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 300°C. In other parts of the Cascades, pervasive alteration has produced mixed layer clays and zeolites, resulting in low‐resistivity 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.

Geoelectrical structure of the central zone of Piton de la Fournaise volcano (Réunion)

“…Consequently, we can provide an interpretation through combined use of both the TDEM and DC data sets. Fitterman et al (1988) used a similar approach in developing a geoelectrical model for Newberry volcano, Oregon.…”

Autocorrelation of the Seismic Wavefield at Newberry Volcano: Reflections From the Magmatic and Geothermal Systems

“…Background colors are fractional change in velocity from the Heath et al () tomography, taken along A to A′ in Figure a. Red, magenta, and green lines show the top and bottom of an electrical conductor (Fitterman et al, ; Waibel et al, ) and the top of a resistive layer (Fitterman et al, ) respectively. The autocorrelation traces for 6 Hz ambient noise (Figure a) are hung from the topography and converted to depth using a 1‐D velocity model, assuming a vertically propagating wave.…”

“…Magnetotellurics studies have characterized three interfaces in the upper few kilometers of the west flank of Newberry. The first is the top of a conductive layer, often associated with smectite alteration (Fitterman et al, ), which is found at depths of 0.5–0.9 km. The bottom of this layer forms a second interface at ~1.5 km depth and is related to a change to greenschist facies metamorphism (Waibel et al, ) and to alteration of smectite to chlorite and epidote.…”

“…These alteration facies are commonly associated with temperatures between 150°C and 220°C (Didana et al, , and references therein). Mineral deposition of chlorite and epidote can clog pore spaces (Fitterman et al, ), and a decrease in conductivity has also been correlated with changes in porosity in other areas (e.g., Wamalwa et al, ). The third layer, at 1.5–3 km depth, is the top of a resistive layer, thought to be a switch to an intrusive dominated zone (Fitterman et al, ).…”

Mapping the Geothermal System Using AMT and MT in the Mapamyum (QP) Field, Lake Manasarovar, Southwestern Tibet

“…Although the QP geothermal field can be characterized as a non-volcanic geothermal region [29], with the exception of its very resistive surface layer, this field has a similar geoelectric structure to Newberry volcano in Oregon, USA [28], and to the Theistareykir geothermal area in Iceland [30], apart from the very resistive surface layer. These sites are also characterized by the presence of a very resistive surface layer (1); a conductive layer (2); a thick resistive layer (3); and a lower-crustal conductive layer (4) [31]. Comparison of the example MT curve between Newberry volcano in Oregon and QP in Tibet is shown in Figure 7.…”