The Life of a Geophysical Program
The mission of a Geophysical Program is to provide information for decisions.
The Value of Information depends on whether and how decisions are affected.
A Geophysical Program starts from the decision makers.
Typically the decision makers are not geophysicists and they need information about
what is reasonable to expect from which geophysical method.
A feasibility study that includes a review of available legacy data follows.
Both signal and noise in legacy data must be analysed.
If time allows, the feasibility study includes modeling that may go beyond subsurface coverage.
A survey design follows. The survey design requires good understanding of the needs and
detailed knowledge of applicable geophysical methods.
It is important to note that the survey design covers not the acquisition but also the processing.
The Acquisition is tailored to a data processing plan.
For example, velocity model building with certain methods such as Full Waveform Inversion
require long offsets.
Next is the data acquisition. One or more crews execute the design.
The data must be quality controlled onboard to make sure that the crew
would not leave the area without good data.
If the data acquired is very different from what is expected based on modeling, the earlier we know the better.
If possible, data processing starts onboard during the data acquisition.
Next us data processing onshore.
Data interpretation should starts during the data processing.
Note that we describe a Geophysical Program rather than a Seismic Survey.
Obviously Geophysics includes also non-seismic methods such as Electro Magnetic waves and Gravity.
The other distinction between a Program and a Survey is because each survey is part of a program.
The program usually start with a 2D seismic survey and continues to exploration 3D seismic survey and
EM (MT and Controlled Source EM). Then high resolution Development 3D which provides information for
development and also baseline data for 4D monitoring.
Monitoring surveys support production decisions by better understanding and prediction of the Dynamic Reservoir Model.
However, because our understanding of the area and geophysical technology are developing, the monitoring
surveys provide increasing resolution and quality that add value by improving the Static Reservoir Model.
In addition to legacy data from the same area, it is often useful to review analog case histories.
For example, if another field was successfully imaged or monitored with nodes, it may encourage the operator
to try the same nodes; after a review of how similar are the local streamer data to those of the analog case history.
The handover challenge
Different people, often different companies, perform different stages.
Poor handover of material from one stage to another destroys value.
- Acquisition problems or just oddities that are well understood onboard
become mysteries that take weeks to unravel onshore in data processing.
- Onshore data processing starts from scratch instead of from where
onboard processing got.
- Under-utilization of modeling. Fresh synthetic data are always used in survey design,
but their value is arguably even greater to prepare the data processing and in interpretation.
If the data processing people, programs, and computers have processed real-size synthetic
before they get the real data, time is saved, and the risk for undetected errors is mitigated.
- Legacy and analog data may be under-utilized.
Modeling may be better in predicting the signal but not the noise.
- Interpreters may not be aware of consequences of features of the source or
receiver equipment, survey design features, and decisions taken
during data processing with respect to processing sequence and processing parameters.
How can Totum Geo help?
Vertical Integration: smooth handovers
from survey design to acquisition to processing to interpretation
add value by saving time and improving quality.
It is better to learn from other people's experiences!
- Simultaneous Sources:
The cost of data acquisition is the cost per day times the number of days.
(For now, let's just note that mob/demob, transits, and data processing costs will be considered later.)
If the time it takes to shoot a unit area is not in balance with the
time it takes to move the receivers in the same unit of area,
then either the source will have to wait for the receivers, or vice versa.
With towed streamers it is not a problem because the sources and the receivers move together,
but special care in the design of Land and Seabed surveys must be taken.
Increasing receiver motion capacity depends on the cost of the crew.
For example, if one ROV vessel can move 100 nodes per day, then two such vessels would be able to
move 200 nodes per day, when both of them are working together.
Source motion capacity, however, is more complicated.
Increasing source capacity used to be limited by the record length.
The number of seconds per day is 86400 and if we have to wait 10 seconds
between shots we will not be able to shoot more than 8640 shots per day.
Simultaneous shooting has changed this limit. The 10 seconds per shot above
is typically a balance between the time it takes for the echoes to reflect,
the time it takes to move the source from one point to the next, and the time
it takes the compressors to charge the airguns, or the sweep length in Vibroseis.
It is by now well known that Simultaneous Sources can help. This can either reduce
cost by reducing the time it takes to shoot a certain area
and/or improve quality by increasing the number of shots per area.
However, a simple idea may still lead to a complicated decision.
The main complication is the availability of resources and the implications for a project over-all time.
A risk is that the time saved in acquisition would be more than lost in processing,
or the quality of the results would be compromised.
Balancing sources and receivers is easier than balancing cost, quality, and schedule.
It is important to understand and predict the implications for quality and schedule in Data Processing.
Without Data Interpretation, we can not know how significant it would be to compromise
quality while reducing cost or to increase quality while keeping the cost at a certain budget.
Modeling can help, but often a plan is a basis to changes, and onboard processing can help
not only to reduce the onshore processing time, but also to review and possibly revise the
shooting plan as early as possible during the acquisition.
- Offset and Azimuth Ranges and sampling.
It's been demonstrated that Wide Azimuth improves the ability to image under complex overburden such as salt.
It's been demonstrated that Long Offsets improve the ability to build velocity models.
Yet, these simple ideas lead to complicated decisions.
Modeling can help, but usually the solution is iterative: start from 2D, then Narrow Azimuth 3D, then Wide Azimuth 3D.
Offset sampling in terms of offset and azimuth ranges and distributions
is associated with source and receiver sampling intervals.
For example, do we need the far offsets sampled as densely as the near offsets, or
do we need finer spatial intervals for imaging with the near offsets and coarser spatial sampling
intervals for velocity model building with the far offsets?
- Shear Waves. Shear-waves see things that P-waves alone don't see well.
But again, a simple idea leads to a complicated decision.
On the seabed, for example, all receivers except hydrophones record shear waves.
But the spatial sampling interval for good imaging with shear waves may cost a lot of time and money.
So, more often than not, surveys are designed for P-wave imaging.
Yet, shear waves are recorded.
Shall we attempt to get value from the incidental shear data?
If so, we must be careful to not compromise the schedule of the P-wave data analysis.
The next question is whether and where are high priority areas where extra receivers can be deployed
to increase the value extracted from shear waves.
Mis-management of expectations is probably as big a challenge as coupling to the seabed,
shear waves processing, and joint interpretation of P-waves and shear waves.
People tend to expect too much and then too little.
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