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Marine Geology 215
Seismic inversion for acoustic impedance and porosity of
Cenozoic cool-water carbonates on the upper continental
slope of the Great Australian Bight
M. Huusea,*, D.A. Fearyb,1
a3DLab, School of Earth, Ocean and Planetary Sciences, Cardiff University, Wales, UKbAustralian Geological Survey Organization, Canberra, Australia
Received 30 September 2004; received in revised form 17 December 2004; accepted 21 December 2004
Abstract
Seismic inversion is used to estimate detailed seismic and rock properties, such as acoustic impedance and porosity, from
seismic data. Themethod is widely used and proven successful by the petroleum industry, but has hitherto not beenwidely adopted
for academic studies. This paper outlines a workflow and reports the application of model-based seismic inversion to a Cenozoic
cool-water carbonate succession on the upper continental slope of the Great Australian Bight that was cored during Ocean Drilling
Program Leg 182. Acoustic impedance data and the derived porosity distribution facilitate detailed studies of lithology,
compaction and fluid flow in the shallow subsurface (0–500m). A comparison of reflection and impedance data support the notion
that seismic reflection events arise from bed boundaries rather than from lateral changes in impedance, even where these are
significant. The uppermost continental slope of SWAustralia is swept by a strong (N0.5 m/s) geostrophic current, the Leeuwin
Current, and seismic profiles across the upper slope show geometrical similarities with contourite drifts. Cores taken through a
conspicuous mounded seismic facies at ODP Site 1131 suggest that bryozoan build-ups nucleated on top of contourite mounds on
the uppermost slope. Core recovery at three sites on a transect across the uppermost continental slope systematically decreased
with increasing acoustic impedance and depth of the drilled section regardless of age. Because of the enhanced interpretability
afforded by acoustic impedance and porosity data, and the possibility of predicting core recovery, the workflow outlined here
should be of use in a broad spectrum of continental margin studies.
D 2004 Elsevier B.V. All rights reserved.
Keywords: seismic inversion; acoustic impedance; carbonate; contourite; Great Australian Bight; ODP Leg 182
0025-3227/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.margeo.2004.12.005
* Corresponding author.
E-mail address: [email protected] (M. Huuse).1 Present address: Board on Earth Sciences and Resources,
National Research Council, Washington, DC, USA.
1. Introduction
Seismic reflection profiles provide images of
relative subsurface variations of acoustic impedance,
(2005) 123–134
M. Huuse, D.A. Feary / Marine Geology 215 (2005) 123–134124
i.e., they show the distribution of interfaces between
layers with different acoustic properties. Acoustic
impedance is the product of rock density and
compressional (P-wave) velocity. It is thus a measure
of physical properties that are commonly measured in
boreholes, such as bulk density and sonic velocity, as
well as being qualitatively observed in core and
outcrop as the dhardnessT of the rock. Seismic
inversion is a method of deriving seismic parameters,
such as acoustic impedance, from reflection seismic
data constrained by borehole data (e.g., Sheriff and
Geldart, 1995; Veeken and Da Silva, 2004). This
information may be used to derive the detailed
porosity and lithology structure of the subsurface,
provided there is a link between porosity (or
lithology) and acoustic impedance (Anselmetti and
Eberli, 1997; Marion and Jizba, 1997).
Seismic inversion for acoustic impedance is widely
used for subsurface interpretation by the petroleum
industry, particularly in settings where a unique
relation between acoustic impedance and porosity
can be established (e.g., Marion and Jizba, 1997). A
suite of inversion methods has been developed in
response to the need for detailed subsurface mapping
of porosity and pore-fluid variations as part of the
petroleum industry’s hunt for increasingly subtle oil
and gas reservoirs (Maver and Rasmussen, 1995;
Buiting and Bacon, 1999; van Riel, 2000; Story et al.,
2000; Veeken and Da Silva, 2004). The increasing
number of oil discoveries that have been made using
inverted seismic data has confirmed that inversion is
indeed a powerful means for enhancing the interpret-
ability of seismic data (e.g., Buiting and Bacon, 1999;
Vejb&k and Kristensen, 2000). Apart from their use
for standard reservoir interpretation, rock property
data and their derivatives (such as porosity) are useful
for a range of other studies that include fluid flow and
continental slope stability (e.g., Dugan and Flemings,
2000; Bunz and Mienert, 2004). Consequently, there
has been a recent shift in emphasis from inversion
focused at petroleum reservoir levels to also include
the overburden to prospective intervals in the inver-
sion. Such studies have proven particularly useful for
inferring drilling hazards caused by shallow gas or
overpressured intervals (e.g., Dutta, 2002; Dai et al.,
2004; Salisbury et al., 2004). However, despite the
advantages of impedance data for shallow hazard
detection and for seismic facies and lithology inter-
pretation, inversion methods have so far not been
widely adopted for non-petroleum research, such as,
e.g., the majority of the ocean drilling (DSDP, ODP,
iODP) activities. This is despite the fact that boreholes
are tied to detailed site surveys and regional seismic
surveys using synthetic seismic and seismic modelling
as an integral part of both academic and industrial
drilling programs (e.g., Lorenzo and Hesselbo, 1996).
