ABSTRACT
As part of ongoing environmental studies on the island of Guam, a natural-potential geophysical survey was conducted in the area of Tarague Beach within Andersen Air Force Base, on the north coast of Guam, a United States Territory in the Western Pacific. The purpose of the survey was the identification of zones of increased hydraulic conductivity in the coralline limestone to optimize monitoring locations for dye-tracer studies. Since most of the groundwater flow in karst terrane is concentrated in solution caves and conduits, it becomes necessary to intercept the stream conduits in order to sample the groundwater system reliably. The natural-potential (NP) method is currently the only proven geophysical technique that responds to the movement of fluids, rather that their mere presence.
The natural-potential method measures natural voltage at the earth surface resulting from ambient d.c. electric currents that occur everywhere on land and sea. Localization of natural potentials is the result of discrete physical phenomena acting underground and not some random process. The currents can arise from a variety of sources; e.g., redox reactions around a metallic conductor intersecting the saturated zone; fluid diffusion across soil and lithologic contacts; subterranean chemical and temperature gradients; and most commonly, the flow of water through pores, fissures and caverns in the ground, the electrokinetic effect.
Ten natural-potential anomalies were selected as drill sites and drilled to depths ranging between 9 and 28 meters (30–92 ft). Five holes were located inland and the remaining five, along the beach. Eight of the ten holes intercepted cavernous zones and water-filled cavities up to 2 meters (6 ft) in height within the limestone (one hole did not penetrate limestone).
Results of the drilling demonstrate that well-defined positive natural-potential anomalies are associated with solutionally- enhanced permeable zones within the coralline limestone groundwater system. From the well logs alone, however, one cannot assess the lateral extent of the solutional activity, or whether cavities would be encountered if one drilled off of a significant NP anomaly. At Tarague Beach, groundwater movement toward the sea and concentrated within solutionally enhanced permeable zones or karst conduits is the most evident mechanism responsible for the observed electrical manifestations at the surface.
