The term seismic activity for most people is associated with earthquakes, for those who have a more geographical background, it is about ground vibrations to be more specific. Seismic waves, just like water or light waves are subject to the laws of reflection when the wave front comes into contact with an interface between two dissimilar media. This change in direction of the waves at the boundaries of bodies of rocks of different densities is what is defined as seismic reflection.
Seismic reflection, also known as reflection seismology, is an exploration method used by a geophysicist to study the earth’s subsurface by estimating the properties from the reflected seismic waves. This method of study is dependent on the principles of seismology and is similar to sonar and echolocation technology. For seismic reflection to be made possible, the following are required; a controlled seismic source of energy, usually in the form of a dynamite explosion or a seismic vibrator and a range of receivers, geophones, placed on the earth’s surface.
At its simplest form, this technique involves generating seismic waves and determining the amount of time taken for the wave to travel from the source, come into contact with an interface and be reflected so as to be detected by the array of geophones. With this timings, a geophysicist can reconstruct the path of the wave so as to build an image of the subsurface.
Seismic reflection surveys can be used to produce either 2 or 3-dimensional imagery of the underlying stratigraphic borders and the geologic structure of the subsurface up to depths of several kilometers. The accuracy of this method has made it the most common methodology in the search for oil and gas, in both marine and onshore explorations.
Other than oil exploration, seismic reflection is used for identifying and mapping faults, mapping the bedrock topography, exploration of ground water and determining the thickness of landslides. The much we know and understand of the earth subsurface, its structure, hydrogeology and its stratigraphy is as a result of seismic reflection studies.
Factors to consider
While carrying out a seismic reflection survey, here are some of the factors you should keep in mind:
• The depth to which the wave penetrates to is more dependent on seismic refraction than on the spread length. Advisably, the length of the active spread should be equal to the penetration depth required.
• For shallow seismic reflections, using a hammer and a plate is preferred if the ground is relatively hard, for softer grounds, a downhole seismic vibrator is used. For deeper explorations, small explosives such as dynamite are ideal. When working close to roads, especially in the urban centers using the mini-Sosie technique is advisable.
• To ensure the quality and accuracy of the survey’s results, an experienced seismic processor is an absolute necessity.
Limitations of seismic reflection
Some of the limitations of this methodology when carrying out sub surface surveys include the following:
• In order for it to be applicable, a relatively continuous reflecting horizon must exist. This is not usually the situation. For example in fluvial environments.
• It is much easier to map deeper reflectors compared to shallow ones since it is harder to transmit high frequencies through the ground.
In a bid to understand the earth better, seismic reflection as a method has been used to map out the sub surface of the earth. Compared to other geophysical methods such as the ERT and GPR, seismic reflection surveys have proven to be more expensive but of higher accuracy.
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