Can new acquisition methods improve signal-to-noise ratios with seismic refraction techniques?

Abstract

A fundamental issue which impacts on the inversion of refraction data using any method, is the large variations in the accuracies of the traveltime data. These variations are related to the wide range of signal-to-noise (S/N) ratios along the refraction spread, and in turn, are a result of the large amplitude variations caused by geometric spreading. For shallow refraction investigations, the geometrical spreading can be more rapid than the generally accepted inverse of the distance squared function. At any particular location, a detector will be close to a source point, and the traveltime will be comparatively accurate, because the S/N ratio is high. However, for other shot records, generally in the reverse direction, the source to receiver distance will be much larger, and the accuracy will be greatly reduced. As a result, computations at each detector location are carried out with data which have large variations in accuracies. This adversely effects the quality of the processing using any inversion method. Most approaches to the processing of seismic refraction data perceive the problem as achieving satisfactory, rather than uniform S/N ratios, and commonly, a simple gain function is applied to adjust amplitudes to a desired level. However, this does not address the issue of the large range in accuracies of the traveltime data due to the variations in S/N ratios. This study demonstrates that, to a very good first approximation, the convolution of forward and reverse seismic traces compensates for the large variations in S/N ratios due to geometric spreading. This facilitates the use of signal enhancement techniques analogous to the common mipoint (CMP) stacking methods, which are an integral component in reflection seismology. Examples demonstrate the improvements in S/N ratios with stacking of convolved refraction data. However, in order to achieve suitable data multiplicity or fold, as well as more efficient field methods, it is necessary to employ CMP acquisition techniques, instead of the standard static refraction spread with multiple source points which is the norm in most geotechnical and groundwater investigations. This will require substantial re-capitalization of most shallow refraction operations as very few currently employ sufficient numbers of recording channels or roll along capabilities.