Magnetotelluric interpretations based upon new processing and display techniques

Warner, Barry N.; Bloomquist, Marvin G.; Griffith, Paul G.

doi:10.1190/1.1893794

Cited by 11 publications

(3 citation statements)

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“…Zonge and Hughes (1991) in the CSAMT chapter in this volume describe several empirical techniques for spatially smoothing those data. Warner et al (1983), using data from many closely spaced sites, calculated an average of log @a at each site and then adjusted each curve so that these averages varied smoothly between sites. The adjustment was justified on the basis that sites were only a few hundred meters apart in a relatively undeformed sedimentary basin.…”

Section: Gives Model Resistivities Too Large By Pij/pl and Depths Toomentioning

confidence: 99%

8. The Magnetotelluric Method

Vozoff

1991

Electromagnetic Methods in Applied Geophysics

381

249

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In the magnetotelluric (MT) method, natural electromagnetic fields are used to investigate the electrical conductivity structure of the earth. Natural sources of MT fields above about 1 Hz are thunderstorms worldwide, from which lightning radiates fields which propagate to great distances. At frequencies below 1 Hz, the bulk of the signal is due to current systems in the magnetosphere set up by solar activity. In both cases the electromagnetic (EM) fields at the surface of the earth behave almost like plane waves, with most of their energy reflected but with a small amount propagating vertically downward into the earth. The amplitude, phase, and directional relationships between electric (E) and magnetic (H or B) fields on the surface depend on the distribution of electrical conductivity in the subsurface. By use of computed models, field measurement programs can be designed to study regions of interest within the earth from depths of a few tens of meters to the upper mantle.Equipment to carry out the measurements consists of magnetometers for the frequency range of interest; pairs of electrodes separated by suitable spacings to sense the electric field variations; plus amplifiers, filters, and suitable digital recording and processing systems to permit the signals to be captured and analyzed. The magnetometers in particular must have very low noise and great stability because those signals are so weak.Once the signals have been recorded they must be processed and analyzed. Processing is normally done in the frequency domain because the theory is simpler than in time domain. Hence processing begins with Fourier transformation, from which earth impedances to the incident waves as functions of frequency, direction, and position are computed. Processing in many systems is now done in real time, while the signals are being acquired.These computed impedances are next interpreted in terms of electrical conductivity versus position and depth. Numerical models for one-dimensional, twodimensional, and three-dimensional structures are used for this last step. Interpretation is the most difficult part of the method because information is seldom complete and the models are never complex enough to represent a real earth. For that reason, and to use the MT data to best advantage, other available external data such as well logs, seismic, and any other electrical data are commonly utilized to help with interpretation.The main shortcoming of the method is the difficulty of obtaining data in electrically noisy areas or where the surface is unstable.The strengths of the method are its unique capability for exploration from very shallow depths to very great depths without artificial power sources and with little or no environmental impact. At high frequencies, audio frequency MT (AMT) has been used to map groundwater and major base metal deposits at depths from 50-100 rn to several kilometers. However, the major application of MT is to petroleum exploration in areas where reflection seismology is very expensive or ineffective, such ...

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Section: Gives Model Resistivities Too Large By Pij/pl and Depths Toomentioning

confidence: 99%

8. The Magnetotelluric Method

Vozoff

1991

Electromagnetic Methods in Applied Geophysics

381

249

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“…Processing corrections (Warner et al, 1983;Sternberg et al, 1984;Zonge, 1985) show considerably more promise. These approaches involve various spatial filtering techniques.…”

