DC Resistivity
Presentation Highlight: Introduction of DC Resistivity to soundings
Concepts in DC resistivity form much of the foundation for the rest of the day. The physical property that we are sensitive to in a DC resistivity experiment is electrical conductivity (or equivalently, its reciprocal, resistivity). Electrical conductivity is a diagnostic physical property in a variety of settings, including applications such as mineral exploration (mineralized regions are often more conductive than the host rock), hydrocarbons (hydrocarbons are more resistive than the host reservoir which is saturated with saline formation fluid) the locating a source of water flowing into a mine, or geotechnical applications such as locating a tunnel.
In a DC resistivity experiment, the source is composed of two current electrodes, one positive and one negative. Currents flow through the subsurface from the positive to the negative electrode. The distribution of currents depends on the conductivities of the geologic structures. Currents preferentially flow through conductors and avoid resistors. At interfaces between units with differing conductivities, charges accumulate. According to Coulomb’s law, electric potentials are generated, and these are what we measure at the surface with a pair of potential electrodes.
So now that we have an idea of what our data are, how do we use those data to obtain the resistivity of the earth? To answer that, we go back to Maxwell’s equations. The DC problem is governed by Maxwell’s equations in the electrostatic regime, giving us a div-sigma-grad system as our governing equations. For a homogeneous halfspace, the exact resistivity of the earth can be extracted from an idealized 2-electrode system or from a real 4-electrode system.
The earth is never as simple as a homogeneous half-space. If, for example, we have a two layer model, then we are not computing the true resistivity from measured potential, it is an apparent resistivity.
If we have a conductive overburden over a halfspace, that reduces the apparent resistivity — we are “seeing” a more conductive earth. The apparent resistivity that we measure depends upon the distribution of currents in the subsurface.
If the current electrodes are very close together, then most of the currents flow in the top layer, thus the apparent resistivity is very close to the resistivity of the top layer (100 Ωm). As we move the electrodes further apart, we sample more of the second layer. For the example above, the lower layer is more resistive, thus the observed apparent resistivity increases as we move the current electrodes further apart. You can explore this in more detail in the DC app: DC_LayeredEarth.ipynb.
A case history: Mt. Isa
On the first day, we covered a number of case histories, including Mt. Isa, which is introduced with DC Resistivity and later appears in the course during the discussion on Induced Polarization. As with all of the case histories used for the DISC, it is presented in a seven step framework. We start with the setup, the question to be addressed; identify the physical properties that are diagnostic; select a survey; examine the resulting data; process those data, often through an inversion; interpret the results including geologic and other available information; and synthesize the interpretation within the context of the original question.
The Mt. Isa region in West Queensland, Australia hosts a number of mineral deposits. The data considered in this case history consist of 10 lines of DC and IP data collected over what is now the Cluny Mine. Of particular interest is the Native Bee Siltsone; it is a potential host for mineralization. It has a moderate electrical conductivity compared to the background (Surprise Creek Formation and Eastern Creek Volcanics). What makes this region challenging for electrical methods is the presence of the highly conductive Breakaway Shale, which is not of economic interest.
Ten lines of DC and IP data were collected over the area of interest (2km x 5km). Two configurations were used (Pole-Dipole and Dipole-Pole). DC data are often viewed as pseudosections, which is a useful tool for quality-controlling the data and identifying outliers or possible faulty electrodes. To interpret anything from the data though, an inversion is required. For this example, all of the DC data were inverted together in a 3D inversion, producing a voxel-model of electrical conductivity of the subsurface. A large conductor striking N-S through is clearly visible — this is the Breakaway Shale. There are other regions of moderate conductivity in the model that are potentially interesting. To identify regions of potential mineralization, more information is still needed. Later in the course, we return to the Mt. Isa case history and examine a chargeability model recovered from the IP data. Check back in on future blogs to hear more, or read the full case history on EM GeoSci.
