Basic AVO Workflow

The following is a very basic guide for the use of the AVO functionality in the new GeoSyn 1D program. This functionality is capable of both reverse and forward modeling. Reverse modeling requires seismic data of the common offset type.

Forward Modeling

1. Import well logs, P Sonic, Density and Shear sonic if available.

2. Re-datum the synthetic by adding new section at the top. This critical step is necessary to ensure that the incident angle for any given interface calculated within the GeoSyn ray tracing algorithm matches the seismic section. Ray tracing must be done from close to the surface where the seismic was acquired.

Two methods may be used to accomplish this. The older method, using the Insert Section dialog box in the Log Editor, is no longer recommended. A new dialog box, Time Datum properties’, handles the problem much better.

Using the Time Datum properties dialog box

Open the Time Datum Properties dialog box, Edit/Time Datum… (). A single entry typically indicates that the top of the log suite is time zero. Click Add a layer, to create a new layer with zero thickness. Next change the KB or SS value of the top of the new layer and set it to near the elevation of the ground surface or seismic datum.
Update the velocity of the new zone to create enough time delay to push the GeoSyn models down in time to the correct position in time relative to observed seismic or checkshot data. Notice that the top of the visible model now has a time matching the bottom of this new layer. GeoSyn’s AVO calculation honours this new zone (or zones) as if it were genuine log section added to the top of the log suite (as in the older method below).

Using the Insert Section dialog box

Calculating the amount of section to add: The depth datum of the seismic section should not be used to calculate this value, as the incident angle field in the seismic is a function of the shot point and receiver elevations, in other words, the ground surface. In the idealized case of flat stratigraphy and flat ground surface, the depth value of the first log sample (relative to the KB) is a decent value to use. Using the Insert Section dialog box, add this section to the top of the log suite.

Calculating the velocity of the new section: The velocity/transit value of the new section is also an issue. It must match the average velocity of the section from the top of the well log to the surface. Calculate this value before adding section to the well log.

Correlate the synthetic to the seismic and determine the difference, the time shift, between seismic zero and synthetic zero. Calculate the seismic time datum shift applied by the processor and remove this from the value. Divide the Time Shift by two to change from 2-way to 1-way.
Calculate the required average velocity or transit value by dividing the required amount of depth section by the Time Shift.

3. Assign AVO parameters to a model. Right click over a model and select the Model Parameters dialog. Set the model type to 'Multi Offset' and adjust the following parameters:

Ray trace method - Ray trace (slow). ( Full Snell's law calculation).

Vertical sampling interval - Use 'Constant time'.

Geometry - CDP interval, Num traces and Near trace offset.

Adjust the three parameters, adding or subtracting traces, adjusting the CDP interval and offset until the required maximum offset is reached. Remember that the CDP interval is half the surface offset.

Apply NMO correction - Set to yes.

Mute - Something around 28 degrees or so will work.

4. Apply an offset dependency formula. By default when a multi-offset model is created the offset dependency for reflection coefficient calculation is set to none. To apply a dependency select the Reflection Coefficient (Ref Coef) tab of the Model Properties dialog and select one of the three options, Aki Richards 2 or 3 term or Shuey. If a shear sonic is unavailable GeoSyn will prompt you to create one based on Castagna's well known relationship between shear and p sonic curves.

Step 4 completes the basic setup for forward AVO modeling. The model is ready for cross plot analysis.

5. Cross Plot analysis. While some information may be gleaned from visual analysis of the model the best way to analyze the data is with a cross plot.
Cross plots of interest fall into two categories:

Analysis of a single or at most 2 or three sample points involving amplitude plotted against one of the following; offset, incident angle, sine incident angle or sine squared incident angle. This method is most appropriate for detailed analysis of an AVO response at a particular interface of interest.

Analysis of the entire data set, or significant portion of it, by using the gradient vs. intercept method. This method is convenient in a reconnaissance sense as the entire data set may be analyzed at once with a view toward identifying zones of interest.

Reverse Modeling

Reverse modeling is defined as a modeling effort involving real data.

1. Import well logs. P Sonic, Density and Shear sonic if available.

2. Import seismic offset data. A common offset stacked panel, sorted to display near trace on the left and far trace on the right, works well. Set the synthetic sampling rate to match the sample rate of the seismic.

3. Re-datum the synthetic by adding new section at the top. Please refer to step 2 in the Forward modeling section.

4. Tie seismic to synthetic. This critical step adjusts the time datum of the seismic section to tie the synthetic. Open the 'Tie seismic to model dialog by right clicking over the seismic or selecting from the Seismic properties dialog. This dialog initially requests a model to tie against then automatically calculates a best fit time shift.

5. Creating an AVO model using parameters from the seismic. Select an unused model using the 'Link to AVO trace model' list box in the Seismic properties dialog. The model will have its AVO properties automatically adjusted to match the seismic offset panel. If there is a problem reading the seismic headers the model properties must be set manually.

Step 5 completes the basic steps required to prepare a reverse model for analysis. The model is ready for cross plot analysis.

6. Cross Plot analysis. Please refer to step five in the Forward modeling section for instructions on cross plot usage. The cross plotting effort is identical except that all offset and angle selections must come from the model and all amplitude selections must come from the seismic.