Norman Harthill

Megasource, time-domain electromagnetic sounding methods

The Colorado School of Mines time‐domain electromagnetic (EM) sounding system makes use of a grounded length of cable powered with high‐amplitude current square waves to generate an EM field for probing the earth. The vertical component of magnetic induction is detected at a sounding site located at a relatively large distance compared to the desired depth of investigation. With a source moment of a million ampere meters or greater, offset distances of several tens of kilometers can be achieved easily, providing depths of investigation of up to 10 km. The recorded induction field versus time curves are routinely interpreted by comparison with computer‐generated theoretical curves for a layered earth. Megasource EM surveys have been carried out at The Geysers in northern California and near Yakima in central Washington, providing apparently meaningful information on the electrical structure in these areas at depths as great as 10 km.

The dipole mapping method

In the bipole-dipole mapping method, a current field is set up by the use of a bipole current source. The current field is then studied by making measurements of electric field intensity with dipole receivers at many locations around the bipole source. The values for electric field intensity may be used to compute apparent resistivities if we assume that the earth is uniform or to compute apparent conductance if we assume that the earth resembles a conducting sheet. Maps of apparent resistivity values or apparent conductance values may be interpreted by comparing them with similar maps computed analytically for various simplified earth models. The bipole-dipole mapping method is useful mainly in locating areas where ground resistivity varies rapidly in the horizontal direction. It has found application mainly in exploration for geothermal reservoirs but also has been used for mining exploration and engineering studies, and an example of each is described.

A quadripole resistivity survey of the Imperial Valley, California

A quadripole resistivity survey of the Imperial Valley, California was carried out from the Salton Sea in the north to the Mexican border in the south. The east and west boundaries of the survey were the topographic limits of the valley. The quadripole resistivity method consists of sequentially energizing two orthogonal bipole sources with a square wave of electric current and measuring the resultant electric fields with a pair of orthogonal wire receivers. Two resultant electric fields are measured and by combining them in different proportions, their resultant can be made to rotate through 360 degrees. By performing this procedure, an ellipse of resistivity can be calculated at each measurement location. The arithmetic mean of the maximum and minimum axes of the ellipse is a tensor invariant resistivity. It was this value which was used to define the variation of electrical resistivity over the Imperial Valley. The quadripole survey of the Imperial Valley was undertaken to compare its known geothermal ...

Statistical analysis of airborne gamma‐ray data for geologic mapping purposes: Crixas‐Itapaci area, Goias, Brazil

Q-mode factor analysis, K-means clustering, and G-mode clustering were used on digitized gamma-ray spectrometer data from an aerial survey of the Crixas-Itapaci area, Goias, Brazil. The data points including seven variables--eU, eTh, K, total count, U/Th, U/K, and Th/K--were digitized for a 2 km square grid. For the northwest corner of the area the data were gridded at 1 km.The Q-mode classification method supplied results that do not show a good correspondence with the known geology. The K-means clustering procedure barely identified the main lithologic features of the area. The G-mode technique produced results that correlate well with the known geology and identified the greenstone belts present in the area by discriminating their ultramafic and mafic components from adjacent felsic rocks.Statistical analysis of aerial gamma-ray spectrometer data can be very helpful in mapping geologic units in poorly known areas. It can also be used for mineral exploration purposes if mineralization is known to be associated with lithologies that can be identified by the techniques used in this study.

Interactive Modeling of Potential Fields in Three Dimensions

The goal of the research is to develop a full space, real time and interactive modeling technique for potential fields in three dimensions. The modeling of potential fields in three dimensions includes three aspects: gravity field computation, physical properties, and geometry model construction. This paper focuses on analysis of the field computation algorithm and geometric model construction.

THE CSM TEST AREA FOR ELECTRICAL SURVEYING METHODS

Testing of geophysical equipment and methods over a region of known geology has long been considered desirable if not actually necessary. It is the purpose of this note to bring to the attention of the geophysical community the development of a test area for the evaluation of electrical prospecting methods. The test area is a strip, 19 km broad and 280 km long, made up of the tiers of townships 1N and 2N stretching between R42W and R70W in eastern Colorado (Figure 1). Access to the strip is easy as the area is relatively level and county roads border most sections; population is sparse.

Reply by the author to Discussion by Tsvi Meidav

In his discussion of the subject paper, Meidav expresses a number of opinions about the use of bipole‐dipole surveys which differ from mine. I should like to expand upon the technical merits of some of the comments that he makes.

A quadripole resistivity survey of the Imperial Valley; discussion and reply

An unfortunately large number of misstatements and misconceptions regarding dipole resistivity and the Imperial Valley characterize this paper, resulting in a simplistic and, in my opinion, erroneous interpretation of the resistivity data gathered by the author, which may have already led to less‐than‐satisfactory results when the ultimate test—deep exploration drilling—was completed. The paper represents a specific school of thought regarding interpretation of dipole data which is not commonly accepted by other dc resistivity practitioners, for technical reasons enumerated below.

Time-domain electromagnetic survey for gold exploration, Nevada

A time-domain electromagnetic (TDEM) sounding survey was carried out on the western basin-range boundary of Smith Creek Valley, Nevada. The survey achieved its goal of mapping depth to volcanic basement along five traverses that crossed the boundary. In addition to mapping the electrical basement, the TDEM survey outlined significant cross faulting within the alluvium. Seventy five soundings were made over a five day period using a Geonics EM37 system that shows the usefulness and effectiveness of the TDEM technique to determine depth to electrical basement under heterogeneous range-front alluvium.