of transmission losses, the match to the transmission loss
curve at 5 m sampling is probably coincidental. However,
the change in slope around 2500 m KB on the modelled
curves, which is due to the transition from high contrast
basalt beds to low contrast hyaloclastites, is also evident
on the VSP. By reciprocity, the transmission seismogram
going back up through the basalt sequence is the same as
that going down through the sequence. Hence, the reflec-
tion seismogram should be the time-delayed, one-sided
correlation of the down-going wave. Strictly speaking, the
two-way loss estimates should be made after convolving

Fig. 9. Contoured power spectrum of the energy in a 150 ms win-
dow around the down-going first arrivals in the VSP. The up-
going wavefield was removed first. The contour levels are dB down
from the peak value.

Fig. 10. Losses from RMS amplitudes
estimated in 150 ms window around the
first arrivals in the VSP, after correcting
for geometrical spreading using the log-
derived spreading loss estimates from
Fig. 5. The first VSP level amplitude is
arbitrary and has been selected visually to
overlie the upscaled log curve of modelled
transmission losses with the most similar
slope. Regression lines have been fitted
over the VSP intervals indicated to esti-
mate effective Q for a wavelet dominant
frequency of 36 Hz.

200

160

120

1320

1720

2120

2520

2920

3320

-30

-20

-10

-40

-30

-20

-10

F
r
eque

n
cy Hz

Depth KB (m)

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

Measured depth KB (m)

Losses (dB)

3600

3000

2000

1000

-45

-25

-20

-15

-10

-5

Regression (i) Q = 35

Regression (ii) Q = 112

Transmission losses

from upscaled logs

VSP estimated losses

10 m

5 m
3 m

-40

-30

-35

the down-going wave with itself. Since the amplitude ef-
fect of cascaded filters is to take the product of their gain
functions, we have approximated the amplitude effect of
the two-way propagation by doubling the dB loss esti-
mated from the one-way measurement of RMS time-do-
main amplitudes.

Because of the change in slope of the transmission loss

curves around 2500 m KB, we estimated two effective Q
factors to represent the scattering loss. Over the interval
of periodic layering, from 1350 m KB to 2450 m KB, we
fitted a linear trend and estimated losses corresponding to
an effective Q of 35 for a dominant period wavelet of
27.5 ms. The 95% confidence limits on the regression map
to a range in effective Q of 32 to 40.

From 2450 m KB to 3510 m KB, the loss curve is re-

duced and a linear regression resulted in a loss correspond-
ing to an effective Q in excess of 100. The inverse correla-
tion of effective Q values with the change in impedance
contrasts further supports the inference that the volcanic
sequence has low intrinsic loss and that its attenuation is
principally due to scattering. This is consistent with the
results of other, similar studies (e.g. Pujol & Smithson
1991) which found that spreading and scattering losses
could account for observed VSP amplitude behaviour in
basalt sequences. Evidently, we can find a log sample rate
such that the ray-theoretical losses match the VSP loss
estimates. However, given the large variability in the trans-
mission losses with sampling, we undertook further anal-
ysis to test the hypothesis that scattering loss is the domi-
nant mechanism.

We estimated spectral ratios of windowed VSP down-

wave traces at 2420 m, 2980 m, 3044 m and 3500 m KB,
using the downwave at 1880 m KB as a reference. The
spectral ratios are plotted in Fig. 11 with the amplitude
spectrum of the reference level. Over the rather limited
frequency interval of the strongest signal (17-40 Hz), the
slopes of the spectral ratios are effectively the same and
rather flat. In a lossy medium, the slopes are modelled by

where c is the average velocity over the interval

z . The

four slopes in Fig. 11 correspond to depth intervals of
548 m, 1100 m, 1164 m and 1620 m, but there appears
to be little variation of slope with the depth interval. The
real

VSP

amplitude

loss

not

well

modelled

frequency-

dependent attenuation.

