4. DATA COLLECTION AND EXPLORATION METHODS
4.3. Phase 2: Exploration Methods
4.3.4. Geophysics
4.3.4.3. Magnetotelluric Surveys
The magnetotelluric (MT) method responds to the earth’s electrical resistivity structure (Simpson and Bahr, 2005). The method involves taking a time series recording of
natural, low frequency, orthogonal electric and magnetic fi elds at the earth’s surface, then interpreting the data in the frequency domain. Natural fl uctuations in the earth’s magnetic fi eld are generated by lightning, ionospheric resonances or variations in the solar wind.
These fl uctuations induce electric currents (or telluric currents) beneath the surface of the earth. The ratio of the electric fi eld to the magnetic fi eld in the induced electromagnetic (EM) wave is a function of the frequency of the signal and the bulk electrical resistivity of the ground. Lower frequency magnetic fl uctuations induce currents through a greater thickness of ground (Figure 4.25). Recording data over a wide frequency spectrum effectively gives information about a great thickness of ground. Lower frequency records (i.e., information about greater depths) require longer collection times.
The MT method is one of the very few geophysical techniques that can provide information about rock units deeper than about 1,000 meters. This makes it useful for geothermal exploration, where target depths are typically in the range of 1,000-3,000 meters for convection-dominated geothermal plays and even deeper for conduction-dominated plays. The MT method is particularly useful for convection-conduction-dominated plays because it can potentially image low resistivity and low permeability smectite clay units that often cap high enthalpy geothermal reservoirs (Melosh et al., 2010). For this reason, the MT method is often used to reduce uncertainties about reservoir depth, geometry, and areal extent.
During an MT survey the horizontal electric and magnetic fi elds at the earth’s surface are measured using electrodes and magnetometers buried in the ground (Figure 4.26, left-hand side). The non-polarizing electrodes often contain solutions of copper sulfate or cadmium chlorate. Metal electrodes can be used, but electric fi eld data quality can be low since they generate electrical noise as they corrode. The magnetometers are induction coils for frequencies above 0.01 Hz and fl uxgate magnetometers for lower frequencies.
Figure 4.26 shows examples of coil preparation, storage, and data acquisition in the fi eld.
Figure 4.25.
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Recorded MT data are processed and represented as a complex impedance tensor relating the electric and magnetic fi eld values and expressed as the apparent resistivity and impedance phase (Vozoff, 1986). Interpretation involves estimating the shallow resistivity structure using the higher frequency information, then deriving progressively deeper resistivity structure from the longer wavelength bulk resistivity information and the shallow resistivity estimates. By its nature, MT interpretations become less precise at greater depths.
MT surveys can be performed at a regional scale. In these cases, the station spacing may be less than one per square kilometer. It is usually more cost effective to identify a prospective area with other methods and then conduct an MT survey with relatively high station spatial density in that area, with perhaps as many as 10 to 15 stations per square kilometer.
Carrying out a time domain electromagnetic (TDEM) survey (see below) covering the same MT station locations (Wameyo, 2005) is a good practice in order to apply a “static shift correction” to the MT data for more robust interpretations (Irfan et al., 2010). The TDEM survey effectively provides high frequency electromagnetic information that the MT survey is unable to record. This allows greater resolution of the resistivity structure at shallow depths (typically a few hundred meters), hence improving the structure’s interpretation at greater depth from the MT data.
Unaltered volcanic rock generally has high electrical resistivity. Hydrothermal fl uids tend to reduce the resistivity of volcanic rocks in three ways:
• By altering the rocks to clay
• By increasing the salinity of the fl uids in the rocks
• By increasing the temperature of the rocks
Hydrothermal alteration has a dominant effect on resistivity in high enthalpy reservoirs.
In volcanic areas, acid-sulfate water can interact with the surrounding volcanic rocks to produce different alteration products depending on the temperature and hence on the distance from the heat source. With basalt as the surrounding rock, low-resistivity smectite becomes the dominant alteration product in the temperature range from 100°C to 180°C. At higher temperatures mixed layer clays are produced (Figure 4.27).
