Step By Step Procedure for Seismic Modeling
We will start with four basic definitions:
   1. forward seismic modeling - making a synthetic seismic trace from EDITED sonic and density log data.
   2. inverse seismic modeling - making a synthetic acoustic impedance log from a seismic trace.
   3. seismic interpretation - making correlations, picking horizons, and mapping seismic data, with geological and well log data for control.
   4. modeling a log - calculating what a log should read in a given rock and fluid mixture, including the creation of synthetic sonic and density logs from other logs or geological/seismic data.

Just as log analysts use core data as ground truth to calibrate their log analysis results, geophysicists use synthetic seismograms created from edited log data as their ground truth.

Structural interpretation of seismic sections involves identification of reflective horizons and picking of seismic travel times for each trace on each horizon. From these data, time, and subsequently depth, maps are made for each horizon. Choosing the exact horizon time is sometimes difficult, since the bed boundary may not be on a peak or a valley of the seismic trace. A synthetic seismogram is often used to calibrate these time picks.

Stratigraphic interpretation is based on the ability to correlate seismic character to subsurface geology, instead of just seismic two-way times. The correlation is established at a well with a synthetic seismogram computed from edited well log data. The shape and amplitude of the reflection on each trace is important in its interpretation.

The steps in making a synthetic seismogram are:
   1. edit the sonic and density log for borehole and recording problems, based on regional trends, offset logs, and mathematical models of log response.
   2. model sonic and density logs in formations which have been affected by invasion or rock alteration, based on a comprehensive quantitative log analysis.
   3. model effects in carbonates caused by porosity type or density contrast, based on log analysis and geologic data.
   4. integrate the modeled and edited sonic log.
   5. interpolate equal time increment values for sonic and density (and other log) values from depth data.
   6. calculate acoustic impedance and reflection coefficients from modeled and edited logs.
   7. generate an appropriate wavelet.
   8. convolve wavelet with reflection coefficients.
   9. plot synthetic seismogram on an appropriate time scale.
   10. check results against real seismic data.
   11. revise edits or log models over intervals that do not match real seismic, OR improve seismic processing, OR change wavelet characteristics.
   12. make "What-if" models to test alternate interpretations and sensitivity to fluid, porosity, and lithology assumptions, as well as wavelet shape and frequency.
   13. remodel zones which do not tie as to time, amplitude, or character, and check again If possible, the wavelet to be used is based on frequency and phase measured from the seismic data we are trying to match. A seismic wavelet can be extracted from real seismic data by autocorrelation of a seismic trace. If this is not done, a wavelet can be generated from various mathematical expressions, or by filtering a unit impulse with a band pass filter..

Synthetic seismograms derived from unedited and un-modeled data are common commercial products from all vendors. Two typical examples are shown below. They are useful for gross correlation and major reflector identification. Most vendors allow the purchaser to customize the product to include the type of editing and modeling described in the following Sections. However, the responsibility for the model parameters rests with the client as few suppliers have the log analysis skills to prepare adequate log models.


Commercial synthetic seismogram with integrated sonic log and interval velocity table

Using the log response equation we can inject new layers, delete existing layers, replace existing layers with new ones, change the fluid content from water to gas or oil (or vice versa). This is called "What-if" modeling. Models of this type will show why some water zones have bright spots, why some coal beds have no reflections, and why some carbonate porosity is visible on seismic (and some is not). What-if models allow the interpreter to test different structural and stratigraphic solutions against the actual data before committing to one interpretation.

Another variant of this modeling method is to replace data, or add data to the bottom of one well, using log data from another well which more closely represents the interpreted seismic section. This data must also be modeled and edited to reflect the unaltered rock/fluid mixture. This is often done to aid in design of a sidetrack or whipstock well to find the fault, reef, or salt dome structure that was missed by the original hole.

As far as can be determined, FEW existing seismic modeling workstation performs the necessary log analysis and log recalculation. Therefore YOU must do it first, offline from the modeling package, and enter the edited/modeled logs into the system.
 

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