Seismic Research at the KGS

Survey researchers have experimented with seismic reflection techniques to produce sharper images of subsurface features previously considered too small to image, especially in the few hundred feet closest to the surface

CONTENTS

Seismic Research–page 1

From the Director–page 2

New Maps–page 3

Website Expands–page 3

A Place To Visit–page 4

The earth shook at the Lawrence headquarters of the Kansas Geological Survey early this spring. It wasn’t a rare Kansas earthquake that rattled windows and jiggled furniture. Instead, the vibrations came from a specially designed truck called a seismic vibrator that Survey researchers were testing outside the building.

Seismic vibrators are one of the tools used in seismic reflection, a technique commonly used in oil exploration which produces images of the subsurface. Seismic reflection is a relatively noninvasive technique—that is, it doesn’t require drilling into the earth to find out what’s below the surface.

Here’s how seismic reflection works. A vibration of some sort, usually from an explosive device or a seismic vibrator, is put into the earth. The energy travels as waves underground (much like waves on a pond), reflects or echoes off rock layers, and bounces back to the surface where it is recorded by microphone-like devices called geophones. Because different rock types reflect the energy in different ways, the returning signals can be processed by computers to produce an image of the underground rock layers.

“It works much like a sonogram or fish finder,” said Survey geophysicist Rick Miller. “It’s a snapshot of what the rock layers look like at a particular moment in time.” In recent years, Survey researchers have experimented with seismic reflection techniques to produce sharper images of subsurface features previously considered too small to image, especially in the few hundred feet closest to the surface.

Survey researchers have applied seismic reflection technology to a number of problems in Kansas. In Pawnee and Stafford counties, the technique is being used to study sinkholes. These sinkholes developed in oilfields where old wells allow water to move underground and dissolve away salt layers, creating voids. At one sinkhole, the ground above the void settled slowly. Twenty miles away, near Macksville, the earth dropped suddenly in what geologists call a catastrophic collapse. By imaging the rocks around and below these sinkholes, Survey researchers can get a better feel for how sinkholes developed and what can be done to predict and remediate them.

At an industrial facility in Olathe, the Survey used seismic reflection to assist a study of potential ground-water contamination. Using high-resolution techniques developed at the Survey, KGS researchers were able to image the bedrock limestone eight to ten feet below an asphalt-covered ground surface. The three-dimensional map of the rock layers revealed a channel in the limestone, information that was used to guide the placement of monitoring wells to track potential contamination. “We can begin to identify underground flow paths and predict how contaminants will move, where they will spread,” Miller said.

Survey researchers also use seismic reflection techniques to assist petroleum research in the state. In a study near Elk City in Montgomery County, seismic reflection is producing a two-dimensional image of rock layers that occur 400 to 1,000 feet below the earth’s surface. Because these rocks crop out at the surface about 15 miles away, the picture produced by seismic reflection can be compared with the revealed rock layers. This gives the Survey researchers a way to test the accuracy of their imaging techniques, Miller said. “The thinner the rock layers we can image, the better we can map small reservoirs of oil and gas that are now invisible using conventional seismic techniques.”

Seismic reflection is just one geophysical tool researchers use to learn about the rocks below the surface. “Eventually, we have to use invasive techniques to verify our interpretations, but tools like seismic reflection can help us use drilling more effectively,” Miller said. “Otherwise, it’s hit and miss, like throwing darts at a board.”

Online February 10, 2003

Comments to: lbrosius@kgs.ku.edu

Kansas Geological Survey

URL:http://www.kgs.ku.edu/Publications/GeoRecord/2000/vol6.2/Page1.html