The following is an article that appeared in the March 1999 Report to Stakeholders newsletter published for the Edwards AFB Installation Restoration Program:

 

The Sounds of Science
Principles of Sound Waves and How They Interact with Different Geologic Structures

In the world of environmental cleanup, knowing the underground geologic structure and makeup of a geographical area can play a critical part in determining how a cleanup process will progress.

For example, it is difficult to determine where hazardous materials and wastes may have migrated after being released on the ground surface or just below the surface. If the underground structure is porous, the materials and wastes may have migrated straight down into the groundwater. However, if the underground structure is fractured bedrock, the materials and wastes may have migrated through the fractures to the underlying groundwater far from the initial source of the contamination.

To give cleanup project officers a more accurate picture of the fate of the contamination, environmental officials at Edwards are making use of two relatively new seismic surveying techniques to create an underground blueprint.

Seismic refraction and seismic reflection surveying are nonintrusive imaging techniques that use sound waves to map the underground geologic surface structure and makeup.

The techniques are based on the principle that sound waves will bounce or reflect off different geologic structures. The effect can be similar to an echo off a canyon wall or sounds absorbed by piles of loose dirt.

Bedrock can contain many small pockets and fractures that can trap rainwater and contamination that have migrated from the ground surface.

"When this happens, it makes cleaning up the contamination next to impossible because we don’t have an accurate picture of what we are dealing with," said Robert Wood, chief, Edwards AFB Installation Restoration Program.

"Many sites on base have fractures running in several different directions. High contaminant concentration usually occurs in fracture zones."

Drilling and installing wells in bedrock is time consuming and expensive. Without some idea of the geology at a site, it is difficult to determine where groundwater and contamination may be trapped. It could be necessary to install many wells before identifying the location of the contamination. "We need to identify the optimum location for wells in these situations," Wood said.

Performing seismic surveys requires three components: an energy source, seismic sensors, and a geophone (a high-speed data acquisition and recording instrument).

Here’s how the technique works:

Impacting the ground surface with a hammer or power-assisted-weight drop imparts acoustic energy into the earth. The sound waves spread into the earth on a spherical wavefront. When the sound encounters a change in the physical properties of the material in which it is traveling, it will do one of three things:

reflect to the ground surface,
penetrate deeper into the bedrock, or
encounter other changes and then reflect to the ground surface.

Areas of structural deformation, such as fractures, produce different reflections than a continuous rock surface. Acoustic energy is disrupted, or "diffracted," by fractured rock surfaces in much the same way that an image is distorted by a shattered mirror.

Using an aluminum impact plate as a consistent impact surface for the hammer can further enhance the energy source. The aluminum plate produces higher frequency seismic energy than impacting the ground surface directly.

The geophones are placed along the ground surface to collect the sound waves. The data is then converted into a seismic record.

Impacting the ground several times at the same location stacks the resulting signal and increases the data quality of a seismic record. Stacking the signal reinforces coherent reflections and weakens or cancels random noise.

Analyzing the seismic record can provide a blueprint of the underground geologic structure and makeup that will help with the design of a cleanup technology.

"This technology has been extremely beneficial to us," said Keith Dyas, North Base project officer. "It has provided key data for building the groundwater model of the aquifer at Operable Unit 10."

Seismic reflection survey data were used in the model to determine if a fault cuts through the OU10 area. This added to the accuracy of the model.