Electrical structure of Newberry Volcano, Oregon

Abstract

From the interpretation of magnetotelluric, transient electromagnetic, and Schlumberger resistivity soundings, the electrical structure of Newberry Volcano in central Oregon is found to consist of four units. From the surface downward, the geoelectrical units are (1) very resistive, young, unaltered volcanic rock, (2) a conductive layer of older volcanic material composed of altered tuffs, (3) a thick resistive layer thought to be in part intrusive rocks, and (4) a lower-crustal conductor. This model is similar to the regional geoelectrical structure found throughout the Cascade Range. Inside the caldera, the conductive second layer corresponds to the steep temperature gradient and alteration minerals observed in the USGS Newberry 2 test hole. Drill hole information on the south and north flanks of the volcano (test holes GEO N-1 and GEO N-3, respectively) indicates that outside the caldera the conductor is due to alteration minerals (primarily smectite) and not high-temperature pore fluids. On the flanks of Newberry the conductor is generally deeper than inside the caldera, and it deepens with distance from the summit. A notable exception to this pattern is seen just west of the caldera rim, where the conductive zone is shallower than at other flank locations. The volcano sits atop a rise in the resistive layer, interpreted to be due to intrusive rocks. The intrusive material has served as a heat source to produce enhanced hydrothermal alteration and, perhaps in the case of the west-flank anomaly, elevated fluid temperatures. While no public drill hole information is available to confirm this hypothesis, the west-flank anomaly appears to be a good geothermal target. In addition to the possibility that a region on the west side of the volcano could be favorable for prospecting, part of the resistive structure under the center of the volcano could be due to a vapor-dominated environment with temperatures above 300øC. In other parts of the Cascades, pervasive alteration has produced mixed layer clays and zeolites, resulting in lowresistivity anomalies. Low resistivities cannot be assumed to indicate high-temperature pore fluids. The use of electrical methods that measure resistivity as a function of excitation frequency, such as spectral induced polarization, may provide a way of obtaining information about the type and extent of alteration. This paper is not subject to U.S. copyright. Published in 1988 by the American Geophysical Union. Paper number 7B1054. sient elcetromagnetic, and Schlumberger soundings. The interpretation of these data is compared with available drill hole data to determine the rock types associated with the various geoelectrical units. These data are used to produce an electrical model of Newberry Volcano. GEOLOGY Newberry Volcano is a large Quaternary volcano located east of the main axis of the Cascade Range and is very similar to Medicine Lake Volcano in northern California, both having summit craters, large volumes of basalt, and very young (600-to 1550-year-old) silicic flows. Its geolog...

Interpretation of long‐offset transient electromagnetic data from Mount Merapi, Indonesia, using a three‐dimensional optimization approach

“…Another explanation for the extensive high conductivity is that minerals produced by hydrothermal alteration lower the bulk resistivity of the rock. This has been observed in geothermal systems of other volcanoes, for example Newberry volcano [ Fitterman et al , 1988] or Piton de la Fournaise [ Lenat et al , 2000]. However, the extension of the conductor to the south [ Kalscheuer et al , 2004], the occurrence of generally low density sediments under the volcanic deposits [ Hoshino and Sunoto , 1978], and the observation that temporal gravity changes below the volcano edifice can be explained by fluid transports within a porous domain [ Jentzsch et al , 2004] all suggest that the conductor contains hot saline fluids.…”

Internal structure of Mount Merapi, Indonesia, derived from long‐offset transient electromagnetic data

“…This would allow a speculative interpretation: altered minerals could exist independent of the central conduit. An example was given by Fitterman et al [1988] for Newberry caldera.…”

Inner structure of La Fossa di Vulcano (Vulcano Island, southern Tyrrhenian Sea, Italy) revealed by high‐resolution electric resistivity tomography coupled with self‐potential, temperature, and CO2 diffuse degassing measurements

“…Because of their sensitivity to porosity, water saturation, and the presence of clays and zeolites minerals, DC‐electrical resistivity and electromagnetic methods (e.g., time domain electromagnetics) are efficient tools to image active volcanoes [ Fitterman et al , 1988; Zohdy and Bisdorf , 1990; Lénat et al , 2000]. DC‐electrical resistivity measurements were obtained along the nine profiles displayed in Figure 2.…”