Swept impact seismic technique (SIST)

Abstract

A coded seismic technique is developed that can result in a higher signal‐to‐noise ratio than a conventional single‐pulse method does. The technique is cost‐effective and time‐efficient and therefore well suited for shallow‐reflection surveys where high resolution and cost‐effectiveness are critical. A low‐power impact source transmits a few to several hundred high‐frequency broad‐band seismic pulses during several seconds of recording time according to a deterministic coding scheme. The coding scheme consists of a time‐encoded impact sequence in which the rate of impact (cycles/s) changes linearly with time providing a broad range of impact rates. Impact times used during the decoding process are recorded on one channel of the seismograph. The coding concept combines the vibroseis swept‐frequency and the Mini‐Sosie random impact concepts. The swept‐frequency concept greatly improves the suppression of correlation noise with much fewer impacts than normally used in the Mini‐Sosie technique. The impact concept makes the technique simple and efficient in generating high‐resolution seismic data especially in the presence of noise. The transfer function of the impact sequence simulates a low‐cut filter with the cutoff frequency the same as the lowest impact rate. This property can be used to attenuate low‐frequency ground‐roll noise without using an analog low‐cut filter or a spatial source (or receiver) array as is necessary with a conventional single‐pulse method. Because of the discontinuous coding scheme, the decoding process is accomplished by a “shift‐and‐stacking” method that is much simpler and quicker than cross‐correlation. The simplicity of the coding allows the mechanical design of the source to remain simple. Several different types of mechanical systems could be adapted to generate a linear impact sweep. In addition, the simplicity of the coding also allows the technique to be used with conventional acquisition systems, with only minor modifications.