The main principle of the seismic method is based on the measurement of the arrival time of the reflected “echos” of the artificially generated elastic waves, emerging back to the surface of the earth from the boundaries of different rock layers. (Similar principle is applied by the bats in the air, and by the cetacea in the water for navigation.)
(We see a schematic geological section in the lower part of the Figure, containing a buried horst with an “oil and gas cap” below. The emerging vertical yellow lines in the upper part of the Figure, above the eight sensors (so called geophones laid out on the soil) symbolise the vibrations of the earth recorded against the ARRIVAL TIME, after proper processing in the computer of the recording truck. The horizontal sliding bar in the lowermost part of the Figure can be used to control the “velocity” ["v"] of the propagation of wavefronts.)
The elastic wave energy may be generated by dynamite blasts or by vibrators. The latter, most frequently applied, environmentally friendly method makes no any “wound” in the soil. (In practice, to increase the energy, groups of four perfectly synchronised vibrators are used.) The dynamite blasts may take place in shallow holes (not deeper than some multiples of 10 m) drilled along the survey lines.
The measurement of the reflections (the “echoes”) is carried out by small sensors (geophones) planted into the surface of the soil along the survey lines (several thousands, 2-5 meters apart). The geophones are small, nut-sized devices sensitive to the tiny vibrations of the earth (pricked into the soil by the attached spike). The vibrations of the soil measured by the geophones are transformed to electronic signals which are transferred to the truck mounted recording unit by the cables laid out provisionally on te surface along the survey line(s). The observed signals are recorded on magnetic tapes or DVD discs and further computer data processing transforms them into “time sections” revealing the subsurface geology. Promising structures – potential oil or gas reservoirs – are selected as targets for later deep drillings.