Nevertheless, the variation in spectral power with depth

in Fig. 9 does show a loss of high frequencies with depth,
mainly in the shallower part of the section, with little ap-
parent bandwidth change over much of the deeper part of
the volcanic sequence. We therefore modelled the propa-
gation of a VSP pulse generated at ground level (GL)
through a 185 m uniform layer on top of the pile of ba-
salt flows logged from 185 m below GL and emerging
into a uniform half-space at 2160 m below GL (Fig. 12).
The 1975 m interval was modelled using a full-elastic,
1D modelling code based upon the Kennett algorithm
(Kennett 1974, 1983) and developed at Schlumberger
Cambridge Research. The 15 cm log data were upscaled
to 3 m by Backus averaging to reduce the computational
cost. Equivalent medium averaging, using windows of
approximately 1/20th of the dominant seismic wavelength,
provides a convenient method of upscaling logged data to
allow efficient elastic waveform modelling, while retain-
ing fidelity of both the travel time and amplitude infor-
mation of the log scale model (Folstad & Schoenberg
1992, 1993).

Propagation was modelled both with and without mul-

tiple scattering, with 3D geometrical spreading from a
point source. The injected wavelet at the top of the stack
is a zero-phase, 60 Hz Ricker wavelet. (The wavelet ap-
pears not to be zero phase because of near field effects.) At
the base of the stack, the escaping wavelet modelled with-
out multiples is zero phase, but its amplitude has been
diminished, undergoing a one-way loss of 46.9 dB due to

Fig. 11. Spectral ratios computed from
four VSP levels relative to a reference level
at 1880 m KB. The amplitude spectrum of
the reference trace is also shown to
indicate the signal frequency band.

- log

Spectral a

plitude

Frequency (Hz)

Spectral ratio (dB)

400

800

1200

-25

-20

-15

-10

-5

2420

2980

3500

3044

1880 spectrum

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

the combination of one-way geometrical spreading and
transmission losses. The escaping wave modelled with all
internal multiples has been phase rotated within the 40 ms
analysis window and has a higher RMS amplitude than
the wavelet propagated without multiples, because short-
period multiples have served to boost amplitudes within
the analysis window. The RMS amplitude of the escaping

wave with multiples is -37.5 dB relative to the input wave-
let at the top of the stack. However, energy has been re-
moved from the front of the wavelet, its trough-to-trough
duration has increased and a coda has developed. These
effects are due to scattering by the high-contrast layering.

We ran a further model, with multiples, to study the

frequency-dependent effects of multiple scattering by the
periodic layering by using a wide-band source signature
defined in the frequency domain from DC to the Nyquist
frequency at 125 Hz, corresponding to 4 ms sampling (in
contrast to the real VSP which was acquired at 2 ms sam-
pling). We also altered the model by adding a lower half-
space simulating a massive sand unit and placing the

deeper receiver at 300 m into the sand below the base
basalt which was again modelled at 2160 m.

The Fourier domain amplitudes of the deep- and shal-

low-receiver down-going compressional waves are display-

ed in Fig. 13a, where the low-pass filtering effect of the ba-
salt sequence can be seen, with quite deep notches ap-
pearing from 39 Hz, although higher frequency peaks al-
so appear, such as that at 85 Hz.

The spectral ratios of the wide-band synthetics are

shown in Fig. 13b with a linear regression to 80 Hz, avoid-
ing the higher frequency side-lobes. The regression pro-

vides an effective Q estimate of 32, with 95% confidence
limits of 26 and 42, which is consistent with the time
domain estimate of 35 from the real VSP over the inter-
val of basalt flows shown in Fig. 10. However, we note
that while the real VSP displayed little or nOFrequency-
dependent loss over the interval from 1880-3500 m KB,
the full elastic modelling does support the assertion that
the amplitude loss can be modelled by scattering and ge-
ometrical spreading alone.

Loss estimates from the literature

Rutledge & Winkler (1989) made estimates of attenua-
tion from VSP data in the Upper Basalt Series in the
Vøring Plateau area of the eastern Norwegian Sea. From
451 to 1111 m below the sea-floor in 1289 m of water,
they found 105 basalt flows with about 10% of the sec-
tion comprising volcaniclastic sediments. Their estimates
of

overall

scattering attenuation of 2.7

-4

dB/m

(effec-

tive

25)

could

be accounted for by scattering loss mod-

elled from the sonic and density logs recorded in the well,
leaving an intrinsic attenuation of less than 0.6 × 10^-4

dB/m (Q

115). Their Q of 25 is somewhat less than the

estimated effective Q of 35 from the Lopra-1/1A VSP.

