Multi-Shot Processing For Better Velocity Determination

Abstract

We perform a technique called multi-shot processing on a section of 12-channel sonic logs in order to better resolve compressional and shear velocities. The data are from the ODP Leg 102 cruise, which occupied drill site 418A near the Bermuda Rise in 1985. Multi-shot processing has been done on a 9 meter section of this data, using different combinations of numbers of shots vs. numbers of receivers in an attempt to compare the vertical resolution and stability of this processing method. The method is stable only with certain shot-to-receiver subarray combinations. This paper demonstrates that the optimum combinations using this set of data are 4 shots with 6 receivers apiece, and 3 shots with 8 receivers each. While a combination using 5 shots with 4 receivers is possible, the method produces spurious results. This may be because of spatial aliasing over too few receivers, or it may be a result of poor outside control over the entire experiment (ship heave, etc.). It is hoped that an optimum subarray combination can be used to resolve velocities over shorter array lengths using the redundancy in the sonic data. This would result in a greater ability to characterize fracturing and alteration in the oceanic crust, since velocity variations have been shown to correlate with fracture zones.