When well-to-seismic ties have been made, and a
wavelet has been extracted, the final step of inverting
the data is comparatively straightforward.
This paper presents the workflow and application
of model-driven seismic inversion to estimate the
acoustic impedance and porosity of Cenozoic cool-
water carbonates in the Great Australian Bight, using
seismic data collected during site surveys (Feary,
1997) as well as core and borehole log data acquired
during Ocean Drilling Program (ODP) Leg 182 (Feary
et al., 2000, 2004). After outlining the method for
deriving the acoustic impedance section and estimat-
ing porosity, we discuss the enhanced interpretability
afforded by the impedance data by looking at the
internal structure of carbonate mounds and discussing
the origin and significance of reflections within the
Pleistocene slope succession. The advantages for
interpretation and the relationship between core
recovery and acoustic impedance makes seismic
inversion a powerful tool that could be useful for
most integrated studies of borehole and seismic data.
2. Geological setting: the Great Australian Bight
The Great Australian Bight is a siliciclastic-
starved, passive continental margin bordering the
Southern Ocean. The uppermost continental slope
along SW Australia is swept by a strong geostrophic
current, the Leeuwin Current, and associated coun-
ter-flow and eddies, the intensities of which vary on
multiple time scales (Cresswell and Peterson, 1993;
Creswell and Griffin, 2004). The target of the ODP
Leg 182 drilling campaign was an improved under-
standing of the factors that controlled cool-water
carbonate deposition of Eocene to Quaternary
sequences on the continental shelf, slope and basin
(Feary and James, 1998; Feary et al., 2000, 2004).
The shelf edge to upper slope in the Great
Australian Bight consists of a spectacular succession
M. Huuse, D.A. Feary / Marine Geology 215 (2005) 123–134 125
of progradational clinoforms forming an approxi-
mately 500 m thick wedge of Pleistocene cool-water
carbonates (Fig. 1; Feary and James, 1998; Hine et
al., 1999; Feary et al., 2000, 2004; James et al.,
2000). Leg 182 Sites 1127, 1131 and 1129 form a
closely spaced dip transect across this uppermost
slope wedge (Fig. 1). The sediments forming the
slope wedge are packstones to wackestones contain-
ing a wide variety of skeletal grains produced on
the adjacent shelf (Hine et al., 1999; Feary et al.,
2000, 2004).
Apart from the unusually thick Pleistocene
succession, Leg 182 also cored uppermost Cenozoic
bryozoan mounds located at present-day water
depths of ~200–350 m (Fig. 1; Feary and James,
1995, 1998; Feary et al., 2000; James et al., 2000),
Fig. 1. Seismic line illustrating the progradational clinoforms and moun
beneath the Eucla Shelf and upper slope. Vertical scale is two-way travel ti
m on the slope to about 170 m on the shelf. The extent of the strong, eas
reference (cf. Cresswell and Peterson, 1993). ODP Leg 182 Sites 1129, 11
curve) and reflectivity (denoted R~white curve) logs. Interpreted seismic ho
indicated by arrows in the upper right corner of the inset, which also sho
sequence subdivision are according to Feary and James (1998) and Feary
and intersected abnormally high concentrations of
hydrogen sulfide and methane within the sediment
succession (Swart et al., 2000; Mitterer et al.,
2001).
