Assessing and Improving the Quality of Dispersion Images Using the MASW Technique for Ground Exploration

Abstract

An approach has been proposed to assess the quality of the dispersion plot extracted from the dispersion image obtained based on the multichannel analysis of the surface waves (MASW) technique. The variation of the quality factor Q associated with a given mode has been presented as a function of frequency. A sensitivity analysis has also been performed to determine the influence of different input parameters on the quality factor. The phase-shift wavefield transformation method was used for generating the dispersion image(s). Dispersion images were obtained by generating synthetic signals as well as by performing field tests at three different sites with the help of 48 geophones. An attempt has been made to examine the effects of: (1) the stacking/padding of data in the time domain; (2) the number of geophones; (3) spacing between the receivers; and (4) the signal-to-noise ratio (SNR) on the dispersion images for the purpose of ground exploration for a target depth of approximately 30 m. It is observed that a good quality dispersion image in the lower frequency range (<10 Hz) can be obtained by increasing the receiver spread length using a large spacing interval (2–5 m) between receivers, increasing the SNR of the signal, and the stacking of the data in the time domain. However, the usage of a smaller spacing (approximately 1 m) between receivers still becomes essential in overcoming the presence of any spurious (dummy) dispersion mode(s) at higher frequencies. Greater numbers of sensors, 96 or even more, are recommended to examine more accurately the dispersion images to detect the presence of any complex mode(s) while obtaining the nonregular dispersive ground profile. It is observed that Q becomes quite sensitive to changes in the spacing and number of geophones especially in the lower frequency region (around 10–15 Hz).