Geophysical studies on the Shallow Groundwater Aquifer in drought prone Somavathi River basin, Ananthapuramu district (Andhra Pradesh), India: A case study

Hydrogeological study of the Somavathi River basin in Ananthapuramu district is carried out, which is a drought prone district of Andhra Pradesh. This River basin is investigated using combined geophysical surveys consisting of electrical resistivity components, for shallow sub-surface mapping, however, there are constraints in distinguishing the chemical characteristics of the medium. This complexity can be resolved by integrated investigations, where resistivity anomalies are validated with groundwater available in fractures zones at the interface between the overburden and bedrock. Such methods are useful in the selection of suitable locations for drilling of bore wells, to improve the success rate. Surface resistivity methods are commonly employed in groundwater investigations and the interpretation of the resistivity data gives thickness and resistivity of subsurface layers. Due to heterogeneity in lithological units, the electrical resistivity also varies locally. In the present study, the three-layer sounding curves are found to be of four types, depending on the resistivity value, these are A-type (p1<p2<p3), K_type (p1<p2>p3), H type (p1>p2<p3), and Q type (p1>p2>p3). Depth to basement in somavathi river basin, varies from 12 m. to 76 m.At the confluence of the somavathi river, the depth of the basement is maximum at about 76 m.