Figure 4.26.
Left: preparing coils (blue tubes) for an MT station.
Middle: MT coils.
Right: acquisition unit at an MT station.
Source: HarbourDom GmbH, Germany.
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Magnetotelluric data are normally interpreted through an “inversion” process, whereby a semi-automated algorithm determines the simplest and most likely “apparent resistivity”
structure consistent with the collected data. Inversions can also be carried out in 1D, 2D or 3D, referring both to the spatial distribution of recording stations and the dimensions of the model simultaneously solved. A 1D inversion produces a vertical “sounding” from a single station; a 2D inversion, a profi le from a line of stations; and a 3D inversion, a self-consistent block model from an array of stations (Siripunvaraporn et al., 2005). Higher dimension inversions require signifi cantly greater computing power and time to complete.
Inversions might be carried out by the MT contractor or by an independent third party.
Inversion algorithms typically need to be constrained in some way, usually through limiting the allowable number of discrete layers and/or the depths between layers. For this reason, inversion results are subjective because they depend on input from the data processer. The results from 1D, 2D and 3D inversions can differ signifi cantly from each other for the same set of data, because the models depend on the dimensionality and complexity associated with the magnetotelluric responses. The resolution and accuracy of inversion models in terms of both depth and apparent resistivity decrease with depth.
The results of magnetotelluric inversion are normally presented as apparent resistivity on 1D soundings, 2D profi les (Figure 4.28) or maps (Figure 4.29), or 3D block fi gures (Figure 4.30).
Figure 4.27.
Diagram of a generalized
geothermal system.
Note: Geothermal system (modifi ed after Johnston et al., 1992). The smectite cap or “claycap” typically displays resistivity of around 2 Ohm*m, and the mixed layer around 10 Ohm*m.
Source: HarbourDom GmbH, Germany.
Figure 4.28.
Cross section showing apparent resistivity from MT data.
Source: HarbourDom GmbH, Germany.
Depth (m)
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Figure 4.29.
MT resistivity map with station locations shown.
Source: GNS Science, New Zealand.
Figure 4.30.
3D MT resistivity block model.
Source: GNS Science, New Zealand.
Depth (m)
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A phenomenon called magnetotelluric polarization is of particular and growing interest for geothermal exploration as it has the potential (when interpreted jointly with other data sets) to reveal the dominant orientation of fractures (e.g., Onacha et al., 2010). Joints or fractures lying in a preferred orientation and fi lled with conductive brine tend to conduct electricity more effi ciently (i.e., lower resistivity) parallel to their strike, compared to perpendicular. If the electrical and magnetic fi elds are each recorded along perpendicular axes, then deriving a resistivity tensor is possible in describing how the electrical
resistivity varies with direction. A preferred direction of higher apparent resistivity within a particular depth interval might be interpreted as being perpendicular to a dominant fracture orientation. The evidence becomes more compelling when combined with other directionally sensitive techniques such as “seismic shear-wave splitting” (see below).
Table 4.6lists parameters that should be recorded during an MT survey
DATA ACQUISITION DATA PROCESSING INTERPRETATION
• Field report
• Noise sources
• Instruments used
• Map(s) showing all data points and remote reference station(s)
• Coordinates (xyz) of all data points, time and date, and values
• Data on fi le (EDI format)
• Processing report
• Detailed information about applied corrections and software used
• Inversion results
• Apparent resistivity maps and profi les
• Processed data on grid-ded data on fi le
• Report
• Geological information
• Detailed information about assumptions and software used
• Detailed information about the modeling and interpretation process
• Assessment of how well the data fi t the model
• Interpreted anomaly maps and/or profi les
• 2D/3D images of inter-preted structures
A good outcome from an MT survey is a 3D apparent resistivity block model derived from stacked 1D or 2D inversions (incorporating static shift corrections from a TDEM survey), or a full 3D inversion. The inversion model should suggest regions of contrasting electrical resistivity consistent with the conceptual model of the geothermal system.
Table 4.6.
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