What is Seismic Reflection?
Seismic Reflection is commonly used to provide images of the subsurface, particularly in oil and gas exploration. In this technique, energy is put into the ground, then recorded as it reflects off of underground rock layers. Because different rocks reflect the energy in different ways, the returning signals can be used to produce images of the subsurface.
What are its applications?
Seismic reflection technology has been applied to characterizing the shallow geology at locations that are environmentally contaminated; in detecting shallow subsurface voids that might be related to sinkholes, tunnels, or construction; in mapping faults or bedrock surfaces; and in other situations.
How does seismic reflection work?
A vibration of some sort, usually from an explosive or a specially equipped truck, is put into the earth's surface. The reflections bounce back to the surface and are recorded. Seismographs, connected to microphone-like devices called geophones, convert the energy into images. The results are then processed by computers into images of the subsurface.
What research has the survey undertaken?
This section has been very active in the 1990's. Research has been done at numerous sites across the country and around the world. Countless applications for seismic reflection have been studied. Among the more noteworthy, in 1995 research was done at Oak Ridge National Laboratories to investigate potential environmental contamination. More recently, near Ellsworth, KS seismic reflection was used to examine the cause of a large sinkhole that threatened to damage a nearby highway.
What equipment is used?
Typical seismic acquisition systems consist of the following components.
- Seismic Source--This is nothing more than an apparatus for delivering seismic energy into the ground. Sources can vary greatly in their size and complexity. All, however, share the following characteristics:
- They must be repeatable. That is, the nature of the energy delivered into the ground (its amount and the time duration over which it is delivered) should not change as the source is used in different locations and
- Time of delivery must be controllable. We must be able to tell exactly when the source delivered its energy into the ground. In some cases, we can control the time of delivery. In others, we simply note the time the source delivered its energy.
- Geophones--These are devices capable of measuring ground motion generated by the seismic source. These typically convert the ground motion into electrical signals (voltages) that are recorded by a separate device.
- Recording System--This actually consists of a number of components. In essence, this entire system does nothing more than store the ground motion detected by a number of geophones. This number could be quite large. Today, it is not unusual for oil exploration surveys to record ground motion detected by 1000's of seismometers at a time. In addition to recording ground motion, this system must also control the synchronization of the source. It consists of not only a "black box" to store information but also numerous electrical connections to the geophones and the source and usually a device to select subsets of the installed geophones to record.
Who funds the research?
Research is partially funded by the U.S. Geological Survey, Exxon Production Research, Burns and McDonnell, U.S. Army Corps of Engineers, Martin Marietta Energy Systems, and others.