Fold Balancing to Preserve Amplitude of Land‐Based Shallow Seismic Reflection Data

Seismic reflections typically contain four types of information: travel time, amplitude, phase and frequency. In most shallow reflection seismic studies, amplitude is ignored because the true amplitude is difficult to preserve when conventional automatic gain control (AGC) is applied. Recent, floating point technology lets geophysicists process data with a greater dynamic range and apply band pass filtering without AGC, thereby retaining amplitude information in the data set. Also, the usual muting operations that erase the part of the trace affected by normal move-out (NMO) stretch have the effect of changing the fold and decreasing the amplitude. In general, shallow reflectors have a smaller fold than deeper ones. The affected traces can be normalized by “fold balancing” which consists of normalizing the stacked traces as a function of the number of non-muted traces at each time sample. After the trace shows a true amplitude, an exponential gain can be applied to compensate for spherical divergence and absorption. The effects of processing data with AGC, the new fold balancing and exponential gain are compared using two shallow seismic sections referenced to borehole data.