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Electromagnetic Geophysics

Electromagnetic measurements reveal conductive (yellow) and resistive (greens) subsurface layers.

This slide shows a regional resistivity (inverse of conductivity) structure across southern British Columbia. The units (in colours) of resistivity (r) are Wm. The arrowheads on top of both sections indicate the position of the recording stations. Note the vertical exaggeration is 5:1.
Scientists like checking up on their own work, preferably through methods different from what has led them to their conclusions. For instance, a chemist can establish through a chemical analysis that the nugget he is handed consists of pure gold; but so can a physicist by precisely measuring the nugget's volume and specific gravity (the latter is the Archimedes principle we have learned about in physics).

So, LITHOPROBE scientists who probe Earth's crust by employing seismic (sound) waves and their reflection by different layers in the crust use entirely different types of measurements to check up on the seismic results. Gravity data are one way of doing this, but these data rely on physical properties of the measured rock (namely their densities) which also influence the rocks' seismic responses.

Thus, geophysicists employ electromagnetic (EM) studies which reveal subsurface structure in terms of a physical property, electrical conductivity (or its inverse electrical resistivity), that is independent of the other physical properties on which seismic and gravity measurements rely.

Most of us have experienced electrical conductivity through an electric shock, which is even more effective when we are unfortunate enough to touch live wires with wet hands or a wet towel. Don't try it, but believe me that it would work even better if the fluid we have on our hands was salty. And if your worried buddy would grab you by the hand that same moment, both of you would be conductive. (It's advisable to turn the electricity off instead.) So, this is how effective a force electric conductivity can be.

Electrical conductivity also is extremely sensitive to composition, texture and fluid content within the rocks of the lithosphere and thus provides another facet to integrated programs of crustal study.

In the continental crust, saline water in interconnected pores and fractures is probably the most widespread cause of high electrical conductivity. Alternate explanations involve the presence of graphite or partial melt (molten rock), both of which are conductive. So, high electrical conductivity alerts us to the possible presence of fluids in the measured rock, or else graphite or melt. All of this can be measured without having to dig a hole down to where these electric responses are coming from.

Now, we already know that fluids within the continental crust are vitally important, both in the mode of deformation of rocks and in the genesis of ore deposits. The roles of volatile fluids in the crust are discussed further in the subsection on Geochemistry.

In tectonically active regions, such as the Cordillera west of the Rocky Mountains, other fluids, silicate partial melts, can cause high electrical conductivities in the lower crust and upper mantle. Rocks are composed of a number of mineral constituents which melt over a range of temperatures. At a temperature within the melting range, a partial melt may occupy cavities in the solid matrix of the remaining minerals. As most silicate melts are excellent conductors, a rock with a few percent of melt in interconnected cavities may have high bulk conductivity. Thus EM geophysics may detect and map partial melt in anomalously hot regions, such as those along the Southern Cordillera Transect.

By the way, where does the electricity (whose impact we are measuring here) come from? Glad you were going to ask. Also, let's cut corners here, because you touched a difficult spot of science, where not all there is to know is known yet. One reasonable explanation is that the core of the earth, together with heat-generated convection streams, rotate at speeds different to surrounding matter, thus friction and electricity is induced, i.e. the dynamo effect. This, in turn, is thought responsible for the electromagnetic field, or magnetosphere, surrounding Earth. This magnetosphere, in turn, induces electricity into the crust (from above). Since the surrounding magnetosphere is subject to solar winds (protons from the sun), it shows variations. These, in turn, provide all sorts of opportunities one can measure. At any rate, where does the electricity come from? From Earth's core dynamo, via the induced magnetosphere around Earth, which, in turn, induces electricity into the crust. Within the crust, different rocks and various fluid contents provide differing conductivities; thus these differences can be measured. Pretty neat, I 'd say!

LITHOPROBE's electromagnetic studies generally comprise two types of field surveys. Broadband tensor magnetotelluric (MT) soundings, which rely on the effects of Earth's magnetospheric field, is one. The other measures varying the effects of controlled, man-induced electricity, which is the electro-magnetic (EM) method.

The former, magnetotelluric soundings (MT) are recorded with station spacings of a few to tens of kilometers, and provide regional coverage along the transects. These make use of natural time-varying EM fields due to electric currents in the magnetosphere and ionosphere. These fields are recorded over a wide range of frequencies enabling derivation of conductivity structure from near the surface to mantle depths. Recently, the GSC has developed extremely "long-period recording magnetotelluric instruments" (LIMS), collecting data which can provide conductivity information to depths in excess of 500 km! Thus, these can provide a “probe” into the sub-crustal lithosphere and upper mantle of Earth.

When specific geological targets or other features have been identified, high-resolution, controlled-source, electromagnetic methods are employed to better delineate conductivity anomalies in the upper crust.

Reconnaissance surveys and some special projects are carried out by university and GSC scientists who have the necessary equipment. Analysis of the complex data sets and presentation of results are greatly enhanced by the recent availability of powerful computer workstations with interactive and colour capabilities.

LITHOPROBE results and those from other countries have shown that in the study of a continental crust in which fluids are important, electromagnetic geophysics, reflection and R/WAR seismology are much more powerful together than each is alone.

Heat Flow and Geothermal Studies
We already have discussed that Earth itself is our biggest nuclear power station. The Earth's internal heat drives tectonic processes and is ultimately responsible for the formation of mineral deposits and the maturation of hydrocarbons in sedimentary basins. Thus any studies of these processes must include the contributions, past and present, of heat flow. Within the Earth, heat is produced by the decay of naturally occurring radioactive elements and by mantle cooling. Many tectonic processes generate local heat perturbations, as evidenced directly by volcanoes and hot springs.
Heat flow measurements are commonly made in drillholes on land or at sea with special probes into the soft sediments of the sea bottom and also in drillholes. While some holes are drilled specifically for heat flow measurements, many heat flow values are determined from previously drilled "boreholes of opportunity". To calculate heat flow values, temperature gradients and thermal conductivities of the rocks in which the temperature gradients are being measured must also be determined. Using measured heat production values and other thermodynamic properties, thermal modeling can provide estimates of temperatures today and in the past.

Temperature-dependent rheological properties (that is how stiff the rocks are; over long periods of time, such as those we deal with in geology, rocks can actually flow) control zones of strength and weakness in the crust and thus depths at which tectonic motions could take place. In sedimentary basins such as that east of the Rocky Mountains, movement of water in the rocks greatly affects measured heat flow. Geothermal studies of the fluid migrations are of great value because they detect water speeds of a few centimeters per year which can be discerned in no other way. Both university and government scientists have the necessary facilities and expertise and are actively participating in thermal studies relevant to LITHOPROBE.

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