List of Figures
Figure 1--A sample of new high-resolution gravity in the deep water Gulf
of Mexico.
Figure 2--Today’s workstations handle integration of seismic , gravity
and magnetics.
Figure 3--Salt body model for determination of required gravity data
accuracy.
Figure 4—Salt thicknesses accuracy chart, using differing accuracies
of gravity data .
Figure 5--High resolution gravity used to refine 2-D and 3-D seismic
velocity.
Figure 6--Flow diagram for integrated seismic -gravity-velocity
interpretation .
Figure 7--Initial density model (cross section of salt feature), deep
water Gulf of Mexico.
Click
here for sequence of Figures 7 and 8.
Figure 8--Revised velocity model after application of gravity of salt
feature in Figure 7, with initial salt outline shown.
Click
here for sequence of Figures 7 and 8.
Figure 9--Full integration: Incorporated into this Gulf of Mexico
interpretation are a 3-D seismic volume, 3-D velocity volume, 3-D
density volume, calculated and observed gravity fields, interpreted well
data , and interpreted seismic horizons. Display courtesy of
CGG-Petrosystems.
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Recent Advances
Brief
History
Historically,
gravity and magnetic data were primarily used as a basin reconnaissance
tool for determining gross features of an area. Key elements are depth
to high-density basement (often coincident with economic basement),
sediment thickness, fault delineation, mapping of volcanics, and salt
modeling. A common approach was to have a gravity and magnetic
“guru” on staff, or as a consultant, who would disappear with all
the data and return some time later with an “answer.”
If
the results of the gravity and/or magnetic work did not agree with the
seismic interpretation , the “guru’s answer” would generally be
disregarded. Although much good work was achieved, results had been
limited by the resolution of the recorded gravity and magnetic data and
the lack of cohesive integration.
Advances
in Resolving Power
At
a recent technical meeting in Houston, Ed Biegert, non- seismic methods
specialist for Shell, asked the question: “Why do we re-acquire
gravity?” His own answer to the question was: “For the same reasons
we re-acquire seismic data .” Although the gravity fields mapped in
prior years have not changed, our ability to measure and process gravity
accurately on a ship has improved dramatically – just as we have
improved our ability to shoot, record and process seismic data (Figure
1).
Recent
advances in gravity measurement at sea include:
-
Upgrading from analog to digital
control and acquisition systems.
-
Higher data sampling and
recording rates (200 Hz sampling, 1 Hz recording).
-
Precise DGPS positioning for
removal of ship accelerations.
-
More accurate measurements of
water depth.
- New
data
processing developments (signal to noise enhancement,
micro-leveling, etc.).
With
these advances, industry has seen stunning improvements over data
recorded as recently as 10 years ago. In many cases, there is an
increase of up to 10 times the data per unit area in new surveys over
older data , with a correspondingly higher level of confidence in
interpreted geological results.
Many
operators are routinely incorporating new high resolution gravity into
their interpretation projects, particularly in the deep water Gulf of
Mexico. This integration is facilitated by new workstation software
applications (Figure 2).
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Gravity
Data Accuracy
One
very experienced oil company gravity and magnetic interpreter quotes the
following approximate interpretable accuracies of data in the Gulf of
Mexico:
1.
Sidney Schafer Water Bottom Gravity (shelf to 600-foot water depths),
over 2,000- to 4,000-foot horizontal distances; primary limitation is
sampling – station spacing: 0.1 to 0.3 milliGal (mGal).
2.
Legacy Deep Water Marine Data (over 10 years old) over 10,000-foot
horizontal distances: 0.5 to 1.0 mGal.
3.
New High-Resolution Deep Water Marine Data (1991 or newer), over
1,500-to 3,000-foot horizontal distances: 0.1 to 0.5 mGal.
