A novel weight-drop seismic energy source for subsurface characterization

Abstract

Multichannel Analysis of Surface Waves (MASW) studies for subsurface characterization in Central Ganga Basin of India, where thick cover of alluvium absorbs most of the vibrations generated by a sledgehammer source, give low quality seismic data at large offsets and generally provide imaging depth lesser than 30 m due to low quality of dispersion curves at lower frequencies. Therefore, a novel seismic energy source is designed and developed to produce large amplitude surface waves even at longer offsets and for deeper depths of imaging using dispersion analysis of surface waves. The developed source for seismic data acquisition is low-cost, repeatable, portable, and easy to operate with no environmental impact. Unlike the sledgehammer source where the quality of surface waves generated also depends on the person hitting the metal plate, the seismic data quality from the novel weight-drop source is independent of the operator.The developed source is an impact type where a free-falling mass accelerated due to gravitational pull hits onto a base plate coupled with the earth generating vibrations in the ground. The performance of the source was evaluated in a field trial. A receiver array of 24-channel cabled geophones with 3 m spacing was planted on the ground. Seismic data were acquired using the source at 5 m negative offset inline from the first geophone. Seismic data were also recorded with the sledgehammer as a source for comparison. Five shot gathers for each source were acquired with the same acquisition geometry and source position.Shot gathers were analyzed and compared using trace attributes, 1D and 2D Fourier spectra, signal-to-noise ratio, relative energy, and dispersion images. The study shows that the developed source produces low frequency high amplitude signal and better dispersion image, notably in the lower frequency range, in comparison with the sledgehammer source. Therefore, the developed source is adapted to investigate deeper subsurface velocity structure with dispersion curve from high amplitude surface wave analysis. Moreover, the repeatability study suggests that the repeat shot gathers acquired from the developed source have remarkably high degree of similarity. Therefore, seismic data acquisition with the developed source does not require repeat shots for vertical stacking as often done in case of a hammer source. Sound pressure level measurement indicates that the developed source has no adverse environmental impact on the operator. From the field trial of the developed source, it is convinced that the source is cost-effective, portable, easy to operate and handle and is environmentally friendly.