Seismic Techniques Help Define Kansas Sinkholes

"We're usually looking for pollution or void spaces, things that nobody wants to find, but they need to know about."

CONTENTS

Seismic Techniques

From the Director–page 2

New Publications–page 3

Education Award–page 3

Series Goes Online–page 4

A Place To Visit–page 4

Drive 15 miles south of Ellsworth on Kansas Highway 14. East of the highway, just before you get to the Ellsworth–Rice County line, you’ll see a bowl-shaped depression, about 75 yards across and, at most, about 10 feet deep. The depression often holds water. Along one edge, trees in a shelterbelt have sunk along with the ground.

The depression’s source is about 600 feet underground, where ground water has created a void in a layer of salt, causing the surface to slowly settle.

The Kansas Corporation Commission wanted to know if the depression—a sinkhole—would continue to slowly expand, if the settling might reach the highway, and if there was chance that the sink would collapse suddenly. To find out, they needed to see beneath the surface, something none of us can do without drilling a well. But seismic reflection, a technique widely employed in the oil industry, offers images of the underground without digging. By fine-tuning seismic reflection techniques, researchers at the Kansas Geological Survey have produced a sharper vision of the subsurface, especially the few hundred feet closest to the surface.

Here’s how seimsic reflection works. A vibration of some sort, usually from an explosive or a truck equipped with a special vibrating pad, is put into the earth’s surface. The energy moves underground, reflects off rock layers like an echo off a canyon wall, then bounces back to the surface where it’s recorded by microphone-like devices called geophones. The results, processed by computers, are images of underground rock layers.

“Seismic reflection is ordinarily used for finding oil and gas,” said Survey geophysicist Rick Miller. “We’re usually looking for pollution or void spaces, things that nobody wants to find, but they need to know about.”

The Survey’s seismic reflection techniques are often focused on sites of underground contamination. At the Cherry Point Marine Air Station in North Carolina, solvents spilled at the surface had reached a shallow aquifer. Using seismic reflection, the Survey provided an image of the near-surface geology, pinpointing the path of contamination and helping determine locations of new wells that could correct the problem.

Since the early 1990’s, the Survey’s seismic-reflection group has averaged about $300,000 per year in grant and contract work, much of it for the Department of Energy and the Department of Defense. They’ve experimented with various energy sources, from shotguns and sledgehammers to vibrator-equipped trucks.

“People come to us because we take the time to get better resolution in our images of underground rock layers,” said Miller. “We can show people details they’ve never seen before, except in road cuts or cores from drilling.”

In Kansas, the technique has been used to find abandoned coal mines, shallow limestones that might hold oil, and the rock beds beneath sinkholes. Many of the sinkholes in Kansas develop where old wells allow water to move underground and dissolve away salt layers, creating voids. The ground above usually settles slowly. Sometimes it drops suddenly, creating (in the language of geologic hazards) a “catastrophic” collapse.

“Seismic methods help reveal the configuration of rocks around and below sinkholes, showing places where rock layers are altered or missing,” said Miller. That information can help geologists predict the sink’s behavior and help guide remediation.

And the Ellsworth County sink? Seismic surveys in 1984 showed that it probably won’t fail catastrophically, but will continue to subside and grow, at the rate of a few inches per year. Twelve years later that’s proved to be the case.

Online February 10, 2003

Comments to: lbrosius@kgs.ku.edu

Kansas Geological Survey

URL:http://www.kgs.ku.edu/Publications/GeoRecord/1997/vol3.1/Page1.html