S. H. Ward

MINERAL DISCRIMINATION AND REMOVAL OF INDUCTIVE COUPLING WITH MULTIFREQUENCY IP

In‐situ complex resistivity measurements over the frequency range 10-2 to 10+5Hz have been made on 26 North American massive sulfide, graphite, magnetite, pyrrhotite, and porphyry copper deposits. The results reveal significant differences between the spectral responses of massive sulfides and graphite and present encouragement for their differentiation in the field. There are also differences between the spectra of magnetite and nickeliferrous pyrrhotite mineralization, which may prove useful in attempting to distinguish between these two common IP sources in nickel sulfide exploration. Lastly, there are differences in the spectra typically arising from the economic mineralization and the barren pyrite halo in porphyry copper systems. It appears that all these differences arise mainly from mineral texture, since laboratory studies of different specific mineral‐electrolyte interfaces show relatively small variations. All of the in‐situ spectra may be described by one or two simple Cole‐Cole relaxation mod...

Magnetotelluric responses of three-dimensional bodies in layered earths

The electromagnetic fields scattered by a three‐dimensional (3-D) inhomogeneity in the earth are affected strongly by boundary charges. Boundary charges cause normalized electric field magnitudes, and thus tensor magnetotelluric (MT) apparent resistivities, to remain anomalous as frequency approaches zero. However, these E‐field distortions below certain frequencies are essentially in‐phase with the incident electric field. Moreover, normalized secondary magnetic field amplitudes over a body ultimately decline in proportion to the plane‐wave impedance of the layered host. It follows that tipper element magnitudes and all MT function phases become minimally affected at low frequencies by an inhomogeneity. Resistivity structure in nature is a collection of inhomogeneities of various scales, and the small structures in this collection can have MT responses as strong locally as those of the large structures. Hence, any telluric distortion in overlying small‐scale extraneous structure can be superimposed to ar...

State-of-the-art geophysical exploration for geothermal resources

At the present stage of development, use of geothermal energy saves about 77 million barrels of oil per year worldwide that would otherwise be required for electrical power generation and direct heat applications. More than a dozen countries are involved in development of geothermal resources. Currently, only the moderate- and high-temperature hydrothermal convective type of geothermal system can be economically used for generating electric power. Lower-temperature resources of several types are being tapped for space heating and industrial processing. Geophysics plays important roles both in exploration for geothermal systems and in delineating, evaluating, and monitoring production from them. The thermal methods, which detect anomalous temperatures directly, and the electrical methods are probably the most useful and widely used in terms of siting drilling targets, but gravity, magnetics, seismic methods, and geophysical well logging all have important application.Advances in geophysical methods are needed to improve cost effectiveness and to enhance solutions of geologic problems. There is no wholly satisfactory electrical system from the standpoint of resolution of subsurface resistivity configuration at the required scale, depth of penetration, portability of equipment, and survey cost. The resolution of microseismic and microearthquake techniques needs improvement, and the reflection seismic technique needs substantial improvement to be cost effective in many hard-rock environments. Well-logging tools need to be developed and calibrated for use in corrosive wells at temperatures exceeding 200 degrees C. Well-log interpretation techniques need to be developed for the hard-rock environment. Borehole geophysical techniques and geotomography are just beginning to be applied and show promise with future development.

Distribution, Morphology, and Origin of Ridges and Arches in Mare Serenitatis

Lunar mare ridges and arches in Mare Serenitatis were mapped to understand better their mode of formation. Mapping of these features indicates that several pre-mare impacts in the Serenitatis area may be responsible for the localization of the circular ridge systems and that the subsurface, pre-mare topography is more complex than previously recognized. Apollo Lunar Sounder cross sections of ridge systems in southern Serenitatis indicate 50 to 100 m of local relief on these features. Ridges in the southwestern part of the basin mark the boundary of a bench 200 m above the local mare level. As reflected in their orientation, arches and ridges are possibly controlled both by rings of pre-mare basins resulting from impacts and by a more widespread global stress system. Small-scale features of ridges, such as medial lineations and lobate margins, do not conclusively define the origin of the ridges. However, estimates of crustal shortening from Lunar Sounder data and the coincidence of the major ridge system with the Serenitatis mascon suggest that ridges and arches were formed by gravitational readjustments of the mare fill along four probable impact structures and along a north-trending fracture pattern.

FOREWORD AND INTRODUCTION

The motivation for preparing a special issue of Geophysics devoted to electromagnetic scattering lies largely with a sudden and dramatic improvement in our ability to model the earth in theoretical analyses of the electromagnetic exploration problem. This improvement was in turn generated by a need for better interpretation of electromagnetic data, a need for better design of electromagnetic systems, and a need to develop better criteria for selection of electromagnetic systems suited to a given prospecting problem.