3. Subsurface database
A grid of high-resolution, multichannel reflection
seismic profiles was used to choose drilling loca-
tions for the ODP Leg 182 drill sites (Feary and
James, 1998; Feary et al., 2000, 2004). The stacked
and time-migrated seismic data are zero phased,
sampled at 2 ms and have a dominant frequency of
about 100 Hz. This corresponds to a dominant
wavelength of 15–25 m at subsurface interval
ds that constitute the upper Cenozoic cool-water carbonate wedge
me (TWT). The water depth along the profile varies from about 750
tward flowing Leeuwin current at the uppermost slope is shown for
27 and 1131 are shown with acoustic impedance (denoted AI~black
rizons are shown as black lines. The location of the seismic profile is
ws the location of all Leg 182 boreholes. Seismic stratigraphy and
et al. (2000).
M. Huuse, D.A. Feary / Marine Geology 215 (2005) 123–134126
velocities between 1.5 and 2.5 km/s and is
equivalent to an approximate vertical seismic
resolution (one quarter of the dominant wavelength)
of about 5 m.
The drilled section mainly consists of Pleistocene
carbonates of varying grain size and lithification
(Feary et al., 2000, 2004). Composite logs of sonic
(P-wave) velocity and formation density were
compiled for each site, primarily using downhole
log data. The uppermost ~100 m of each hole could
not be logged with downhole instruments, so the
downhole data were supplemented with a range of
shipboard core measurements for this interval-along-
core sonic velocity measurements, Vp(z), and
gamma-ray attenuation porosity evaluation
(GRAPE) density. Site 1127 was not logged with
the downhole density tool, and consequently, ship-
board density measurements were used for the
entire density log at this site. Because the sediment
cores expanded as they were brought to the surface,
the laboratory-measured values of velocity and
density are slightly lower than expected from
extrapolation of the downhole measurements. This
is seen as a minor (~10%) shift in the absolute
acoustic impedance at ~100 m subsurface, and
shallower values thus represent minimum values of
acoustic impedance.
Fig. 2. Flow diagram illustrating the major steps in the model-driv
4. Seismic inversion—methodology
A wide variety of seismic inversion methods exist,
ranging in sophistication and precision from basic
integration of the seismic traces over model-driven
inversion of post-stack seismic data to inversion of
pre-stack seismic gathers (see review by Veeken and
Da Silva, 2004). The seismic inversion procedure
employed in this study is outlined in Fig. 2 and
described below; it uses commercial software devel-
oped for use in the petroleum industry (Maver and
Rasmussen, 1995; Rasmussen and Maver, 1996).
The inversion procedure involves log to seismic
calibration, wavelet estimation, background impe-
dance-model estimation and seismic inversion using
simulated annealing (Maver and Rasmussen, 1995).
This is a deterministic inversion methodology that is
relatively simple to employ, but generally thought to
be more precise than other deterministic approaches
such as trace integration, sparse-spike inversion or
coloured inversion (Veeken and Da Silva, 2004). It
should be noted that inversion of seismic reflection
data is non-unique in that a range of (relative)
acoustic impedance results will fit a given reflection
profile. In the methodology employed here, the
available well logs are used to provide the overall
level of acoustic impedance, and the final inversion
en inversion of the seismic data to yield acoustic impedance.
40000
-6000
Amplitude
0
2
4
6
8
10
12
Tim
e (s
ampl
es)
Fig. 3. The wavelet derived at Site 1129 and used in the inversion.
Very similar wavelets were derived at Sites 1127 and 1131,
indicating that one wavelet can be used to invert the entire seismic
profile.
0.2
0.4
0.6
1
1.2
1.4
TW
T(s
)
1.38e+06 Impedance (kg/m s2) 7.39e+06
0.2
0.4
0.6
1
1.2
1.4
TW
T(s
)
1.38e+06 Impedance (kg/m s2) 7.39e+06
Water column
Core-basedacousticimpedance
Log-basedacousticimpedance
Relative acousticimpedance variationsderived from seismic data
Borehole-derived acousticimpedance at ODP Site 1131
Low-frequency AI model
Inversion-derived AI
Fig. 4. Quality-control plot for the seismic inversion at Site 1131. A
denotes acoustic impedance. Note the good overall fit between the
measured and the inversion-derived acoustic impedance, except fo
the abrupt impedance increase at the seabed (~0.45 s TWT). Also
shown is the low-pass filtered impedance log used in the absolute
(dpriorT) impedance model.
M. Huuse, D.A. Feary / Marine Geology 215 (2005) 123–134 127
result is thus relatively well constrained compared to
inversion results derived without absolute impedance
information.
