Amplitudes: reducing acquisition footprint for prestack migration

Bill Kamps, president, Tsunami Development, discusses development work by his company to reduce the distortion caused by the acquisition footprint in seismic surveys in order to improve the analysis of migrated amplitudes. Users of seismic data today are more interested in the amplitudes of the migrated data than ever before. The amplitudes of the output seismic gathers are used for distinguishing geologic and reservoir properties as well as for AVO analysis. The effective use of these technologies depends a great deal on the relative accuracy and stability of the amplitudes in the migrated gathers. Regardless of how careful we are in preserving the amplitudes within the migration algorithm, the acquisition geometry of the traces can distort the migrated amplitudes and render amplitude analysis very risky. The acquisition pattern of 3D surveys can have a great effect on the resulting amplitudes, especially in land datasets. In the ideal situation we would like each offset to be acquired on a regular grid, and for each cdp to contain all the offset ranges. This of course only happens in synthetic surveys. The distortion created by the acquisition geometry is called the acquisition footprint. It is the imprint left on the amplitudes by the irregular spacing of the acquired traces. In this article we discuss technology that significantly reduces the acquisition footprint as well as research we are doing to make further progress in this area. Two problems not just one There really are two dimensions to this problem. The first is that within an offset bin the traces are not acquired at a uniform density over the survey. This can be compounded by the common practice of merging surveys which have different acquisition geometries and possible overlap areas. The second problem is that the different offset bins usually have very different populations. The near offsets may have one tenth the number of traces in them as the middle offsets. This very large difference in trace population produces corresponding amplitude differences in the migrated gather. The small population offsets have weak amplitudes in the migrated gather; the large population offsets have high amplitudes in the migrated gather. Producing useful amplitudes from the migration requires addressing both of these issues. Figure1 is a trace distribution map of the traces within a single offset bin. The blue areas show holes in the survey while the brighter white areas show high concentrations of traces in the acquisition. Figure 2 is an offset histogram showing a typical land survey offset population. We see very few traces in the near and far offsets, and large numbers of traces in the middle offsets. This type of distribution greatly distorts AVO analysis, since the amplitudes will certainly be distorted by virtue of the large variance in the number of traces within each offset bin.