George V. Keller

Results of an experimental drill hole at the summit of kilauea volcano, Hawaii

Abstract A borehole has been drilled to a depth of 1262 m (4141 ft) beneath the summit of Kilauea volcano, on the island of Hawaii. The purpose was to test predictions made from surface-based geophysical surveys and seek evidence of a hydrothermal system over a known magma body. Nearly all rocks penetrated by the borehole are olivine basalt, with minor amounts of olivine diabase, picrite diabase and olivine-poor basalt. While the rocks are petrographically uniform, their physical properties vary widely from flow to flow. Core samples have progressively more thermal and chemical alteration with increasing depth. The temperature distribution in the wellbore is strongly affected by fluid convection. The temperature distribution can be simulated by two-dimensional convection having half-cell dimensions of 700 by 700 m. However, the match is improved by assuming that heat transport takes place by conduction in the lower half, while convective heat transfer dominates the upper half.

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.

Natural-field and controlled-source methods in electromagnetic exploration

Abstract Electromagnetic methods have been used effectively in minerals exploration for over half a century. However, over the last several years, advances have been made in theory and instrumentation which will lead to the wide use of electromagnetic methods for quantitative determinations of earth structure in other kinds of exploration problems: in engineering studies, in the search for petroleum, in the evaluation of hyperthermal cells for power development, and in the study of the earths deep interior. One advantage of electromagnetic methods is the flexibility in technique that is available; electromagnetic fields may be generated with current-carrying loop sources, with long or short grounded wires, or even by the natural electromagnetic field of the earth. Each type of source may have special advantages with respect to operating ease for different types of exploration problems. Another important feature of electromagnetic methods is that resistivity may be determined as a function of depth merely by altering the frequency content of the electromagnetic field, rather than by moving the source and receiver apart, as is done in direct current methods. This reduces the amount of physical labor involved in electromagnetic methods tremendously in comparison with direct-current methods. Electromagnetic methods also provide a greater depth of investigation than direct current methods for a given source-receiver separation, and surveys may be designed using available equipment to prospect to depths of tens of kilometers.

Computer-assisted interpretation of electromagnetic soundings over a permafrost section

In two‐loop electromagnetic sounding, the electromagnetic coupling between two vertical‐axis loops of wire is measured as a function of frequency, for frequencies ranging from 20 Hz to 20 kHz. If the electrical structure of the earth beneath the loops is horizontally stratified, these data may then be interpreted in terms of a sequence of layer resistivities and thicknesses. This interpretation is accomplished by computing a series of curves for various resistivity profiles and comparing them with the field data to determine which matches best. Calculation of the theoretical models is carried out by applying a linear filter to solve the appropriate integral expression. Interpretation is aided by using an interactive nonlinear least‐squares algorithm iteratively to adjust the model parameters. This procedure was used to interpret two‐loop induction soundings made along the Arctic Slope of Alaska during 1969 to determine permafrost thickness and character. The results indicate that two‐loop induction soundi...

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 Comparison of Two Electrical Probing Techniques

Electromagnetic soundings were made using two techniques in an area around the summit of Kilauea Volcano, Hawaii. The target for the soundings was a conductive zone at a depth of about one kilometer, associated with geothermal heating. In one sounding method, an electromagnetic field was generated by passing current steps through a grounded-wire transmitter, while in the other, an electromagnetic field was generated by passing current steps through an ungrounded loop. The field was detected at a receiving site by measuring the vertical component of magnetic induction using an ungrounded loop. Both methods accurately reflected the presence of conductive rocks at depth. However, the use of a grounded-wire source had a strong advantage in that the field decayed with distance more slowly than that from the loop source, and a larger area could be surveyed using a single grounded-wire source.

Seismic velocities and electrical resistivity of Recent volcanics and their dependence on porosity, temperature, and water saturation

Variation of P‐wave velocities and electrical resistivities of several suites of water‐saturated recent volcanics was investigated. Both P‐velocities and resistivities exhibited strong dependence on porosity. Resistivity was also dependent upon degree of water saturation and temperature. P‐wave velocities, while showing a strong dependence on porosity, appear to be independent of water saturation and temperature. Volcanics, in general, exhibit higher resistivities compared to other igneous rocks and sediments. Electric resistivity of fine‐grained basalts is anomalously low, probably due to higher content of disseminated iron. Pyroclastics and volcanic breccia, on the other hand, exhibit higher resistivities in relation to fine‐grained basalts.

Principles of time-domain electromagnetic (TDEM) sounding

The term time‐domain electromagnetic sounding can stand for any of a variety of electrical exploration methods in which the transient response of the Earth to a pulse‐like magnetic field is measured. Such methods have been described in the geophysical literature since the 1930s and are currently being used quite successfully in mineral exploration and civil engineering studies. It has become standard practice to call them “TEM” (for transient electrical methods). The TDEM technique (time‐domain electromagnetic method) described here is included within the broader TEM category.

Statistical Study of Electric Fields from Earth-Return Tests in the Western States Compared with Natural Electric Fields

Preliminary grounding tests have been carried out by the Bonneville Power Administration and by the Bureau of Reclamation in three regions of the western states, in preparation for the installation of a HVDC transmission system linking the Pacific Northwest and Southwest regions. Electric fields observed about ground mat locations in the Washington-Oregon area, the southern Nevada area, and the central California area were analyzed statistically as a function of distance from the ground mat. These fields are compared with the probable levels of natural fields induced by slow variations in the earths magnetic field-in order to estimate the possible hazards involved in using ground return for current in a HVDC system.

Electrical and electromagnetic methods in areas of complex geology

Abstract In the application of electrical methods in areas of complex geology, two levels of complexity can be recognized. At lower levels of complexity, a fundamental dependence of resistivity on depth can be recognized. In such cases, the profile of resistivity with depth can be studied using various forms of sounding, in which array size, signal frequency, or a combination of both can be used. The complex subsurface structure can then be estimated by comparing a series of soundings. At higher levels of complexity, soundings are severely distorted by the complexity, and this approach is no longer useful. In such a case, it appears that the most effective approach is by observing electric or electromagnetic field behavior at many observation points over the surface of the earth. Examination of the spatial spectra of the field behavior then adds to the interpretability of the data, making interpretation possible.