Three‐dimensional resistivity inversion using alpha centers

The method of alpha centers represents a class of solutions to the three‐dimensional (3-D) dc conduction equation based on certain nonlinear substitutions for electric potential and earth conductivity. A solution is obtained which is sufficiently fast to make the inversion of 3-D resistivity data practical. The inversion routine results in a conductivity distribution defined by α centers which simultaneously fits the data from several parallel or perpendicular dipole‐dipole profiles. To illustrate the characteristics of this modeling approach, we apply the inversion algorithm to three theoretical and four field data sets. The four field data sets represent samples from massive sulfide and geothermal environments. The technique, when applied to theoretical data from prismatic bodies, gives fairly good estimates of the positions of conductive inhomogeneities but poor estimates of their actual conductivities. The entire inversion algorithm requires less than 15,000 words of computer memory, thus making it tr...

Ridge regression inversion applied to crustal resistivity sounding data from South Africa

Ridge regression inversion has been used to test the applicability of various one‐dimensional crustal models to the interpretation of deep Schlumberger sounding data from southern Africa (Van Zijl and Joubert, 1975). Four main models were investigated: a simple three‐layered earth, a layered earth with a transition zone exhibiting a linear decrease in log resistivity with depth, a similar earth with the transition zone determined by cubic splines, and a model having exponential resistivity behavior at depth. The last model corresponds to temperature‐dependent semiconduction through solid mineral grains (Brace, 1971). It was found that all of these models are capable of fitting the sounding data from southwestern Africa, while all except the semiconduction model fit the data from southeastern Africa. One is, thereby, immediately alerted to the problem of lack of resolution in Schlumberger sounding data where geologic control is not available. A major with the inversion of Schlumberger data alone is that ac...

A summary of the geology, geochemistry, and geophysics of the Roosevelt Hot Springs thermal area, Utah

The Roosevelt Hot Springs thermal area is a newly discovered geothermal power prospect in Utah. Seven production wells have been drilled with a maximum per well flow capability averaging 4.5×105kg of combined vapor and liquid per hour at a shut‐in bottom hole temperature near 260°C. The thermal area is located on the western margin of the Mineral Mountains, which consist dominantly of a Tertiary granitic pluton 32 km long by 8 km wide. Rhyolitic tuffs, flows, and domes cover about 25km2 of the crest and west side of the Mineral Mountains within 5 km of the thermal area. The rhyolitic volcanism occurred between 0.8 and 0.5 m.y. ago and constitutes a major Pleistocene thermal event believed to be significant to the evaluation of the Roosevelt Hot Springs thermal area. Thermal waters of the (now) dry spring, a seep, and the deep reservoir are dilute (ionic strength 0.1 to 0.2) sodium chloride brines.

INTERPRETATION OF APPARENT RESISTIVITY DATA FROM APODI VALLEY, RIO GRANDE DO NORTE, BRAZIL

One hundred and twelve Schlumberger vertical electrical soundings were made as part of a hydrogeological study in the Apodi Valley, Brazil. Most of the data have been interpreted using an automatic ridge regression inversion algorithm in conjunction with a fast digital filter forward algorithm. As a result, the inversion costs are very low. The increase in speed and accuracy in the evaluation of the forward problem has also allowed calculation of the Schlumberger apparent resistivity from potential differences, instead of the electric field. Consequently, there is no difficulty in the interpretation of data where very large receiver electrode (MN) spacings have been used or where discontinuities have been introduced by changing the MN spacing on a layered earth containing large resistivity contrasts.The soundings were conducted primarily to map the thicknesses of a known alluvial aquifer and a potential sandstone aquifer. These thicknesses have been determined to within an error of 20 percent as estimated from analysis of the parameter standard deviations and comparison with available drill hole information.

Statistical Evaluation of Electrical Sounding Methods. Part I: Experiment Design

Traditional experiment design techniques, widely applied to both linear and nonlinear problems in many scientific fields, is applicable to the design of exploration geophysical surveys. The design technique is formulated using the mathematics of the generalized inverse and its construction via eigenvalue decomposition. The design technique is demonstrated by the designing of electromagnetic sounding surveys for a horizontal loop source.The experiment is designed whereby it is determined (1) which one of the electromagnetic field quantities, vertical and horizontal magnetic field amplitudes or phases, and polarization ellipse quantities, tilt angle, and ellipticity, and (2) which set of transmitter and receiver separations and transmitter frequencies, best resolve the conductivities and thicknesses of a given layered earth model. Model resolution is sensitive to the data error. As an example, for different assumed data errors, a model is best resolved in one instance by the phase of the two components of t...