College - Author 1
College of Science and Mathematics
Department - Author 1
Physics Department
Degree Name - Author 1
BS in Physics
Date
6-2026
Primary Advisor
John Jasbinsek, College of Science and Mathematics, Physics Department
Abstract/Summary
Identifying a productive groundwater source in hard rock terrain is often a challenge. Conventional hard rock aquifer models commonly describe a weathering derived layered system consisting of a laminated layer, an underlying fractured zone, and a competent granitic basement. This study investigates the applicability of this conceptual model at Lockshaw Ranch, a granitic site within the Salinian Block of central California located near the Rinconada Fault.
Previous electrical resistivity tomography (ERT) surveys and time domain electromagnetic (TDEM) data were integrated with a newly acquired ERT profile (ERT-06) are used to characterize the subsurface structure and identify potential groundwater bearing zones. The geophysical results show no evidence of a laminated layer or a distinct competent granitic basement. Instead, the groundwater occurrence appears to be associated with fracture controlled zones bounded by structural discontinuities. Resistivity values of approximately 150–200 Ω·m correspond to regions of enhanced fracture connectivity and permeability, including the location of a previously successful groundwater well (CP1, 2024).
The interpreted structural framework suggests that the subsurface is compartmentalized into multiple hydrogeologic zones with limited hydraulic connectivity between them. These findings indicate that groundwater occurrence at Lockshaw Ranch is controlled by localized fracture networks rather than a continuous weathering derived profile. The successful identification of groundwater bearing zones using ERT demonstrates the need for geophysical methods in groundwater exploration for tectonically complex granitic terrains, as well as providing a site specific conceptual model for future groundwater development.
URL: https://digitalcommons.calpoly.edu/physsp/239