4.1. Log to seismic calibration
First, seismic and borehole data are imported to
the inversion project, and the borehole data are
converted to two-way travel time using check shot
and/or integrated velocities from the sonic logs. The
logs are re-sampled to the sample interval of the
seismic data (2 ms). The composite sonic and
density logs are multiplied to yield an acoustic
impedance log, which describes downhole variations
in impedance. The impedance log is used to derive
a log of zero-offset reflectivity, R, which is the
parameter that governs the (zero-offset) seismic
reflection response of the subsurface. The reflectiv-
ity between two samples is defined as R=(AI2�AI1)
/(AI2+AI1), where AI2 is the acoustic impedance of
the lower sample and AI1 is the acoustic impedance
of the upper sample. The depth to time conversion
using check shots and/or sonic information yields a
rough calibration of seismic and borehole data.
After wavelet estimation (see below), the calibration
experiment is repeated using synthetic seismic data
to recalibrate the logs to the seismic data. After
readjustments of the depth to time calibration, a
new wavelet is estimated. This iterative loop is
continued until no further improvement is observed.
4.2. Wavelet estimation
Once the initial calibration is performed, a
wavelet is defined by deriving the convolution
operator between the reflectivity log and the
I
r
M. Huuse, D.A. Feary / Marine Geology 215 (2005) 123–134128
seismic section at the drill site using a least squares
approach. No assumptions are made about the
phase, polarity, delay, or length of the wavelet.
The optimal wavelet is the one that has the best
trade-off between signal and noise and is derived
using Akaike’s Final Prediction-error criterion
(Maver and Rasmussen, 1995). The optimal wavelet
is used to recalibrate the log and seismic data, and
the improved tie is then used to estimate a new
best-fit wavelet, and this is repeated iteratively until
no further improvements can be made. A best-fit
wavelet was extracted at each site and tested at the
other two sites. The derived wavelets were almost
identical, indicating that one wavelet may be used
to invert the entire section. In this case, the wavelet
extracted at Site 1129 (Fig. 3) was found to provide
Fig. 5. Acoustic impedance section derived by deconvolving the seismic se
frequency information contained in the absolute impedance model (see Fi
1131 and 1129 are also shown. Impedance values below the boreholes sho
low-frequency information is available. On the original seismic reflection p
and low-impedance (soft) intervals on the clinoform slopes and on the she
and density (and porosity) has been quantified, thus facilitating correla
impedance section is directly related to sediment porosity (right column,
mainly as a function of depth below seabed, and that absolute impedance v
(For interpretation of the references to colour in this figure legend, the re
the best overall fit, and accordingly, this wavelet
was used for the inversion.
4.3. Absolute impedance level
The absolute level of acoustic impedance in the
investigated section is defined by the dprior modelT(Fig. 2), which in this study is based on low-pass
filtered well log information extrapolated along the
interpreted seismic horizons. The prior model intro-
duces sub-seismic frequencies (b10–20 Hz) to the
seismic inversion. This widens the seismic bandwidth
and thus enhances the seismic resolution. The
impedance logs from the three boreholes were first
low-pass filtered to only include sub-seismic frequen-
cies (see Fig. 4). The resulting impedance logs were
ction (Fig. 1) with the optimal wavelet (Fig. 3), with addition of low-
g. 2). Impedance traces from borehole measurements at Sites 1127,
w only relative values of acoustic impedance (orange colours), as no
rofile, it was difficult to differentiate between high-impedance (hard)
lfward side of the carbonate mounds. Inter-well variation of velocity
tion between boreholes penetrating different facies. The acoustic
see Fig. 6). Note that the impedance increases (porosity decreases)
aries markedly within stratal units depending on their depth of burial.
ader is referred to the web version of this article.)
M. Huuse, D.A. Feary / Marine Geology 215 (2005) 123–134 129
then extrapolated along interpreted seismic horizons
to yield the absolute level of acoustic impedance
along the seismic profile. Constant values of absolute
impedance were assigned to the water column and to
the section below the drilled interval from which only
relative impedance values were derived (Fig. 5).
