Abstract
In this paper, we demonstrate that Common Mid-Point (CMP) cross-correlation
gathers of multi-channel and multi-shot surface waves give accurate
phase-velocity curves, and enable us to reconstruct two-dimensional
(2D) velocity structures with high resolution. Data acquisition for
CMP cross-correlation analysis is similar to acquisition for a 2D
seismic reflection survey. Data processing seems similar to Common
Depth-Point (CDP) analysis of 2D seismic reflection survey data,
but differs in that the crosscorrelation of the original waveform
is calculated before making CMP gathers. Data processing in CMP cross-correlation
analysis consists of the following four steps: First, cross-correlations
are calculated for every pair of traces in each shot gather. Second,
correlation traces having a common mid-point are gathered, and those
traces that have equal spacing are stacked in the time domain. The
resultant cross-correlation gathers resemble shot gathers and are
referred to as CMP cross-correlation gathers. Third, a multi-channel
analysis is applied to the CMP crosscorrelation gathers for calculating
phase velocities of surface waves. Finally, a 2D S-wave velocity
profile is reconstructed through non-linear least squares inversion.
Analyses of waveform data from numerical modelling and field observations
indicate that the new method could greatly improve the accuracy and
resolution of subsurface S-velocity structure, compared with conventional
surface-wave methods.
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