Electrical Methods
From Geophysics 300
The Electrical Methods wiki
Contents |
History
Theory
Electricity can be used in many ways to obtain data for geophysics. It is most often used to help us see what we cannot see with our eyes, the subsurface. When an induced electrical current is shot into the ground, it travels very rapidly through many layers of the subsurface, reflecting and refracting, until it reaches back to the surface. The basic path can be estimated by travel times and velocities, and anomalies may be found. Use of electrical currents is a very powerful tool and it can be used to find groundwater, ore, and even oil. Below is an image of all three of the main types of electrical electrode arrays.
These setups all have a pair of current electrodes, which are the red arrows and a pair of potential electrodes, represented by the green electrodes. The red arrows, one having a positive and the other a negative charge, act as a sink and source for the induced current to flow. The potential electrodes or green arrows, measure the actual current flowing at that point.
The point of these electrodes is to measure the potential at certain points, in order to spit out data. The data is a measure of both travel time and potential. This data can be manipulated, using computer programs, to produce a pseudo section. A pseudo section, as seen below, is a computer image representation of the subsurface.
The computer used Ohm's LawV = IR to map the information recorded in the experiment. When it is mapped in the pseudo section, it shows the subsurface as a function of resistivity. This gives us an excellent idea of the apparent resistivities of certain sections, which can lead to an understanding of what is down there below our feet.
Application
Researchers use Electrical Methods (EM) for a variety of applications including investigation of groundwater and subsurface stratigraphy, contamination detection, archaeological research, geotechnical engineering, geological mapping and location of buried materials. Private, academic and government entities employ scientists profficient in EM data acwusition and interpretation to answer a question or aid in the solution of a problem. After data is collected, a computer processes the information, providing a mathematical representation of the subsurface known as a profile. The profile must be interpreted with considerations of many other factors, such as regional geology, seismic history and anthropological activity. A report is then prepared highlighting the findings of the researcher relevent to the nature of the study. Interpretations vary widely and are subject to the knowledge and experience of the analyst, therefore the report is gernerally reviewed by other experts before being accepted.
Case Study Near the small town of Guernsey, Wyoming resides an obsolete landfill implemented prior to environmental regulations and requirements. Contaminents seep into the groundwater supply beneath the landfill, possibly compromising the public drinking water supply and posing a threat to public health. Growing concern has prompted the USGS to perform a study using DC Resistivity (a form of EM), in conjunction with other methods, to analyze subsurface stratigraphy and lithology. Knowledge of the subsurfacw allows for better computation of contaminent flow out of the landfill and through the ground water system. Scientists constructed horizontal profiles from data collected across several cross sections throughout the landfill. Analysis of the profiles showed a much more complex lithology and stratigraphy than originally thought
References
www.water.usgs.gov
www.cflhd.gov
