A fast iterative method for the automatic interpretation of Schlumberger and Wenner sounding curves is based on obtaining interpreted depths and resistivities from shifted electrode spacings and adjusted apparent resistivities, respectively. The method is fully automatic. It does not require an initial guess of the number of layers, their thicknesses, or their resistivities; and it does not require extrapolation of incomplete sounding curves. The number of layers in the interpreted model equals the number of digitized points on the sounding curve. The resulting multilayer model is always well‐behaved with no thin layers of unusually high or unusually low resistivities. For noisy data, interpretation is done in two sets of iterations (two passes). Anomalous layers, created because of noise in the first pass, are eliminated in the second pass. Such layers are eliminated by considering the best‐fitting curve from the first pass to be a smoothed version of the observed curve and automatically reinterpreting it (second pass). The application of the method is illustrated by several examples.

The Mud Volcano area in Yellowstone National Park provides an example of a vapor‐dominated geothermal system. A test well drilled to a depth of about 347 ft penetrated the vapor‐dominated reservoir at a depth of less than 300 ft. Subsequently, 16 vertical electrical soundings (VES) of the Schlumberger type were made along a 3.7‐mile traverse to evaluate the electrical resistivity distribution within this geothermal field. Interpretation of the VES curves by computer modeling indicates that the vapor‐dominated layer has a resistivity of about 75–130 ohm‐m and that its lateral extent is about 1 mile. It is characteristically overlain by a low‐resistivity layer of about 2–6.5 ohm‐m, and it is laterally confined by a layer of about 30 ohm‐m. This 30‐ohm‐m layer, which probably represents hot water circulating in low‐porosity rocks, also underlies most of the survey at an average depth of about 1000 ft. Horizontal resistivity profiles, measured with two electrode spacings of an AMN array, qualitatively corroborate the sounding interpretation. The profiling data delineate the southeast boundary of the geothermal field as a distinct transition from low to high apparent resistivities. The northwest boundary is less distinctly defined because of the presence of thick lake deposits of low resistivities. A broad positive self‐potential anomaly is observed over the geothermal field, and it is interpretable in terms of the circulation of the thermal waters. Induced‐polarization anomalies were obtained at the northwest boundary and near the southeast boundary of the vapor‐dominated field. These anomalies probably are caused by relatively high concentrations of pyrite.

The so‐called “auxilliary point” method for the interpretation of geoelectrical resistivity soundings has received considerable attention from the European geophysicists. However, almost no mention of it has been made in the American literature. It is the aim of this paper to introduce the method and to establish its mathematical relationship to the so‐called “Dar Zarrouk” parameters. The auxiliary point method is an empirical graphical method by which a multilayer problem is progressively reduced to the simple two‐layer case. Hence, to interpret a multilayer sounding curve, use is made of the available two‐ and three‐layer master curves in conjunction with one, or more, of four charts that represent families of auxiliary curves. The four charts are known as the Hummel (H‐chart), the Anisotropy (A‐chart), the Displaced‐Hummel (DH‐chart), and the Displaced‐Anisotropy (DA‐chart). The mathematical basis for the drawing of these charts is discussed, and furthermore the identity of the Dar Zarrouk point to the anisotropy point is shown. The relation between the various parameters of the auxiliary points and the Dar Zarrouk point is graphically illustrated. Other auxiliary methods for the interpretation of sounding curves, e.g., the Cagniard and the Ono charts are essentially the same as the auxiliary point charts. A theoretical four‐layer case illustrates the practical use of the method, its advantages, and its limitations.

Forty‐five resistivity soundings, using Schlumberger and equatorial dipole electrode configurations, were made on the islands of Oahu and Hawaii to determine the applicability of direct current resistivity methods for locating freshwater aquifers in the State of Hawaii. The soundings were made on the northwestern part of the island of Oahu near the town of Waialua and on the island of Hawaii on the “saddle” area near Pohakuloa and Humuula. Interpretation of 32 sounding curves obtained on the island of Oahu indicates that it is possible to correlate five stratigraphic units underlain by a vesicular basalt basement and that the determination of the approximate depth to the fresh‐water‐saline‐water interface within the basalt is feasible. Two of these Schlumberger soundings with electrode spacings [Formula: see text] reaching 6000 ft yielded sounding curves of the maximum and minimum types whose terminal branches asymptotically approach a resistivity of about 30 ohm‐m, which is believed to be the true resistivity of basalt saturated with sea water. Near the town of Waialua the aquifer is a coral zone as well as parts of the weathered vesicular basalt basement. On the island of Hawaii, near Pohakuloa, an exploratory well drilled in basalt to a depth of 1001 ft (prior to the resistivity survey) proved to be dry. Interpretation of thirteen deep soundings made with Schlumberger and equatorial arrays suggests that the minimum depth to a conductive layer, which may represent basalt saturated with fresh water, is about 2700 ft below land surface. The groundwater appears to be dike impounded.

Electrical soundings using the symmetric AMNB Schlumberger and the bipole‐dipole equatorial arrays were made along two profiles near El Paso, Texas, in support of a groundwater exploration program which included seismic refraction and gravity surveys. Electrode spacings ([Formula: see text] or R) reaching 12,000 ft allowed exploration to depths of about 7000 ft. Geoelectrical information on the subsurface materials was augmented by sounding with the bilateral equatorial configuration and by transforming Schlumberger curves into dipole‐polar sounding curves with formulas developed by Al’pin and by Tsekov. The bilateral equatorial sounding curves were found useful for detecting the direction of dip of highly resistive bedrocks whereas transformed sounding curves were used to evaluate the average longitudinal resistivity, and hence the depth, to the “electric basement.” A few of the Schlumberger sounding curves were either clearly or subtly distorted by nonhorizontal geologic structures. The interpretation of these sounding curves illustrates the requirement for careful analysis in processing electrical prospecting data obtained over complex geologic conditions. The interpretation of a combined Schlumberger‐equatorial sounding curve, which did not agree with the preliminary interpretation of seismic refraction data, was confirmed to be correct by data from a test well drilled to a depth of 4363 ft. The application of electrical sounding data in the El Paso area furnished valuable information on the depth to fresh‐water‐salt‐water interfaces and on the depth to highly resistive impervious bedrocks.

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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...

Direct current resistivity and time domain electromagnetic techniques were used to study the electrical structure of the Long Valley geothermal area, A resistivity map was compiled from 375 total field resistivity measurements. Two significant zones of low resistivity were detected, one near Casa Diablo Hot Springs and one surrounding the Cashbaugh Ranch—Whitmore Hot Springs area. These anomalies and other parts of the caldera were investigated in detail with 49 Schlumberger dc soundings and 13 transient electromagnetic soundings. An extensive conductive zone of 1‐ to 10‐Ωm resistivity was found to be the cause of the total field resistivity lows. Drill hole information indicates that the shallow parts of the conductive zone in the eastern part of the caldera contain water of only 73°C and consist of highly zeolitized tuffs and ashes in the places that were tested. A deeper zone near Whitmore Hot Springs is somewhat more promising in potential for hot water, but owing to the extensive alteration prevalent in the caldera the presence of hot water cannot be definitely assumed. The resistivity results indicate that most of the past hydrothermal activity, and probably most of the present activity, is controlled by fracture systems related to regional Sierran faulting.