Instrumentation

Although familiar with numerous techniques in electromagnetic exploration and monitoring, we have concentrated on the magnetotelluric (MT) method due to its superior depth of exploration and the relatively advanced state of understanding in its numerical simulation. The physics of this method also is forming the basis for new-generation array data collection modes arising in the contracting industry.

The MT method makes use of naturally occuring EM wave fields as sources to probe the electrical resistivity structure of the subsurface. At high frequencies (short periods, T < ~1 s), the fields are due to regional and global thunderstorm activity. At lower frequencies (long periods) the fields result from regional electric current systems in the ionosphere and magnetosphere originating from fluctuations in solar wind intensity.

Due to the remote nature of the sources and the high index of refraction of the Earth relative to the air, these EM fields usually are assumed to be planar in geometry and to propagate vertically downward. However, the scattering of these waves by subsurface structure may be arbitrary in polarization, requiring a tensor measurement of the fields for full representation. The propagation of EM waves in the Earth is diffusive, so high frequency signal penetrate a relatively shallow distance while low frequencies penetrate farther. The collected EM time series are decomposed to spectra, ratioed, and finally reduced to the fundamental response of interest - the tensor MT impedance Z. Simple expressions of apparent resistivity (ra), smoothed version of Earth resistivity below site) and impedance phase (j) are used typically for presentation. This scattering response is measured at numerous site over the surface, often in dense profiles, and over a broad frequency range (e.g., 1 kHz to 0.001 Hz, and even lower). Numerical simulation algorithms have emerged to reproduce the MT response of specified structure (forward problem), or to generate conservative images of the subsurface directly from the data (inverse problem). Finally, resistivity models are interpreted in terms of structures representing fluid content, temperature, melting, and solid phases suchas graphite or sulfides.

Our new MT system design combines simultaneous multi-band collection, to help ensure long data records in the problematic MT mid-band, with high-speed, longer distance telemetry to provide real-time remote referencing with signals completely outside the influence of geothermal production noise. Improved capabilities for bandwidth limiting through analog filtering, and use of 24bit A/D converters to increase dynamic range are important new capabilities which will aid the acquisition of MT data in the presence of substantial locally generated noise. Finally, our real-time processing algorithms provide the highly desirable capability to assess data quality as it is acquired to optimize acquisition while on site.

These are based on discrimination by multiple coherence at each frequency, and can be defined flexibly among different field subsets and independently for each MT polarization mode. New processing approaches which consider two-source (conceptually multi-source) transfer functions are to be generalized for application to geothermal systems. An electronics shop is maintained at the University of Utah/EGI which is used to fabricate, integrate, and test MT and other geophysical equipment. The shop contains electronic test equipment and machine shop equipment sufficient to carry out all aspects of construction and testing, except for printed circuit board layout and fabrication prior to component loading which is farmed out to appropriate vendors. This shop has been operated and maintained continuously for more than fifteen years during development and operation of our in-house MT instrumentation.

An example response acquired using the UU/EGI MT system appears here both to exemplify possible data quality with our platform, and to illustrate basic physical principles of the method. This site in central Nevada was not taken in an active geothermal system but instead reflects mainly regional stratigraphic and Basin-Range structural elements. Two panels of data points are plotted: the upper being apparent resistivity (ra) derived from two of the tensor impedance elements, and the lower being phase of the elements (j). The latter tend to be proportional to the slope of the former versus period (T). The two elements approximately represent current flow along geological strike x (NNE) (xy quantities) and across strike (yx quantities).

Data values at individual periods are plotted as 1 s.d. intervals which in general are quite small. Estimate quality was improved significantly by sorting EM field spectra at each period according to multiple coherence, and by using a simultaneously recording site several kilometers away as a remote reference for noise cancellation. The sounding plots exemplify the smooth nature of the responses w.r.t. T, in keeping with the diffusive nature of EM propagation in the Earth. This occurs despite the probability that some structures in the Earth have sharp boundaries, and is at the heart of the ill-posed nature of the MT inverse problem. Note the divergence of the ra curves as period increases, which is due to the increasing influence of conductive valley sediments lateral to the site. The mid-period minimum reflects influence of conductive, mid-Paleozoic shales while the decrease in ra at the long periods is caused by thermally-induced conductivity increases in the lower crust.