Results of a controlled‐source audiofrequency magnetotelluric survey at the Puhimau thermal area, Kilauea Volcano, Hawaii

Abstract

The Puhimau thermal area in the Volcanoes National Park, Hawaii, has attracted interest because it represents an area with anomalously high heat flow and the heat source may be a shallow magma body. At Puhimau a variety of geophysical data, including heat‐flow measurements, have been acquired. The self‐potential data suggest that the heat source is a result of a magma intrusion which plunges steeply northward. The heat‐flow data can be explained in terms of a shallow, relatively thick magma intrusion with a solidification rate sufficient to supply the surficial heat flow for the period of time since the appearance of the thermal area in 1936. In February of 1984, we performed a controlled‐source audiofrequency magnetotelluric (CSAMT) electromagnetic geophysical survey at the Puhimau thermal area to investigate the electrical nature of the thermal anomaly and estimate the depth to any hot water and/or remaining molten magma. The CSAMT survey consisted of several lines of stations where two orthogonal primary‐field transmitting antennas were used. A technique was developed to correct near‐field CSAMT apparent resistivities to plane‐wave values so that plane‐wave analysis techniques could be applied. Plane‐wave 2-D finite‐difference calculations are used to interpret the field data. The results are consistent with the interpretation that the conducting anomaly is a long, thick, dike‐like feature and within this conducting anomaly there is an excellent conductor (approximately 5 Ω ⋅ m) at a depth of approximately 200 m which may be magma or hot, mineralized water. Above this conductor is a zone 40 times less conductive which probably represents an area with less than 100 percent water saturation. The dike‐like structure is connected to a conducting (approximately 5 Ω ⋅ m) basal layer at a depth of 350 m.