What is Surface Wave Research?
When seismic waves are generated at or near the earth's surface, surface waves are generated. If vertical seismic sources are used, the waves generated are called Rayleigh waves, or ground roll. In seismic reflection, ground roll is considered noise to be eliminated. Rayleigh waves have unique properties, however, which allow them to be used to obtain near-surface velocity profiles.
What are its applications?
Surface waves have been utilized in two distinct ways at KGS. Vs Profiling, or shear-wave velocity profiling evaluates the stiffness of subsurface material. Near surface imaging is a method used to detect anomalies below the ground.
How does surface wave research work?
A vibration of some sort, usually from an impulse or a specially equipped truck, is put into the earth's surface. Energy in the form of Rayleigh waves travels along the surface of the earth. Seismographs, connected to microphone-like device called geophones, convert the energy into images. The results are then processed by computers into images of the subsurface.
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
- 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.