Seismic measurements are a widely used method enabling companies exploring for gas (including shale gas) in Poland to decide where to position the drilling rig and drill an exploration well worth some US$15m (shale gas extraction will be addressed in the next question). This is an important – indeed, critical – stage of exploration. Whether experts succeed in identifying accurately the site of future drilling depends on the precision and accuracy of this testing. An off-the mark shot will lose the company millions of dollars.
The registered image comes out as a seismic profile not unlike that obtained by ultrasound scanning, such as performed in pregnancy. Are the two methods similar?
Ultrasonography, a diagnostic technique known since the mid-1900s, is a method of visualising a cross-section of the body. Ultrasound scanning relies on the phenomenon of spreading, diffusion and reflection of ultrasonic waves. As the movement of these waves inside the human body is recorded, the shape of the tested organ appears on the screen.
Seismic mapping relies on very similar phenomena, including on the reflection of sound waves. In this case acoustic waves reflect from successive geological layers of the Earth. As sound waves penetrate these formations, they refract, reflect or bend. The changes in their trajectory and velocity make it possible to reconstruct the internal structure of the Earth. Seismic testing has been successfully practised worldwide since the 1980s.
How is this done? To generate a subsurface signal, vibration trucks move along pre-charted paths (known as seismic lines), lowering vibration pads onto the ground for about a minute at a time. The pads generate acoustic waves which spread underground, reflect, and return to the surface of the earth. The image produced by recording them is registered by specialist sensors called geophones. The data so collected is subsequently computer-processed and interpreted by specialists, the result of these measurements providing understanding of the geological structure of the region surveyed and enabling experts to precisely locate future drilling sites.
The main difference between the two testing methods is the length of the sound waves generated: in seismics the waves are of the order of thousands of meters, and in medical tomography they are measured in micrometers. Both seismic testing and ultrasound scans are safe for human and animal life and health. The two methods are easily accessible, non-invasive and they capture images in real time.
Source: Polityka Wakacyjny Niezbędnik Inteligenta, by Wiesław Prugar