MT can be used to map a variety of natural resources including:
(i) oil and gas reservoirs;
(ii) geothermal resources;
(iii) groundwater; and (iv) ore bodies.
Meju (2002) presents a number of case studies involving the use of EM methods (including, but not limited to MT) for exploring natural resources. Many more commercially sponsored MT surveys have been conducted than have been published. This is partly because industrial partners generally wish to protect their proprietary interests, and partly because using MT for commercial prospecting generally relies on applying existing methodology, rather than driving scientific innovation. The development of marine MT instrumentation can be seen as an exception to the latter statement. The period-dependent attenuation of electromagnetic fields by the ocean (see Sections 9.4 and Scale bar
SG4 SG2 SF8 SF7 SF5 SF4 SF3 SE6 SE5 SE3
Ey
Ex
By
Bz
Bx
18.9.85 24.9.85 30.9.85
Date
Figure 9.314-day
electromagnetic time series from ten ocean-bottom sites on the Juan de Fuca plate off the coast of Oregon (redrawn from Bahr and Filloux, 1989).
By,BzandBxare magnetic components towards the magnetic east, downwards, and north. The vertical scale bar in the upper right-hand corner represents 32 nT and 2.7mVm1. The water moves mostly parallel to the coast in the N–S direction, and for this particular case Equation (9.5) reduces toE¼vxBzy.
Therefore, the N–S component of the ocean-bottom electric field,Ex, is little affected by oceanic motional fields, while in the E–W component,Ey, the effects of gravitational tides are clearly visible.
9.6 Industrial applications and environmental studies 177
10.4) has generally limited academic marine MT research to sounding periods longer than 1000 s, precluding crustal studies. However, the potential to exploit the MT method for assessing oil and gas reserves below salt domes on the ocean bottom has helped to drive the devel- opment of a marine MT system that is sensitive to shorter-period signals suitable for imaging shallow structures (Constable et al., 1998). In the low-electrical-noise environment of the ocean bottom, electric fields can be pre-amplified by many orders of magnitude without risk of instrument saturation owing to cultural noise of the type experienced when performing MT on land. The ocean-bottom system described by Constableet al.(1998) consists of AC-coupled sensors and pre-amplification factors for the electric fields of 106, and induction coilsfor the magnetic field (Figure 9.1).
MT sounding cannot be used to detect oil or gas (which have resistivities exceeding 105m) directly, but can help to delineate geological structures that can form hydrocarbon traps. The base of a hydrocarbon trap, which is often itself resistive, is frequently marked by a layer of highly conductive sediments or brines, which can be well resolved using MT methods. MT data can be particularly useful for delineating potential hydrocarbon traps below salt domes, volcanics and karsts, which give rise to multiple reflections and scattering of seismic energy. MT can also be adopted for reconnaissance surveys in areas of rugged topography, where large-scale seismic exploration may be prohibitively expensive.
A model study to investigate resolution of subsalt characteristics using marine MT data acquired over salt domes in the Gulf of Mexico has demonstrated that the base of salt structures can be mapped with an average depth accuracy of better than 10%
(Hoverstenet al., 1998). In this study, the salt dome was assigned resistivities of the order 20m, whereas the contacting sediments have resistivities of the order 1m. Marine MT surveys can be completed at a fraction of the cost of marine seismic surveys.
Geothermal regions often have enhanced subsurface conductiv- ities owing to hydrothermal circulation (e.g., Fiordelisiet al., 2000).
However, geothermal systems incorporate a high level of small-scale 3-D complexity such that lateral resolution of plumbing (i.e., con- duits and channels conveying fluids) at depth may be insufficient to obtain more than general constraints on the extent of geothermal prospects (e.g., Baiet al., 2001).
High conductivities modelled below Las Can˜adas caldera (Tenerife, Canary Islands), at depths consistent with the depths to the water table measured in boreholes drilled into the caldera,
178 The special link to other geosciences
have been interpreted as discontinuous groundwater reservoirs (Pous et al., 2002) that are separated by hydrological barriers.
Quantification of the amount of groundwater present is hampered by temperature heterogeneities and unconstrained hydrothermal alteration processes. Integrated EM methods, including AMT, have been used to map groundwater aquifers in the Parnaiba Basin, Brazil (Mejuet al., 1999). A reconnaissance AMT survey of shallow sedi- mentary basins in northern Sudan excluded the presence of exploit- able groundwater resources (e.g., Brasse and Rath, 1997).
MT methods can be used to detect massive sulphide deposits and copper deposits if, as is often the case, the host rocks are more resistive than the ore body and the ore is distributed in intercon- nected veins. Although highly electrically conductive (5107S m1), gold is more difficult to detect, because of its low-grade presence in typical deposits, and gold prospecting therefore requires an in-depth understanding of the nature of host rocks and mineralisation. The geological environments in which various ore bodies are found are described in Meju (2002).
