Friday, May 1, 2015

Seeing Underground with Geophysics

Being able to “see” underground is the basis for the study of geophysics. Within the Wyoming Center for Environmental Hydrology and Geophysics (WyCEHG), geophysics helps scientists understand what is going on underground—making WyCEHG a unique interdisciplinary research center.
Model of seismic refraction datasets, created by Brady Flinchum


Graduate students Ryan Armstrong and Brady Flinchum are part of the Geophysics team at WyCEHG. Ryan is exploring changes that are happening underground in the Snowy Mountain Range in southeast Wyoming, which may be caused by vegetation changes like beetle kill. Brady is gathering information about density and porosity below the surface.

Ryan in the field (photo courtesy of Ryan Armstrong)
In the future, their research can help give WyCEHG a clearer picture of how water travels from the Snowies down into the surrounding areas. 

Ryan is making an underground geologic map of the Snowies using electromagnetic data. He collects data by measuring the speed at which electrical current travel through the ground.

This type of data helps researchers “see” the shape of features on the earth, which in this case it is a mountain. The data can also help researchers understand what the mountain is made of, since different types of rock conduct electricity in different ways.

Geophone (photo courtesy of Brady Flinchum)
While Ryan uses electrical currents to map the ground beneath our feet, Brady uses a technique called seismic refraction which uses vibration to better understand the earth. Brady’s team uses an instrument called a geophone, which is a magnet attached to a spring. When the ground vibrates, the vibration travels through the spring and the magnet. Then the geophone converts the vibration into voltage that can be measured by researchers.

Brady’s team places geophones throughout the area and then hits the ground with a sledgehammer. The geophones measure the speed of the vibrations from the impact.  Vibrations travel at different speeds depending on the density, porosity, and rock type. High velocity translates to more dense rock; low velocity can indicate porous rock like sandstone. These data sets provide cross-sections of the area, allowing Brady to create a three-dimensional image.

Interpreting this data takes careful examination. Geophysicists often see data that can be caused by many different factors.  It can be difficult, for example, to tell underground water and ore apart.  As Ryan says, “We’re seeing many contrasts between different bedrock properties, and we’re trying to tease those differences apart and see how we can use that information.”
Sledgehammer (photo courtesy of Brady Flinchum)

Every researcher involved in watershed modeling also runs into the problem of scale.  For example, the Laramie range dataset shows smaller geological features that might not translate to a larger-scale map.  Something that shows up in one place, like the striking granite hills of Vedauwoo, may not appear a few miles down the road. 

“Big picture” information is a major part of geophysics research, particularly for projects covering a large area.  These different data sets help form a detailed picture of a complex region. As Brady says, “It’s pretty neat because we’re both seeing similar stuff in our data.  Since I cover a smaller area, I’m essentially on one rock type. The weathering process and changes in underground water content are more important than the overall geological structure. Our goal is to use geophysical methods to learn more about the hydrologic and geomorphic processes within a few hundred meters of the surface.”

Brady in the field (photo courtesy of Brady Flinchum)
When asked how he became interested in Earth science, Brady said that a near-drowning experience during a Hawaiian surfing trip introduced him to the grandeur of nature. “I remember getting out of the water and looking at the ocean and thinking, ‘Wow, the Earth is so powerful.’”

His interest in geophysics stems from learning about Wyoming’s role in the water system: “We’re at the headwaters of the Midwest. The snow that falls on these mountains feeds the rivers.” 

Posted by Jess White

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