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NEWGEN FORGE Laboratory – A Resource for Graduate Education and Research

August 2, 2016

 

GRAD

The internal structure of Newberry Volcano is revealed in slices through a preliminary 3D electrical resistivity model. Warm colors indicate where the earth is conductive, blue resistive.

One of the skills I’ve gained as a graduate student studying geophysics at Oregon State University (OSU) is the ability to visualize miles of solid rock and model the internal structure of volcanoes. This is not easy to do when (aside from a limited number of deep wells) we’re only able to access the earth’s surface. To create an image of the subsurface, we take a variety of surface measurements at hundreds of locations and create a model of the subsurface that reproduces the signals we measured. We assess the performance of the model by comparing synthetic data calculated from the model with observed data. We can use data collected from deep boreholes to directly assess model predictions.

At the Newberry Volcano, we are very fortunate to have an enormous amount of data to constrain our models. There are over 40 years of extensive geophysical and geological surveys, information from four deep wells and numerous boreholes around the Newberry Geothermal Energy (NEWGEN) area. This extensive set of data allows us to create an accurate representation of the subsurface.

Some of the geophysical data used at Newberry come from small variations in the strength of the Earth’s gravity caused by variations in the density of rocks, differences in how fast seismic waves move through different types of rock, how much the ground surface is moved when fluids are injected deep underground, characteristics of tiny earthquakes that have been detected in the area, and how electric current flows through different materials underground. My focus is on the electrical properties of Newberry, where we use a geophysical method called magnetotellurics (MT).

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Using MT, we look at the electric current that is induced by small fluctuations in the Earth’s magnetic field. This signal is driven by electric currents in the ionosphere that cause the Aurora borealis and distant lightning strikes, and we measure it using magnetometers along with an extremely sensitive voltmeter with electrodes extending into the subsurface.

Researchers at OSU have developed data processing tools that are used by scientists around the world to create models of the earth’s electrical resistivity, not only at geothermal sites, but also in tectonic studies of continent-scale geologic structures. I use these tools to determine what parts of Newberry are more electrically resistive and more conductive, which tells us about the composition of the subsurface. It is especially sensitive to the presence of fluids that fill permeable areas of the rocks, as well as to the presence of clays and various minerals that may indicate the presence (past or present) of fluids circulating underground. Sensitivity to this kind of target is really important to our goal of locating where the rocks at Newberry are hot and dry.

Comparing models developed using MT data with models derived using complementary measurement techniques (gravity and seismic) allows us to assess the validity of our predictions. General agreement in model predictions between the various techniques increases our confidence in our representation of the site; this high degree of inter-model harmony gives us confidence that jointly interpreting the different types of geophysics results in an accurate model of subsurface rocks and geologic structures, and provides a 3D view of the NEWGEN site without ever leaving the ground surface.

About the Author
Esteban Bowles-Martinez is a Ph.D. student working with Dr. Adam Schultz in the College of Earth, Ocean, and Atmospheric Sciences at Oregon State University in Corvallis, Oregon. His doctoral research involves developing three- and four-dimensional (3-D plus snapshots in time) methods of imaging volcanoes and related geodynamic processes using electromagnetic and other geophysical methods.

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