การวัดค่าสภาพต้านทานไฟฟ้าและการใช้เรดาร์หยั่งลึกชั้นดิน เพื่อหารอยแตกในชั้นหินแข็ง กรณีศึกษา ตำบลทุ่งใหญ่ อำเภอหาดใหญ่ จังหวัดสงขลา

Abstract

A study of fracture zone in hard rock for groundwater in Ban Thung Ngai, Hat Yai district, Songkhla province where drilling well for groundwater is hard to get successful. This is because the study area lies on a flat hill area where the topsoil is sand from terrace deposits in the Quaternary Periods underlain by the granitic rock that has reached the surface. Three wells have been drilled for groundwater in this area with interval about 50 m arranging in W - E direction. Only the well that is in the middle found no groundwater. This means that the subsurface aquifer here is complicate. The aim of this study is to determine the fracture zone in granitic rock by using geological survey and geophysics methods. Two azimuthal resistivity sounding (ARS) positions have been carried out by VES Schlumberger along six different azimuths with a maximum AB/2 separation of 300 m. The fracture rock strike will be considered from the major axis of apparent resistivity variation with azimuths plotting on the polar diagram for each depth of investigation. The orientation of fracture strikes in NE-SW and NW-SE at the shallow depth (AB/2 < 80 m) and the deeper depth (AB/2> 80 m) respectively, were found which coincide with the geological study. The subsurface fractured zone at the AB/2 > 80 m depths corresponds to the location of well-found groundwater and correspond to the 2D-resistivity and GPR results performed perpendicular to the fractured strike. The well in the middle (no groundwater) coincides with subsurface comprise of sand and granitic rock at the shallow depth, while the beside wells found water are correspondent to the zone of subsurface low resistivity layers and high amplitude layers of GPR signal. Thus ARS, 2D-resistivity and GPR measurements have provided an important data of fracture direction in hard rock area that will be the key to recommend the drilling location for groundwater.