Pujol & Smithson (1991) reported values of effective

Q around 48, estimated from VSP data in thick, Colum-
bia Plateau basalt sequences containing some interbed-
ded clay zones. They also reported that scattering was the
dominant loss mechanism, since elastic modelling was able
to account for all the observed loss. The intrinsic losses in

Fig. 12. Full elastic model of VSP
downwave propagation through a stack
of 3 m layers derived from the Backus
average of the V_P, V_S and density logs

between 185 m below ground level (GL)
and 2160 m below GL. Propagation was
modelled with and without peg-leg
multiples and amplitudes were estimated
in a 40 ms window about the wavelets.

600

One-way loss (geometrical spreading and transmission losses with multiples) = 37.5 dB

One-way loss (geometrical spreading and transmission losses without multiples) = 46.9 dB

6 x 10

-3

4 x 10

-3

2 x 10

-3

-2 x 10

-3

-4 x 10

-3

-6 x 10

-3

185 m

RMS amplitude windows

Wavelet

Without internal multiples

With internal multiples

100

200

300

400

500

600

2160 m

Point source

Velocity model

Recorded time (ms)

6 x 10

-5

4 x 10

-5

2 x 10

-5

-2 x 10

-5

-4 x 10

-5

-6 x 10

-5

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

basalt were low and could not be determined from the
field data.

The values of effective Q estimated from Lopra-1/1A

is bracketed by those reported in the literature, and the
general trends and inferred loss mechanisms are consist-
ent. It is almost certain that more data points from west
of the Shetlands will result in a greater scatter of effective
Q estimates, corresponding to the variety of basalt envi-
ronments.

Modelled offset VSP

Figure 14 shows the wide-band P- and S-downwaves sim-
ulated at a horizontal array of receivers spaced 100 m apart
and located 300 m below the base basalt in a halv-space of

sand. The volume injection source is located in a uniform,

elastic layer, which is the overburden above the modelled
basalt interval. The horizontal array of receivers in a 1D
earth simulates a walkaway VSP shot into a single level
geophone. Multiples are included in the simulation. The

direct P arrival is evident and its amplitude decays rapidly
with offset due probably to

combination

strong geo-

metrical spreading and critical angle effects at larger off-
sets in a medium with strong velocity contrasts.

Also evident is a strong, low-frequency event which

dominates the shear record and is probably a mode con-
version, propagating through the basalt sequence and
emerging into the half space below. At the base basalt-
sand contact, it converts to a P-wave and is also recorded
as a strong event on the P-section, with an earlier arrival
time. Both events are visible over the offset interval 2000-
4500 m but neither event can be traced to zero offset.
The asymptotic velocity of about 3.4 km/s is high and
although it could correspond to the shear velocity of the
shallow dolerites, which display the highest interval ve-
locities in the sequence (Fig. 2), the event arrives before
the direct shear arrival curve and it must therefore have a
compressional leg for part of its ray path. The limited off-

set interval makes interpretation difficult and such con-
versions may be sensitive to the particular velocity-depth
function, but the observation offers some encouragement

Fig. 13: a : Input and escaping wave
spectra for a VSP downwave in a model
similar to that in Fig. 12, but with a
half space representing a sand unit at
the base of the basalt sequence. The
source wavelet is white over the full
spectrum to Nyquist. The effects of
scattering on the escaping wavelet are
evident in the loss and the sidelobes. b :
Regression of spectral ratios of the
downgoing wave referenced to the
input wavelet. The estimated effective
Q is 32.

5.0E-4

6E-3

4.0E-4

3.0E-4

2.0E-4

1.0E-4

0.0E+0

4E-3

2E-3

0E-0

Frequency (Hz)

120

100

Down-wave at 185 m

Down-wave at 2160 m

plitude do

-wa

v
e at 185

plitude do

-wa

v
e at 2160

7E-3

5E-3

3E-3

1E-3

-60

-50

-40

-30

-20

-10

Regression estimate Q = 32

Frequency (Hz)

Spectral ratio (dB)

Least squares fit

Spectral ratios

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

for the use of mode-converted shear waves to probe below

piled basalt flows, as described by Emsley et al. (1998).

