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.

Puhimau Thermal Area: A Window into the Upper East Rift Zone of Volcano, Hawaii?

“…A recent three-dimensional velocity structure study of K" lauea using P-and S-wave earthquake arrival times revealed two high V p /V s zones, thought to be magma reservoirs, at 1-4 km depth under the southern caldera rim and under Puhimau thermal area; the two zones appear to be connected by an inverted v-shaped passageway (DAWSON et al, 1999). Yet, an earlier detailed controlled-source audiofrequency magnetotelluric survey at the Puhimau thermal suggests the anomaly there is actually a dike-like feature that may contain either hot, mineralized water or magma (BARTEL and JACOBSON, 1987). Regardless of the exact nature of the anomaly beneath Puhimau thermal area, it is evident from the work of GERLACH et al (2002) that Kıˆlauea's summit region receives magma rich in exsolved CO 2 that is vented to the atmosphere at an area just east-southeast of Halema'uma'u; CO 2 -poor and SO 2 -rich magma is then fed into the upper ERZ in the vicinity of Puhimau thermal area.…”

Interpretation of 1D Vector Controlled-Source Audio-Magnetotelluric (CSAMT) Data Using Full Solution Modeling

“…To avoid effects by the presence of an artificial source, CSAMT data are usually taken at a distance of 3-5 skin depth from the source using the plane wave approach [6]. Yamashita, et al [7] and Bartel and Jacobson [8] proposed a source effect correction and used plane wave approach to interpret the corrected data. Nearfield corrections have received serious attention since they are based on a homogeneous earth model and have validity under question in complicated environments [9].…”

New Geological and Structural Facts under the Lateritic Cover in Garga Sarali, Ndokayo (East Cameroon) Area, from Audiomagnetotellurics Soundings

“…In geophysical prospecting, AMT method is used and considered as a key tool in mineral exploration [5], in geothermal investigation [6][7][8], in potential radioactive waste disposal characterization [9], and in the identification of tectonics structures [10].…”