Seismics

Seismics

Seismic waves, created by some mechanical means (naturally or artificially), travel through the ground at a rate determined by the density of the medium. Different rock and sediment types vary in their density and elastic properties. When a wave encounters a layer with a different density, some of the wave energy will be reflected at the discontinuity, whilst some will be refracted and continue travelling in a different direction. Thus, differences in the make up of the Earth’s interior can have the effect that waves originating from a fixed point arrive at an observer at different times. Geophysicists can create their own, harmless forms of seismic activity and measure the responses to these disturbances using a wide variety of surveys on land and sea, thereby to deduce the properties of materials.

Differences in the density (amongst other factors) of the deposits underground create a range of responses to a controlled source of energy (explosion, air gun, or seismic vibrator) which can be measured using a geophone (on land) or hydrophone (underwater). By analyzing the travel times from the source to the receivers and the velocity of the waves, it is possible to create a 2D profile or 3D map of the underlying strata from the responses received.

There are three major types of seismic surveys: refraction, reflection, and surface-wave. Each technique adresses a different type of wave.

Types of survey available from Bargheer Geophysics

We offer refraction, reflection and surface-wave surveys.

Reflection seismology measures the time taken for seismic waves to travel from a source, reflect off an interface and be detected by an array of receivers.

Refraction seismology measures the time taken for seismic waves to travel from a source to several receivers after being redirected by one or more changes of medium.

The properties of different materials can affect the propagation of a wave as a result of its travelling at different speeds at different points along the wave front. Under certain conditions, seismic waves undergoes what is known as critical refraction. A critically refracted seismic wave travels along acoustic interfaces and generates new waves that can be detected at the surface. Refraction seismology measures the time a pulse of seismic energy takes to travel from a source point to several receivers after being redirected by one or more subsurface interfaces.Illustrations: https://guidelinegeo.com/