Conclusions

The borehole seismic data recorded in the Lopra-1/1A well
show that the average P-wave velocity is high at 5248
m/s from mean sea level to the deepest geophone level at
3510 m KB. Amplitude loss over the stacked basalt flows
is moderate, corresponding to an effective Q of 35. How-
ever, this amplitude loss can be modelled by geometrical
spreading and elastic scattering, implying that intrinsic
attenuation is low. Persistent up-going events are evident
within the interval logged by the VSP, even before wave-
field separation, suggesting that lateral continuity of the
basalt flow contacts is consistent over the several hundreds

of metres that correspond to the VSP Fresnel zone radius.
We observed an unambiguous tie between the VSP and a
primaries synthetic seismogram which allows a detailed
correlation between the stratigraphy revealed by the drill-
bit with the events on the VSP reflected wavefield. Both
the pre-deepening targets at 920 ms and 1350 ms appear
on the VSP at earlier times of 894 ms and 1330 ms. Both
events result from impedance contrasts within the vol-
canic sequence and it is likely that a strong reflection event,
visible at 1412 ms TWT is also within the volcanic se-
quence. Its polarity indicates a decrease in acoustic im-
pedance with depth so it is therefore unlikely to be base-
ment. The prognosed depth of this event is 3732 m KB,
or 167 m below the final TD of the well at 3565 m KB.

The unprocessed horizontal components suggest the pres-
ence of a persistent down-going shear wave, directly gen-
erated by the twin vibrators used as a VSP source, which
in turn gives rise to up-going shear reflections. The V_P / V_S

ratio from the VSP is 1.8 ± 0.1 (estimated error), which is
in good agreement with the estimate of

1.84

0.01 (one

standard deviation) obtained from the P- and S-sonic log
data, and is rather constant over the logged interval. We
estimate layer-induced anisotropy of about 5% due to the

high elastic parameter contrasts in the basalt flows. Azi-
muthal anisotropy estimated from the dipole shear log is
low, but the direction of the anisotropy is consistent with
mapped master joint sets: the well-defined NE-SW di-
rection gives way to the less well-defined NW-SE direc-
tion at about 540 m KB. Our preferred explanation for
the vertical P-wavespeed being 10% higher than the hor-
izontal, as reported by Kiørboe & Petersen (1995), is that
vertical velocities are locally raised by the presence of fast,
dolerite intrusions.

Reflection seismic data are difficult to process and in-

terpret in basalt covered areas. The Lopra-1/1A borehole
dataset offers insight into the seismic properties of basalts
which we anticipate will be of benefit in designing and
processing reflection surveys, a topic which has attracted
considerable interest but brings with it acknowledged chal-
lenges. An immediate result is that 'basalt', in seismic
terms, cannot be represented by a uniform slab of hard
rock a couple of kilometres thick. We observe challenges
in the geometrical spreading, scattering losses, multiple
development and spectral colouring which point towards
low frequencies as the best hope for imaging beneath ba-

Fig. 14. Full elastic model of single level
walkaway VSP with the geophone located
300 m below the basalt-sand contact. The
left panel are P-waves while the right panel
are S-waves. The ray-traced first P and pure
S arrivals are superimposed. A mixed mode
conversion can be seen with significant but
low frequency amplitude over a limited
offset interval.

400

800

1200

1600

2000

2200

5
50

Offset (m)

T
w

o-wa

y ti

(
m

sec)

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

salts. This observation has been reported elsewhere in the
literature but this paper may allow some more specific
numbers to be applied to the basalt flows in the Faroese
area.

On the positive side, intrinsic attenuation is low and

propagation has been demonstrated through 3533 m of
basalts, tuffs and volcaniclastic sequences. Coherent re-
flections have been tied from the VSP to the synthetic
seismogram with confidence and another event below TD
has been prognosed. Modelled offset VSP propagation al-
so gave some hope for mode conversions, though with a
limited offset range and low frequencies.

Acknowledgements

This work was carried out while the first author was en-
gaged on a secondment with BP in Aberdeen. During this
time he enjoyed considerable support from colleagues in
BP, Shell and Schlumberger. In particular he would like
to acknowledge many discussions and guidance from Matt
Luheshi, Cameron Crook and Brian Mitchener. Chris
Chapman

answered

many modelling questions and Fraser

Louden recorded the VSP.

References

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Manuscript received 16 October 2001; revision accepted 14 March 2003.

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

Geological Survey of Denmark and Greenland Bulletin

Nr. 9, Scientific results from the deepened Lopra-1, Faroe Islands, pp. 23-40

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