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.

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This open-file is composed of two parts: (a) The manual you are reading now (including examples and program listings which are given in the appendix), and (b) the disk with computer programs and test examples. Two computer programs are presented. The-first program (ATO.EXE) is based on a new method for the automatic interpretation of Schlumberger sounding curves obtained over horizontally stratified media (Zohdy, in press). The second program (PICKCONT.EXE) is a utility program that reads layering-files created by ATO.EXE and automatically interpolates the depths at preselected resistivity contour values. The resulting list of depths and resistivities facilitates the construction of contoured geoelectric cross sections. The programs were written in Microsoft QuickBASIC 4.O. Each program is composed of several modules. The complete listings of the source code for the various modules are given in appendices A and B. DISCLAIMER Although these computer programs have been tested extensively and every effort has been made to assure their accuracy and performance, no guarantees are expressed or implied. Furthermore, any use of trade names is-for descriptive purposes only and does not constitute endorsement by the U. S. Geological Survey. SYSTEM REQUIREMENTS This version of the program is-for IBM and compatible computers running under MS-DOS version 2.1 or higher. The computer must be equipped with an EGA (Enhanced Graphics Adaptor) or CGA (Color Graphics Adaptor) card and at least 256 K RAM (random access memory). A math co-processor chip is strongly recommended but is not required. If the proper graphics card is not available the program displays a message to that effect and then exits. An Epson or compatible printer is required to dump graphics and obtain a hard copy. DISK CONTENTS The accompanying disk contains the following: 1) ATO.EXE is the executable-form of the automatic interpretation program. 2) ATO.BAS is the main module of the automatic interpretation program written in QuickBASIC version 4.0. It requires two modules: ATOSUB.BAS, DRCT.BAS. (See appendix A for a complete listing of ATO.BAS, ATOSUB.BAS, and DRCT.BAS). 3) ATOSUB.BAS is a module containing several sub programs that are used with ATO.BAS. 4) DRCT.BAS is a second module containing several subprograms that are used with ATO.BAS. 5) ATO.MAK is a make utility created by QuickBASIC 4.0 to call the modules ATO.BAS, ATOSUB.BAS and DRCT.BAS. 6) EGADUMPL.COM is a public domain (PD) program-for dumping EGA graphics on an Epson (or compatible) printer. 7) PICKCONT.EXE is the executable-form of the pick contour program. 8) PICKCONT.BAS is the main module of the pick contour program written in QuickBASIC 4.0. It requires the-following modules: ATOSUB.BAS, CONTSUB.BAS, and DRCT.BAS. The-first and third modules are the same as those used with ATO.BAS. (see appendix B-for a complete listing of PICKCONT.BAS and CONTSUB.BAS). 9) CONTSUB.BAS is a module consisting of one subprogram that contains the preselected resistivity contour values. 10) PICKCONT.MAK is a make...

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...

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On the basis of groundwater quality data, previous investigations have estimated that more than 60 square kilometers of the Oxnard aquifer have been intruded by seawater to distances reaching several kilometers inland from the shoreline of the Pacific Ocean. A direct current resistivity survey was made to investigate whether the increased salinity in the inland wells is caused by lateral invasion of seawater or by downward leakage of saline water from near surface formations through some wells. The resistivity survey, which consisted of 94 Schlumberger soundings, helped solve the problem by mapping the three dimensional geometry of subsurface materials having different resistivities down to depths of about 500 m. The results of the resistivity survey showed that over most of the study area, near-surface layers (down to a depth of about 10 m) often have very-low to low resistivities (<1 to 4.5 ohm-m); but at depths greater than 30 meters, where the Oxnard aquifer is found, the very-low resistivity layers (seawater-saturated layers) do not extend inland as far as previously suspected. The resistivity survey showed that it is possible to detect freshwater aquifers beneath saline-water aquifers, and that the freshwater aquifers in the study area mostly are characterized by resistivities in the 30 to 70 ohm-m range.

Image maps of apparent conductivity values for horizontal magnetic dipole modes for the EM31-D, (a), and EM34-3, (b) and (c), for stations on the grid near the Norman landfill west cell. The small dots in each image are the station locations ... 11. Image maps of apparent conductivity values for vertical magnetic dipole modes for the EM31-D, (a), and EM34-3, (b) and (c), for stations on the grid near the Norman landfill west cell. The small dots in each image are the station locations ..

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