The
above examples are general estimates based on several criteria,
including positioning, instrumentation, sampling, processing techniques
and associated bathymetry accuracy. Recent work has shown that errors of
0.3 to 1.0 mGal or greater can be introduced into data due to use of
incorrect water depth or positioning information.
The
importance of data resolution makes a thorough investigation of the
gravity data prior to interpretation a sound practice. As with any
geophysical technique, ambiguities still exist, and the limitations of
the technique should be thoroughly understood.
What
is a MilliGal?
Not
many of us have a good grasp of what this measurement unit of gravity
means, or in more general terms, the impact of gravity data accuracy on
geologic interpretations. “We already have a gravity map” is often
heard at oil companies.
The
following is an exercise in converting gravity (milliGals) into a
meaningful geologic quantity (thickness of salt – in this case,
thickness of a salt lens):
Modeled
salt thickness
vs. gravity data accuracy – sensitivity models
Using
a generalized density vs. depth curve for the deep water Gulf of Mexico,
a series of sensitivity models have been constructed for a salt
lens, two miles in diameter (Figure 3). The salt was inserted into the
density model at several depths. At each depth the thickness was varied
to establish data points for a salt burial depth and thickness vs.
gravity response chart (Figure 4). The results of these models
quantitatively demonstrate the need for accurate gravity data in deep
water salt modeling.
Admittedly,
this is an over-simplified example, but it is effective in demonstrating
the need for good quality gravity data to obtain meaningful geological
results. To read the diagram in Figure 4, go to the x-axis (depth) and
find the 7,500-foot depth point. Moving upward on the chart to the 0.1
mGal data curve, it is observed on the y-axis that 0.1 mGal data , when
modeled for salt at this depth, will provide approximately plus or minus
300 feet accuracy in modeled salt thickness.
For
0.2 mGal data this range grows up to 400 feet; for 0.5 mGal data results
are plus or minus 1000 feet. For 1.0 mGal gravity data (most older
gravity data sets in the deep water), the results are plus or minus
one-half mile of salt!
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Present
Economics Of Gravity and Magnetics
New
high resolution gravity data costs approximately $1,200 per Gulf of
Mexico OCS lease block, or $12 per line mile for new 2-D high resolution
data (e.g., gravity from TGS-Calibre Phase 45 Program). Costs for 2-D
models are in the $2,500 to $5,000 range, and full 3-D gravity and
magnetic modeling studies can cost from $25,000 to $50,000 or more
depending on the complexity of the model.
In
terms of new data acquisition, crew and equipment costs are in the range
of $1,500 per day or less. In areas like the deep water Gulf, many
companies are finding this a worthwhile investment. When rig rates are
pushing well over $100,000 per day, it is easy to understand why.
Practical
View of Integration Methods
Even with the best quality 3-D seismic
data , an interpreter can have a troublesome task in defining the
salt/sediment boundary at the flanks of a salt dome, salt sheet or other
complex structure. For decades, gravity has been used in the Gulf of
Mexico to address this problem. The major differences in how it was done
then and how it is now done are twofold. It’s better today because of:
By incorporating a co-recorded data set
with each data set (e.g., seismic and gravity) independently measuring a
related property of the subsurface, the interpreter can place a much
higher degree of confidence in the final geologic interpretation (Figure
5). To
quantify this observation, case studies show that incorporating 3-D
seismic with high resolution gravity and magnetics can alter the base of
salt interpretation by several thousand feet from the 3-D seismic
interpretation alone.
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Team-Oriented Exploration Tools
With the trend toward highly focused
exploration teams, the smooth interaction and coupling of multiple
geophysical disciplines is essential. Explorationists are expected to
employ and be familiar with more disciplines on a continuing basis. The
development of workstation applications, which enable the interpreter to
simultaneously refine the subsurface model using seismic , gravity and
magnetic data , has been a giant step forward. A flow diagram for such an
application is shown in Figure 6.