Section: Figure 31 Is a Simplified Illustration Of Static Effects Ovementioning

confidence: 99%

9. Controlled Source Audio-Frequency Magnetotellurics

Zonge¹,

Hughes²

1991

Electromagnetic Methods in Applied Geophysics

208

136

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INTRODUCTION Controlled source audio-frequency magnetotellurics (CSAMT) is a frequency-domain electromagnetic sounding technique which uses a fixed grounded dipole or horizontal loop as an artificial signal source.CSAMT is similar to the natural-source magnetotellurics (MT) and audio-frequency magnetotellurics (AMT) techniques; the chief differences center around the use of the artificial CSAMT signal source at a finite distance. The source provides a stable, dependable signal, resulting in higher-precision and more economical measurements than are usually obtainable with natural-source measurements in the same spectral bands. However, the controlled source can also complicate interpretation by adding source effects, and by placing certain logistical restrictions on the survey. In most practical field situations these drawbacks are not serious, and the method has proven particularly effective in mapping'the top 2 to 3 km of the earth's crust.The CSAMT source usually consists of a grounded electric dipole about ! to 2 km in length, ideally located at least four skin depths (4g = 2012 X/O/o/f; p = resistivity, f = signal frequency) from the area where soundings are to be made. Measurements are made within the 0.1 Hz to 10 kHz frequency band. Magnitude and phase are measured for two to five electric and magnetic field components (Ex, Ey, Hx, Hy, Hz), using either one or two sources. Grounded dipoles detect the electric field and magnetic antennas sense the magnetic field. The ratio of orthogonal, horizontal electric and magnetic field magnitudes yields the apparent resistivity. The difference between the phase of the electric and magnetic fields yields the phase of the impedance. In tensor measurements, these quantities may be treated by standard MT processing techniques. CSAMT depth of investigation is roughly equal to g/X/• m, when the separation between the transmitter dipole and the receiver station is greater than 4g, although we have found that maximum depth of investigation is limited in many cases to about 3 km due to the practical constraints of the measurements. Lateral resolution is controlled by the electric field dipole length, which normally is between 10 and 200 m. Vertical resolution is .5 percent to 20 percent of the depth of penetration, depending upon resistivity contrasts.Since its introduction in the mid-1970s, CSAMT has been used in exploration for petroleum, geothermal resources, massive sulfides, base and precious metals, structure, lithology, and sources of groundwater contamination. Much of this work has been quite successful due to the inherent capabilities of the technique.However, much remains to be learned about CSAMT interpretation, particularly in regard to static and source effects. By exploring the present understanding of the theory and practice of CSAMT we hope to provide a basis for further development. ObjectivesIn recent years the controlled source audio-frequency magnetotellurics (CSAMT) sounding technique has gained acceptance as a viable geophysical exploration tool. Its hig...

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“…Possibly the most significant interpretational problem in MT and CSAMT is the so-called static effect (Larsen 1981;Warner et al 1983;Sternberg et al 1984). As described by Zonge & Hughes (1988), this effect can occur due to the presence of a near-surface, finite, electrically inhomogeneous body.…”

Section: Static Effectmentioning

confidence: 99%

Structure mapping at Trap Spring Oilfield, Nevada, using controlled-source magnetotellurics

Hughes¹,

Carlson²

1987

First Break

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The seismic reflection method has been a highly successful tool in oil and gas exploration for half a century, and it presently accounts for about 98% of all geophysical expenditures worldwide. However, the relatively high cost of seismic exploration and its limitations in certain geologic environments are continuing problems. Some help has been provided by the magnetotelluric (MT) sounding technique, but the cost of MT is also quite high due to the low natural signal strengths being measured. The controlled-source audio-frequency magnetotellurics (CSAMT) technique is a shallower-penetrating variation of MT which uses an artificial signal source. This permits faster and more economical data acquisition. CSAMT has a penetration of about 2 km in typical petroliferous environments. CSAMT does not replace seismic but functions in three specific roles: (1) as a reconnaissance tool to help focus seismic coverage, or to help avoid 'no-record' zones; (2) to assist in static corrections and in interactive seismic interpretation; (3) as a primary tool in certain environments (volcanics, complex thrust areas) where seismic data acquisition is limited. An example of the application of CSAMT to structure mapping comes from data taken over Trap Spring Field, located in the frontier Great Basin of the western United States. The field produces oil from fractured volcanics at the edge of a major graben fault. The CSAMT data delineate the major subsurface faulting and stratigraphic relationships in the area. The resolution of the CSAMT survey is significantly better than previously obtained induced polarization (IP) data. Detailed comparisons with electric log, drill hole, and air-photo data show an excellent correlation between the CSAMT features and known geology. The work suggests that CSAMT could be used in this area for reconnaissance mapping to develop seismic prospects, at approximately one sixth the cost of seismic.

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Magnetotelluric interpretations based upon new processing and display techniques

Cited by 11 publications

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8. The Magnetotelluric Method

8. The Magnetotelluric Method

9. Controlled Source Audio-Frequency Magnetotellurics

Structure mapping at Trap Spring Oilfield, Nevada, using controlled-source magnetotellurics

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