4.4. Seismic inversion
The seismic inversion uses the original seismic
data, the optimal wavelet and the background
impedance model (Fig. 2). To fully exploit the
information contained in the seismic data, the high-
frequency information from the boreholes is not used
in the inversion. The seismic inversion scheme used is
a globally optimized nonlinear method based on a
revised version of the simulated annealing algorithm
(Maver and Rasmussen, 1995; Rasmussen and Maver,
1996). The inversion is implemented by defining five
Acoustic imped
3*106 3.5*106 4*106 4.5*106
80
70
60
50
40
30
20
10
Por
osity
(%
)
Fig. 6. Relation between porosity and acoustic impedance at Site 1131. T
density and porosity logs) with a total of 2803 data points. The data show
caused by poor borehole conditions and resulting spurious data points. M
lines. The best-fit linear regression has a correlation factor of only 0.71 (R2
which is defined as 100% porosity and acoustic impedance of 1.5�106
relation was defined (green line) to enable impedance to porosity conver
colour in this figure legend, the reader is referred to the web version of th
parameters—signal/noise ratio and lateral continuity
of the seismic data, vertical variation in acoustic
impedance, prior model confidence, and the threshold
for layer interfaces (Fig. 2). The main result generated
by the seismic inversion is an acoustic impedance
section (Fig. 5), together with a variety of quality
control plots and other attributes (see Maver and
Rasmussen, 1995; Rasmussen and Maver, 1996).
4.5. Relation between acoustic impedance and porosity
A relation between subsurface acoustic impedance
and porosity can be established based on the Site 1131
downhole log measurements (Fig. 6). Because of non-
optimal hole conditions and resulting spurious down-
hole log measurements, there is a large scatter
superimposed on the natural variability of the acoustic
impedance–porosity relation. For simplicity, we
assume that the porosity to acoustic impedance
ance (kg*s/m2)
5*106 6*1065.5*106 6.5*106
he plot was constructed using unedited downhole log data (sonic,
a porosity–impedance relationship with a large scatter that is in part
aximum and minimum porosity envelopes are indicated with blue
~0.5). This best-fit linear regression does not pass through the origin,
kg/ms2 (corresponding to seawater). Therefore, a slightly modified
sion from the seabed down. (For interpretation of the references to
is article.)
M. Huuse, D.A. Feary / Marine Geology 215 (2005) 123–134130
relationship for the studied section can be described
by a single linear regression. Simple curve fits using
exponential, logarithmic, or power functions did not
yield improved correlations compared with the linear
fit. The best-fit linear regression does not yield a
realistic value for 100% porosity (seawater), and
hence, a slightly modified relation is used to convert
acoustic impedance to porosity: U=125�17AI, where
U is the porosity (%), and AI is the acoustic
impedance (106 kg/ms2). This relation is linear and
valid for porosity values z30% (the green line in Fig.
6). The acoustic impedance to porosity relation can be
used to convert the acoustic impedance section to a
porosity section (Fig. 5), which may be used to infer
lithology distribution, compaction state and fluid-flow
properties for the shallow subsurface. It is important
to note the large scatter (non-uniqueness) of the
porosity to acoustic impedance relationship (Fig. 6).
This means that for any single acoustic impedance
value, there is a wide range of possible porosity
values, e.g., from about 40% to 70% for an acoustic
impedance of 4.5�106 kg s m�2. Consequently, the
porosity conversion should only be considered a first
approximation, and a more refined relationship could
be established by estimating the relationship for
subsets of the data, e.g., based on mineralogy,
cementation, or texture (cf. Anselmetti and Eberli,
1997; Marion and Jizba, 1997).
5. Results and discussion
The main result of the seismic inversion study is a
profile showing the absolute variations in acoustic
impedance and porosity along a depositional dip
transect through Sites 1127, 1129 and 1131 (Fig. 5).
There is a very good fit between the acoustic
impedance values derived from the boreholes at each
drill site and the seismic inversion result. This was
expected from the wavelet extraction results for each
site, which indicated low variability in the seismic
source signature. Importantly, only the absolute level
of the acoustic impedance was included in the
inversion, i.e., no detailed well-log data were used,
and the fine-scale impedance variation is entirely due
to variations in the seismic reflection intensity.
The porosity distribution derived from the acoustic
impedance section (Fig. 5) and the impedance–
porosity relation (Fig. 6) are useful for a quantitative
determination of the lateral and vertical variations in
lithology and bed thickness between boreholes and
the porosity structure, and thus compaction state, of
the shallow subsurface. The latter may have implica-
tions for understanding the seabed stability and fluid-
flow behaviour of the investigated section, which in
turn contributes to understanding the conditions
leading to the unusual pore-fluid chemistry encoun-
tered in this part of the Great Australian Bight (Swart
et al., 2000; Mitterer et al., 2001). The following
sections describe a number of interesting observations
that were not readily apparent on the seismic
reflection profile (Fig. 1), but which can be made
using the acoustic impedance/porosity section (Fig.