In using a new software tool kit,
high-resolution gravity now can be applied to an increasing number of
seismic velocity modeling projects. This technique is employed using the
procedure outlined below.
1. High-resolution gravity is recorded and
processed along with the 2-D or 3-D seismic survey. Present techniques
allow for delivery of processed gravity data in advance of, or in
parallel with, processed seismic data delivery.
2. The seismic velocity data are used to
create a corresponding density section (or volume, in the 3-D case) by
means of a flexible velocity-density conversion tool kit, incorporating:
Gardner’s
Equation.
Nafe/Drake,
Hilterman and other density-velocity relationships.
Use
of available empirical data (e.g. velocity logs, check shot surveys,
gamma-gamma density logs, etc.).
User
defined conversion algorithms or formulae.
Other
approaches.
3. The density model can be as simple or as
elaborate as the corresponding velocity model – up to and including a
discrete value of density for each x-y-z node within the profile or
volume of data .
4. Input of digital horizon data (again,
2-D or 3-D) as interpreted on the seismic workstation. The system
incorporates a “universal translator” for the conversion of one type
of horizon to another to accommodate company partner teams, etc.
5. Computation of the gravity field of the
model, input of gravity data as recorded on the survey, and a direct
comparison between the two fields.
6. Manipulation of the model using both
forward modeling and inversion processes based on minimizing the misfits
between model and measured gravity fields.
7. On completion of the modeling and/or
inversion process, the revised earth model is converted into the
velocity domain, providing an improved starting point velocity model for
depth migration.
8. This iterative process and feedback loop
continues throughout the seismic migration and interpretation process.
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Gulf of Mexico Example of Integrated
Interpretation![Next Hit](/images/arrow_right.gif)
Figure 7 is a cross section through a full
three dimensional model of a salt feature in the Gulf of Mexico. The
density cube is derived from available well control. The top of salt is
typically obtained from a simple initial stretch to depth from the time
interpretation . Later--in the interpretive processing sequence--this is
updated with the post-stack or pre-stack depth migration results.
The base of salt is input from an initial
time interpretation . In many cases the initial base of salt
interpretation is provided with confidence factors; e.g., a 10 might be
assigned to high seismic confidence areas, a 0 being assigned to seismic
blind zones, and grades in between. The gravity modeling can then be
constrained by the high seismic confidence areas, and the low ( seismic )
confidence areas are then of most interest in the search for a better
interpretation using gravity modeling results.
The density and velocity data are analyzed,
typically using cross plots, and a function is derived to convert
between the density and the velocity volumes. The gravity effect of the
density volume is computed and compared with the observed gravity data ,
and the differences are resolved through a series of automated
structural and density inversion techniques. The final model should
contain as much seismic -gravity constraint as possible for optimal
results, often involving close interaction between the gravity
interpreter and seismic interpreter at the same workstation.
Once the final density model is
constructed, the density-velocity function is used to translate the
alterations into an apparent velocity cube. Figure 8 is the final result
of this process. Note the original outline of the salt body (prior to
integration of the gravity and seismic results), shown as a white
outline. In this case, several thousand feet of change in the base of
salt are indicated through the multi-disciplinary approach, as compared
with the seismic -only approach.
A full 3-D view of an integrated
seismic -gravity model with well control is shown in Figure
9. This
process, in addition to providing important and independent
corroboration and improvement to the seismic interpretation of the base
of salt, also provides an important source of long wavelength velocity
information beneath the salt masses. This information, when injected
back into the velocity model used for producing the final base salt and
sub-salt images, can have a dramatic impact on the enhanced quality of
the seismic processing results.
Economic Impact
In today’s team-oriented exploration
environment, the availability and use of real-time interpretation
software tools allow for the integration of gravity and magnetic data at
the same workstation. This approach is now embraced by a growing number
of oil companies for:
To be most effective, the integration of
gravity and magnetics must take place at the earliest stage of prospect
development, and can continue throughout the exploration process.
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