what is seismic refraction method is Seismic refraction method?

Answer 1

Reflection and refraction seismology are both ways to study the structure of the Earth near the surface. Among other things, they are used to search for oil and gas deposits.

Reflection seismology works like sonar. You send a sound pulse into the Earth. The sound pulse is probably most often made by setting off an array of small explosive charges, but it could be from trucks that balance on a plate and vibrate that plate to send a waveform into the ground, or for measuring soil layers near the surface it could even be done by pounding on a metal plate with a sledgehammer. The sound pulse goes down into the Earth. Each time it hits a rock layer, soil layer, or other object with different acoustical properties (sound speed and material density) than the one above, part of the energy reflects back toward the geophones, the sound detection devices, you have arranged across the surface. You can record these reflections. Making a few assumptions about sound speed in the subsurface and other matters, and after doing a lot of computer processing, you can build up a picture of the underground structure.

Refraction seismology uses a sound pulse and a line of geophones extending away from it to the side. The sound pulse goes into the ground. Some of the energy gets refracted into each of the various rock or soil layers in the ground and moves through them horizontally, and some of the energy is always refracting out of those layers again and returning to the surface, where the geophones detect it. In general, deeper rock layers have higher sound velocities than shallower ones. Therefore at first the earliest signal to reach the geophones will be the direct wave through the surface layer, but at geophones further away a wave that goes down into the Earth, gets into a layer that has a faster sound velocity, and after a while returns to the surface will be the wave that reaches the geophones first. You can take the arrival times of different wave paths at your geophones and calculate out a structure of the rock and soil layers, with the thickness and sound velocity of each.

Reflection seismology needs a lot of data processing, so it wasn't that popular until computer power increased enough to make it practical. In general refraction seismology is good for finding the general structure of an area, while reflection seismology is good at finding small details. They really work best when you use them together. The sound velocity data you can get from refraction seismology can be applied to the reflection seismology, and can make it more accurate than it would have been otherwise.