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Factors affecting frequency content in preSDM imaging

GX Technology, Egham, Surrey, U.K.

Corresponding author: ijones@gxt.com

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

This paper evaluates various factors that influence frequency content during migration. The object is assessing the reasons for potential loss of bandwidth in migrated data.

In the following examples, the nature and cause of various factors that can impact the frequency content are summarized and examined to determine their effects on depth migration more than on time migration, or Kirchhoff migration more than wavefield extrapolation schemes.

We demonstrate that there is no inherent reason for the bandwidth of Kirchhoff depth-migrated data to be worse than other migrated data and offer recommendations to ensure optimal frequency content in the processed output image.

The analysis covers spurious differences; aliasing (temporal and spatial); wavelet changes during migration (frequency, velocity, and offset dependent); and Kirchhoff migration as a stacking process (traveltime sampling errors, sensitivity to velocity error, and acquisition footprints).

	Spurious differences

 
A common element of confusion in time versus depth comparisons is the degree of postprocessing. A final time product (with its associated deconvolution and spectral balancing) will naturally look better, in terms of signal content, than a raw time-converted preSDM result. Consequently, it is important to perform appropriate postprocessing on the preSDM output before drawing any conclusions (Figure 1). In the case of designing spectral balancing operators, we must ensure that the preSDM output spectrum extends well beyond the signal spectrum.

View larger version (116K): [in this window] [in a new window]   Figure 1. In the raw preSDM output, the target horizon (faulted sandstones) lying beneath an unconformity appears to have a lower frequency bandwidth than the conventional preSTM result. However, application of similar wavelet processing to the preSDM produced a superior result. Depth results have been converted to time.