5).
5.1. Acoustic impedance variation and seismic
reflections
The first thing to note about the inversion result is
that the absolute values of acoustic impedance and
porosity vary markedly within the individual seismic
sequences defined by Feary and James (1998) (e.g.,
from N45% to b30% porosity within the lowermost
Pleistocene sequence). This variation exceeds that seen
across stratal boundaries, and yet no seismic reflec-
tions arise from these lateral variations. This supports
the long-standing notion that seismic reflections
mainly follow stratal boundaries rather than lateral
facies variations (Vail et al., 1977). In this case, the
reason for the large lateral variation seems to be that
acoustic impedance and porosity are strongly depend-
ent on depth of burial (Fig. 5). Despite the large
magnitude of the variation within a given depositional
unit, lateral transitions are not observed in the seismic
reflection profile, which seems mainly to represent the
vertical impedance variations (compare Figs. 1 and 5).
It should be noted, however, that subtle variations in
horizon amplitude often seen in 3D seismic amplitude
maps may be able to pick up lateral facies changes not
readily observed with 2D seismic data.
High-amplitude reflection events occur on the
updip side of the seabed mounds and on the steeply,
seaward-dipping parts of the large clinoforms making
up the majority of the Pleistocene slope wedge (Fig.
1). The inversion result reveals that high-amplitude
reflections in the lower part of the succession around
M. Huuse, D.A. Feary / Marine Geology 215 (2005) 123–134 131
Site 1131 are due to high-impedance (hard) layers on
the clinoform foresets. These correspond to increased
abundances of grainstones and, to a lesser extent, to
increased abundance of more lithified intervals (Fig.
5; Feary et al., 2000). The correlation of high
impedance with grainstone intervals can be explained
by the greater velocity of grainstones compared to
more fine-grained carbonate mudstones in slope
environments, which may in turn be due to either a
greater stiffness or a greater diagenetic potential of
more permeable grainstones compared with more
muddy carbonates (Anselmetti and Eberli, 1997).
Whereas dbrightT reflections on seismic reflection
images highlight the presence of anomalous litholog-
ical units, the impedance data can be used to quantify
the lateral variations in thickness and/or facies of such
units, and thus can help to decipher the detailed
sedimentary facies distributions along the transect
away from areas of direct lithological control in wells,
as well as in those parts of the succession with poor
core recovery.
5.2. Geometry and origin of the upper slope succession
Seismic profiles across the Pleistocene upper slope
succession in the Great Australian Bight (Fig. 1) show
a 500-m-thick prograding wedge containing shelfward
accreting mound structures on the uppermost slope
and at the shelf break (Fig. 1; James et al., 2004). This
overall geometry is reminiscent of contourite drifts
seen along many continental margins (Faugeres et al.,
1999; Stow et al., 2002). Contourite drifts often
display upslope migration of more or less mounded
deposits that terminate upslope at erosional moats
(e.g., Faugeres et al., 1999). If the upper slope
succession seen in Figs. 1 and 5 were to be interpreted
geometrically in terms of contourite drift deposits, the
overall progradational slope wedge could be termed a
dslope plastered driftT (sensu Faugeres et al., 1999),
and the seabed and buried mounds could be termed
dmoat-related driftsT (sensu Faugeres et al., 1999) or
dmulti-crested driftT (sensu Howe et al., 2002),
depending on whether the moats are due to erosional
or depositional thinning. Given the relatively shallow
water depth of the upper slope and shelf break in the
Great Australian Bight (Fig. 1: ~750–150 m), the
upper slope succession would qualify as a dshallowwaterT contourite drift (sensu Stow et al., 2002).
