CHARACTERIZATION OF SUB-SURFACE STRUCTURE OF MADHYAPUR THIMI AND SURYABINAYAK MUNICIPALITY USING MICROTREMOR ARRAY MEASUREMENT

Abstract

Microtremor Array Measurement (MAM) is a process of determining a subsurface soil characteristic by analyzing Ambient Vibration of the ground. In this method several geophones are deployed on a ground surface either in a linear or grid configuration for recording ambient vibration. In this research, we tried to estimate shear wave velocity (Vs) in Madhyapur Thimi and Suryabinayak Municipality using non-invasive method of MAM. The ambient vibration data obtained by employing seismometer array of different sizes is processed to obtain the dispersion curve using zero crossing method. This is then used to estimate the velocity structure of the ground by inversion employing Genetic Algorithm. The obtained result is compared with the actual soil layer data from soil test report for validation. Further, we obtained the averaged Vs30 by two ways viz: directly from dispersive curve and by inversion process. These results are compared and validated with previous studies. The Vs30 at the south eastern hills of study area ranges from 318 m/s to 522.24 m/s and 262.043 m/s to 521.979 m/s respectively from both approaches which were found to be comparatively higher to that at the western and central regions where Vs30 ranges from 121.97 m/s to 220.741 m/s and 135.186 m/s to 226.013 m/s respectively. Similarly, Vs30 at north-western region of study area ranges from 224.9717 m/s to 251.47 m/s and 209.022 m/s to 249.982 m/s respectively. These results show hard rock site at the edge of basin, loose soft soil at the central, northern and western region of study area and soft soil at the north western region. The Vs30 obtained was used for the classification of soil of the study area and found that the south eastern region of the study area lies in class C with major portion in class D and western region in class E. Moreover, the Vs thus obtained from inversion is used to find bearing capacity of the soil that ranged from 37.711 KN/m3 to 176.861 KN/m3 using empirical relations and possible amplification of the seismic waves in the study area was also determined using microtremor measurement which supported the Vs30 and bearing capacity obtained in our study area.