User:MOJAHID Abdelaziz/Seismic Refraction

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Seismic refraction methods have been often used in petroleum, mineral, and engineering investigations. The principle of this method is to investigate subsurface ground conditions using P-waves velocities. During the acquisition, the data acquired on-site is computer processed and interpreted to produce models of the seismic velocity and layer thickness of the subsurface ground structure.

Seismic refraction survey2.jpeg

The use of seismic refraction is to determine P-wave travel times and velocities, from which we can deduct depths of layers at the subsurface. Thus, we can determine the depth and thickness of the target layers. For the seismic refraction equipment we have: geophones, seismic connector-cables, sledgehammer, weight drops, or buffalo gun, a trigger extension cable, a strike metal plate, an exploration seismograph, a PC control unit, and a GPS.

The acquisition phase starts by setting up the spread lines of multiple channels, usually 24 geophones are deployed with a constant geophone interval, depending of the resolution and depth we aim to reach. The type of the used source will also affect the depth of the elastic waves penetration. The stronger source we use, the deeper waves can travel. For example, dynamite explosives are best for deeper targets. However, for geotechnical investigations, a sledgehammer can generate enough energy that will travel on acoustic waves, and reach down to 20 meters in depth.

Seismic refraction acquisition.jpg

After data acquisition, the stored seismic data will be ready to be processed by the appropriate software in the laboratory. The processing phase consists on (1) picking the first arrival times of the P-waves of each trace, for each shotgather in each seismic line. A table of the first arrival times will be saved. (2) P-waves original and shifted travel times will be calculated using software algorithms. (3) For each trace, depth of reflectors of the layers underneath the surface will be calculated. (4) Finally, a vertical P-wave velocity section will be elaborated, showing the velocity and depth of each layer.

Geophone1.jpeg

Seismic refraction can be used in different fields, such as assessing the bedrock depth and overburden layer thickness. The resulted P-wave velocity section can be compared to the surface geology, which led to a final geological model of the study area. This method can also be used in mapping depth of the groundwater table.

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