Environmental applications of MT include:
(i) space weather;
(ii) landfill and groundwater contamination problems; and (iii) earthquake studies.
Space weather is a term coined to describe spatial and temporal electromagnetic disturbances (e.g., magnetic storms) in the iono- sphere andmagnetosphereassociated with solar wind–magnetosphere interactions. Magnetic storms can be hazardous to orbiting satellites and spacecraft, cause radio and TV interference, and generate current surges in power lines and gas pipelines. To negate corrosion of steel gas pipelines, a cathodic protection potential is often applied to the pipeline. Geomagnetic variations modify pipe-to-soil contact poten- tials and, during magnetic storms, cathodic protection of pipelines may be compromised. MT methods can be used to determine adequate levels of protection potentials (Brasse and Junge, 1984).
In a more general sense, magnetometer arrays can be used to monitor geomagnetic activity for forecasting purposes (e.g., Valdiviaet al., 1996). Figure 9.4 shows an example of enhanced magnetic activity at mid-latitudes owing to a powerful solar flare that impacted the Earth’s magnetosphere on 28th October 2003.
MT sounding can be used as an aid to assessing theporosityand permeabilityof potential landfill sites, and to monitor leaching and leakage of conductive chemicals in pre-existing landfill sites.
Applications of this type are closely linked to hydrological interests.
9.6 Industrial applications and environmental studies 179
Another source of contamination of freshwater supplies arises from saltwater intrusion. The different resistivities of fresh water and salt water can be used to map the interface between them.
Despite an escalating number of claims that earthquakes might be predicted using EM methods, consideration of the chaotic processes that lead to earthquakes lead us to be sceptical.
Magnetotelluric sounding can, however, be used to map faults, and although it is not possible to distinguish between active and passive faults on the basis of conductivity (since both active and passive faults can be conductive for different reasons), knowing the orientation and dip of a major fault with respect to the stress field acting on it may be useful for predicting the potential sense of motion and possible effects of an hypothetical earthquake.
Hypothetical damage models incorporating geophysical, geological and civil engineering data are used by insurance companies in order to set insurance premiums in earthquake regions.
Furlong and Langston (1990) suggested a model for an earth- quake which nucleated in California at a depth of 18 km in which a discrepancy in directions of displacement in the crust and mantle causes a localised accumulation of strain, which is accommodated by decoupling in the lower crust. This hypothesis might be tested using MT data, since different electromagnetic strikes might be anticipated for the crust and mantle. Thus, the frequency depend- ence of electromagnetic ‘strike’, relating to different depth ranges, might potentially offer constraints concerning the disparate motions of the Pacific plate mantle below the north American crust.
28.10.2003 07:52
29.10.2003 07:52
Date Time 100 nT
Bx
By
Bz
Bx
By
Bz
100 nT
100 nT Figure 9.4Three-component
magnetic time series recorded at Go¨ttingen following a solar flare that impacted the Earth’s magnetosphere on 28th October 2003. (Courtesy of Wilfried Steinhoff.)
180 The special link to other geosciences
Chapter 10
Other EM induction techniques
We finally pay a few other induction techniques a short visit. Magnetometer array studies have been conducted in many parts of the world. Interpretation of the data acquired is mostly restricted to an estimation of the amplitude of the vertical magnetic variational field relative to the amplitude of the horizontal magnetic variational fields, displayed as ‘induction vectors’.
The interpretation of these data in terms of lateral conductivity contrasts has been rather qualitative, but if regional differences of the horizontal vari- ational fields of different sites are also evaluated, then ‘geomagnetic depth sounding’ (GDS) provides quantitative information complementary to MT data. Appealingly, the additional information comes at no extra cost: a group conducting an MT campaign with more than one instrument can use the data so acquired for GDS with no additional hardware or field procedures. We demonstrate how the existing processing and interpretation schemes can be modified with only very minor changes. On a more historical note, simultan- eous measurements of the magnetic field at the Earth’s surface and in bore- holes have provided a means of studying the exponential decay of magnetic fields within the Earth (for those who doubt the action of the skin effect).
A fascinating experience is the evaluation of the magnetic daily vari- ation, caused by current vortices in the ionosphere. In this case, the plane wave assumption is violated (the penetration depth – 600 km – is not really very small compared to the size of the ionic current vortices – 6000 km), but this can be turned to advantage. We demonstrate how the MT impedance can be estimated from purely magnetic data. The plane wave assumption is also violated at both polar and equatorial latitudes, imposing both con- straints and challenges on the MT method.
We briefly mention the active electromagnetic methods. Books on applied geophysics cover most of these techniques, and we will restrict 181
this section to an appraisal of the advantages and disadvantages of active methods, compared to passive MT methods.