Analysis of Leg 182 cores suggest that the
dominant provenance for the Pleistocene slope wedge
is the 200-km-wide Eucla shelf located immediately
to the north of the studied transect (Feary et al., 2000;
James et al., 2000, 2004). Mapping the geometry of
the moat-related mounds and mound complexes
shows that individual mounds may be up to 65 m
high and extend laterally for up to 10 km along strike
and up to 700 m perpendicular to the slope (Feary and
James, 1998; James et al., 2000). Seismic imagery of
the sequence of mounds cored at Site 1131 shows that
the locus of carbonate mound growth migrated
upslope towards the shelf (Fig. 1). The inversion
result shows that shelfward migrating high-amplitude
reflection events are due to high-impedance layers
(Fig. 5). These high-impedance layers could indicate
that fine-grained sediment may have been winnowed
from the upslope margin of the mounds, to leave a
coarser lag deposit. The mound that was cored at Site
1131 contained some 20–25 m of floatstone/rudstone
beds with well-preserved bryozoans, interpreted as a
bryozoan reef mounds preserved in situ (James et al.,
2000, 2004). However, the lower part of the core
taken in the mounded seismic facies consists of
bioclastic packstone, which is largely allochthonous
(Feary et al., 2000: chapter 9, Site 1131, p. 30). This
indicates that only the upper one-third of the mounded
seismic facies cored at Site 1131 represents bryozoan
mounds, and that the lower part achieved its mounded
appearance due to some other cause, prior to the
growth of the bryozoans.
The uppermost continental slope of SWAustralia is
swept by a strong geostrophic current, the Leeuwin
Current, which flows east along the uppermost
continental slope south of SW Australia at speeds in
excess of 0.5 m/s (Cresswell and Peterson, 1993;
James et al., 2000, 2004; Creswell and Griffin, 2004).
The eastward current is associated with a westward-
directed counterflow that may exceed 0.2 m/s located
downdip from the Leeuwin Current (Cresswell and
Peterson, 1993). The east–west flow patterns are
frequently distorted due to large-scale eddies (Cres-
well and Griffin, 2004), and any current influence on
sedimentation is thus expected to be highly complex.
Current velocities measured range up to 1.5 m/s with
0.5 m/s being common (Creswell and Griffin, 2004),
and this is strong enough to winnow relatively coarse
sediments and form scours in muddy sediments, and
M. Huuse, D.A. Feary / Marine Geology 215 (2005) 123–134132
certainly enough to shape the geometry of muddy
sediment drifts (Masson et al., 2004: their table 1 and
references therein).
It is thus conceivable that the overall geometry of
the Pleistocene upper slope succession in the Great
Australian Bight resulted from a combination of
abundant supply of carbonate muds and clasts derived
from the Eucla Shelf, and current-controlled redis-
tribution of these sediments on the uppermost slope
where the Leeuwin Current and related current
systems resulted in drift-like stratal geometries. In
this setting, conditions were intermittently favourable
for bryozoan organisms, which may locally have
formed reef mounds, perhaps preferentially nucleating
on the crests of pre-existing, current-controlled
mounds.
5.3. Hydrogen sulphide hydrates in the upper con-
tinental slope of the Great Australian Bight?
Unusually high concentrations of hydrogen sul-
phide and methane were encountered at Sites 1127,
1131 and 1129 (Feary et al., 2000). Swart et al. (2000)
speculated that the high concentrations of hydrogen
sulphide and methane might be due to the presence of
disseminated hydrogen sulphide and methane
hydrates. Gas hydrate-bearing sediments are usually
characterized by relatively high interval velocities due
to the solidity of the hydrate relative to pore water.
Because hydrates are generally associated with the
presence of free gas in the underlying section, their
presence usually causes a large decrease in acoustic
impedance at the hydrate base and a large impedance
increase at the gas–water interface below. The
hydrate–gas interface thus often gives rise to a bright
reflection event known as a bottom simulating
reflection (BSR), and the underlying gas interval
may be seen as a zone of anomalously low velocity
or low impedance (e.g., Bunz and Mienert, 2004; Dai
et al., 2004). However, no BSR is observed on the
high-quality reflection seismic data available (Fig. 1)
and no velocity inversion has been observed in the
wells or in the inversion data (Fig. 5). This supports the
previous suggestions by Swart et al. (2000) that any
hydrate, if present, would be highly disseminated and
that any free gas phase would be distributed with
sufficiently low gradients to not cause any seismic
anomalies.
