CHARACTERISATION OF SUB-SURFACE STRUCTURE OF HETAUDA AREA USING MICROTREMOR ARRAY MEASUREME

Abstract

In order to record ambient vibration, a number of geophones were placed on the ground in a grid or linear pattern. In this research, we tried to estimate average shear wave velocity (Vs30) in Hetauda area using non-invasive method of Microtremor array measurement (MAM). The ambient vibration data was obtained by employing a seismometer array of different sizes as per space available and was processed to obtain the dispersion curve using the zero-crossing method. The ground's velocity structure was then estimated via inversion using a genetic algorithm. The acquired result was compared with the real soil layer data from the soil test report. Additionally, we used two methods to derive the averaged Vs30: the inversion procedure and straight from the dispersive curve and the obtained results were compared with previous studies. The Vs30 in the North-western study area ranged from 286.17 m/s to 431.86 m/s and 259.49 m/s to 470.10 m/s respectively from both approaches which were found to be comparatively higher than that at the south-eastern and central regions where Vs30 ranged from 191.35 m/s to 284.07 m/s and 292.65 m/s to 226.013 m/s respectively. Similarly, these results showed very dense soil or soft rock site at the edge of Hetauda Dun Valley, and loose soft soil at the central, eastern region of the study area due to which fine Quaternary and floodplain deposits of Karra Khola were linked to reduced shear-wave velocity. The soil in the research region was classified using the calculated Vs30 and it was found that the study area's northwestern section was in class C and south-eastern region in class D. Moreover, the Vs30 thus obtained from inversion was used to draw the north-to-south longitudinal cross-section and also the west-to-east lateral soil profile.