Using Self-Potential and Seismic Ambient Noise Methods on Oʻahu’s Groundwater System, to Uncover the Geological Nature of the North Schofield Dam

The Schofield groundwater body on the island of Oʻahu is one of the most important freshwater reservoirs yet has enigmatically high hydraulic head as compared to the adjacent basal water bodies. Various hypotheses have been proposed to explain this so-called North Schofield Dam, yet for the most part the Schofield aquifer’s high head remains unexplained. For this study we collected extensive self-potential and seismic ambient noise datasets in an effort to better understand the geologic and fluid flow conditions across the North Schofield Dam boundary.  We collected 40 km of interconnected self-potential profiles and seismic ambient noise data recorded by 53 stations (Magseis Fairfield) over 18 consecutive days. As a complementary approach, we performed a basic clustering analysis of our self-potential dataset. Our results revealed two distinct domains with different geophysical signatures, demarcated by a NE-SW-striking boundary. The southeastern area exhibits higher self-potential values and seismic velocities, while the northwestern area has lower values for both data types. This sharp boundary, oriented N25°, separates two hydrogeological domains and precisely highlights the North Schofield dam. We interpret that the NW domain contains buried valley-fill material with low permeability, while lava flows from the Wai‘anae and Koʻolau volcanoes dominate the SE zone. We interpret that the mostly impervious valley-fill material acts as a water flow boundary that may explain the abnormally high groundwater levels observed in the Schofield groundwater unit. We thus provide for the first time, the orientation, underground geometry, and geological nature of the North Schofield Dam. This study helps in understanding the mechanisms associated with forming a large aquifer in the elevated central part of the island of Oʻahu. It also emphasizes the importance of geological complexity of volcanic environments in influencing groundwater storage and flow. Our findings will be key to improving hydrogeological models, and thus fresh groundwater resources management on O‘ahu, and it opens perspectives for further comparative studies on other volcanic islands.