|Institution:||University of New South Wales|
|Keywords:||Seismic refraction method; Signal processing – Digital techniques|
|Full text PDF:||http://handle.unsw.edu.au/1959.4/19275|
The refraction convolution section (RCS) is a simple and efficient method for full trace processing of shallow seismic refraction data. It facilitates improved interpretation of shallow seismic refraction data through the convenient use of amplitudes as well as traveltimes. The RCS is generated by the convolution of forward and reverse shot records. The convolution operation effectively adds the first arrival traveltimes of each pair of forward and reverse traces and produces a measure of the depth to the refracting interface in units of time which is equivalent to the time-depth function of the generalized reciprocal method (GRM). The convolution operation also multiplies the amplitudes of first arrival signals. This operation compensates for the large effects of geometric spreading to a very good first approximation, with the result that the convolved amplitude is essentially proportional to the square of the head coefficient. The head coefficient is approximately proportional to the ratio of the specific acoustic impedances in the upper layer and in the refractor, where there is a reasonable contrast between the specific acoustic impedances in the layers. The RCS can also include a separation between each pair of forward and reverse traces in order to accommodate the offset distance in a manner similar to the XY spacing of the GRM. Lateral variations in the near-surface soil layers can effect amplitudes thereby causing 'amplitude statics'. Increases in the thickness of the surface soil layer correlate with increases in refraction amplitudes. These increases are adequately described and corrected with the transmission coefficients of the Zoeppritz equations. The minimum amplitudes, rather than an average, should be used where it is not possible to map the near surface layers. The use of amplitudes with 3D data effectively improves the spatial resolution by almost an order of magnitude. Amplitudes provide a measure of refractor wavespeeds at each detector, whereas the analysis of traveltimes provides a measure over several detectors, commonly a minimum of six. The ratio of amplitudes obtained with different shot azimuths provides a detailed qualitative measure of azimuthal anisotropy. Dip filtering of the RCS removes 'cross-convolution' artifacts and provides a convenient approach to the study of later events. The RCS facilitates the stacking of refraction data in a manner similar to the CMP methods of reflection seismology. It can improve signal-to-noise ratios.