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Pitfalls in synthetics

GeoLearn, Houston, Texas, U.S.

Corresponding author: geolearn@aol.com

Editor's note: This series is designed to update the classic Pitfalls in Seismic Interpretation by Tucker and Yorstan, first published by SEG in 1973. Contact Steve Henry (geolearn@aol.com) for information about contributing to this series.

The first 20% of the full text of this article appears below.

The primary goal of seismic interpretation is to make maps that provide geologic information (reservoir depth structure, thickness, porosity, etc.). However, even ideally acquired/processed seismic data provide only an image of the subsurface. For geologic information, nearby wells must be correlated to seismic reflectors. Synthetic seismograms (synthetics) provide this link by converting rock properties from well logs to a synthetic trace.

Synthetics make "rocks look like wiggles," using the convolution model (T = RC * W), which states that traces (T) are the result of convolving (*) the reflection coefficient series (RC) with the wavelet (W). When seismic data are acquired, a source wavelet is sent into the earth, reflected back (convolved) to the surface at geologic boundaries (RC), and recorded as a trace (T).

RC, however, remains unknown until a well is drilled and logged. It can then be calculated using the measured changes in rock properties (velocity and density). RC for normal incidence is

where v₁, v₂ are P-wave velocities (sonic log) and r₁, r₂ are densities (density log) in the layer above (1) and below (2) the reflecting boundary.

The normal incidence assumption is generally valid, except where velocity and density contrasts are very large (gas sands, coal, hard streaks, etc.). When these exceptions are critical to the interpretation, RC needs to be . . . [Full Text of this Article]