5.4. Acoustic impedance and core recovery
Vertical zones of poor core recovery have been
plotted next to the borehole locations in Fig. 5. These
zones have been subdivided into poor (10–50%) and
very poor (b10%) recovery intervals indicated by
grey and black colour, respectively. The general trend
is one of decreased core recovery with increased
depth and increased lithification, especially where
discrete hard beds occur. Poor recovery at intermedi-
ate depths are most likely due to the use of the
extended core barrel (XCB) coring technique that was
generally employed for coring the intermediate parts
of the section (c. 200–500 m subsea) of Leg 182. This
technique, in combination with sea swells, will lead to
a dhammeringT effect on any lithified horizons, which
may lead to the liquefaction and thus poor recovery
potential of intervening softer intervals. In the case of
Sites 1129 and 1131, extreme lithification was
associated with the appearance of abundant thin
(few centimeters thick) chert layers in lower parts of
the section (Miocene), which made recovery of the
intervening, relatively soft carbonates impossible. At
Site 1129, some shallow intervals of poor core
recovery, not anticipated based from the impedance
data, were caused by shallow water operational
difficulties caused by a combination of long wave-
length sea swells and poor weather conditions (Feary
et al., 2000). Apart from the shallow poor recovery
intervals unrelated to the geology at Site 1129, there
is a clear tendency for recovery to be poor when
porosity decreases below about 40% (yellow-orange
colours in Fig. 5). This correlation seems sufficiently
strong that acoustic impedance data may be used to
predict broad trends in core recovery away from
points of borehole control. If this is relation is a
general one, it might be possible to use seismic
inversion as a cost-effective tool to optimize the
drilling strategy and recovery prediction of future
drilling campaigns, especially where borehole data
are available to calibrate the background impedance
model.
6. Conclusions
Seismic inversion for acoustic impedance is a well-
established tool in the petroleum industry where it has
M. Huuse, D.A. Feary / Marine Geology 215 (2005) 123–134 133
become a widely used method for optimizing the
quantitative analysis of seismic data to yield lithology
and porosity and permeability structure of the moder-
ately deep (~1–5 km) subsurface. Recently, the focus
has been widened to also include the shallow subsur-
face as this may be of interest to hazards studies and to
detailed studies of geological processes and products.
This study provides the first dacademicT application of
model-based seismic inversion using simulated anneal-
ing and demonstrates the enhanced interpretability of
acoustic impedance data as applied to the upper
continental slope of the Great Australian Bight.
Acoustic inversion shows that seismic reflections
in the upper slope succession originate from bed
boundaries rather than from lateral variations in
acoustic impedance, even if such impedance varia-
tions are of larger magnitude than those observed
across the individual bed boundaries. High amplitude
reflections visible on the seismic reflection section
mainly originate from high impedance (low porosity)
intervals within a relatively low impedance (high
porosity) slope wedge and in near-seabed mounds.
The overall geometry of the upper slope succession
in the Great Australian Bight, as seen in the studied
transect, displays many similarities with shallow-water
contourite drifts, including plastered slope drifts and
moat-related or multi-crested drifts. The study area is
presently dominated by a strong (N0.5 m/s) eastward
flowing geostrophic current, the Leeuwin Current, with
its associated counterflow and large-scale eddies,
which provide a complex current regime likely to
affect the deposition of both fine and coarse sediments
on the outer shelf and upper slope. Core data show that
only the upper one-third of the conspicuous mounded
seismic facies calibrated at Site 1131 contains in situ
bryozoans, whilst the lower two-thirds are comprised
of allochthonous muds. This could suggest that the
mounded facies represent contourite mounds capped
by bryozoan colonies.
No indications of gas hydrate or free gas were
found in the well logs or seismic data, and in parallel
with previous studies, it is suggested that any hydrate
present is disseminated and that the amount of free gas
in the upper 500 m of the slope wedge is minimal. A
rough correlation between zones of poor core recov-
ery and intervals of high impedance could suggest that
impedance data may be used to predict coring
performance away from already drilled sites.
Since acoustic impedance inversion for porosity
prediction may be undertaken wherever good quality
seismic and borehole data are available, the method
outlined here should be useful for most studies
dealing with sedimentary facies analysis, compaction
and fluid flow in the shallow subsurface.
Acknowledgements
The seismic inversion study was carried out during
a research stay by MH at the Borehole Research
Group of Lamont-Doherty Earth Observatory of
Columbia University, New York. The hospitality and
assistance by the people there is greatly appreciated.
MH acknowledges the support by Inge Lehmann’s
Legat af 1983 and DONGs Jubil&umslegat. adegaardA/S kindly supplied the seismic inversion software
(ISIS2D) used in the study and provided guidance for
its installation and use. Joe Cartwright and Richard
Davies provided helpful reviews of an earlier version
of the manuscript. The journal referees and editor
David Piper provided many valuable insights that
helped shape the final version of this article.
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