Seismic Exploration

techniques used in geophysical exploration, based on the study of the propagation characteristics of elastic (seismic) waves in the earth’s crust; such techniques are used to investigate the crust’s geological structure. Seismic exploration makes use of reflected and refracted waves and the piezoelectric effect. The use of reflected seismic waves was proposed by the American scientist R. Fessenden in 1913 and independently by the Soviet engineer V. S. Voiutskii in 1923; however, significant technical difficulties prevented realization of the technique until 1928–30. The simplest variation on the use of refracted waves, based on a design by the German geophysicist L. Mintrop (1919), was used in 1922–23; the modern form of this technique was proposed by the Soviet geophysicist G. A. Gamburtsev in 1939. The Soviet geophysicist M. P. Volarovich and others proposed the use of the piezoelectric effect.

Figure 1. Diagram of seismic exploration work using the reflection method: (1) seismic sensors, (2) seismic exploration station, (3) shooting point, (4) shot point, (5) direct wave, (6) reflected wave

The principal seismic exploration techniques are the reflection method and the refraction method; both make use of the difference in the elastic properties and density of rocks. In the reflection method a seismic wave excited by an explosion or by mechanical action propagates in all directions from the source and reaches certain reflecting interfaces in succession (see Figure 1). At each interface a reflected wave arises and returns to the earth’s surface, where it is registered by instruments. The reflection method makes it possible to study geological structure at depths of 0.1–0.2 to 7–10 km and to determine the depth of seismic boundaries with a precision to 1–2 percent. The method detects small angular inconsistencies, pinches, and sectors where facies change. The reflection method is the most precise and detailed method of studying sedimentary strata and is used primarily in the search for petroleum and natural gas; it also is used in studying certain ore deposits and in making regional geological studies.

The refraction method is based on the observation of waves refracted in a layer with an increased propagation velocity of seismic waves. After the initial refraction, the waves cover a significant part of their path in the layer, are refracted a second time, and then return to the earth’s surface (see Figure 2). The

Figure 2. Diagram of the formation of refracted waves: (1) direct and transmitted waves, (2) refracted head wave, (3) multiply refracted wave, (4) supercritical reflected wave

refraction method makes it possible to determine the position and shape of the surface of one or several such layers and to determine the propagation velocity at depths from a few meters to tens of kilometers.

Seismic exploration also makes use of the piezoelectric effect. In this technique the characteristics of the propagation of elastic waves are studied by observing the electromagnetic field the waves excite when they act on pegmatites and certain rocks (the piezoelectric effect). This technique permits detection of rocks that exhibit the piezoelectric effect to a significant degree.

Seismic exploration primarily makes use of compressional waves (P waves), whose velocities in rocks range from 0.4–0.5 to 7–8 km/sec. Shear waves (S waves) are rarely used because of the difficulty in exciting them; the velocities of S waves range from 0.1 to 5 km/sec. The frequencies of the recorded oscillations excited by seismic waves are 3–5 Hz and higher for deep studies; frequencies up to 150–250 Hz are encountered in studying shallow depths.

Seismic exploration is carried out on profiles, along which oscillation sources and sensors are placed at definite intervals. Explosions of charges in shallow wells (at depths of a few tens of meters) are used as sources of oscillations; mobile vibration or percussion units may also be used. Seismic sensors make measurements on the profile for each oscillation source. The sensors convert the mechanical oscillations of the soil into electrical oscillations, which are transmitted by connecting lines or relayed by radio to a mobile seismic exploration station. The oscillations from each sensor are amplified, converted, and recorded in order to produce a field magnetic seismogram. The distribution of the travel time of the wave on the profile makes it possible to determine the wave’s propagation paths, physical type, and other characteristics.

Geological information is extracted from the seismograms by computer processing, which produces seismogeological cross sections that indicate the position of seismic boundaries along the profile; the position is expressed either in the arrival time of seismic waves or in depths. Maps with isochronic lines are drawn from the cross sections. For correct geological interpretation of data from seismic exploration, it is important to have the most complete knowledge possible of the velocities of wave propagation in the cross section. Information on wave velocities can be obtained from data received by using the reflection method and, to some extent, the refraction method. Data from detailed, seismic observations in deep wells is particularly useful. Despite its high cost, seismic exploration is the most widespread technique used in geophysical exploration.

Seismic exploration is used to solve problems of structural geology, most often with the objective of finding structures favorable for the accumulation of petroleum and natural gas and preparing such structures for exploratory drilling; it is also used to predict the presence of oil and gas pools. Data obtained from detailed observations, especially by the reflection method, serve as the basis for selecting sites for deep exploratory petroleum and natural gas wells. Under complex geological conditions, when studying deep-lying structures and where there are strong interferences, the depth and reliability of exploration data are improved by combining seismic methods with structural drilling and making additional seismic observations in deep wells.

Exploration for petroleum and natural gas prospecting is also conducted with the aid of marine seismic exploration. Seismic exploration is used to study the structure of ore fields, detect and trace large faults, and determine the shape of the bedrock beneath detrital beds. The phenomenon of piezoelectric effect makes it possible to detect and identify the boundaries of pegmatite bodies and quartz veins. Seismic exploration techniques make it possible to study certain engineering properties of solid grounds and to determine the position of confining beds and the water table. Seismic exploration is combined with the other geophysical techniques, such as gravimetric, magnetic, and electrical exploration, to improve geological and economic effectiveness of the exploration work, especially in regional studies. This makes geological forecasts highly reliable. Seismic exploration makes it possible to study the regional abyssal structure of the earth’s crust down to the Mohorovičić discontinuity. Abyssal seismic sounding is used in this case.