Method and system for generating shear waves and compression waves in the earth for seismic surveying

摘要

A seismic source is disclosed which is particularly useful for generating horizontally polarized shear waves in the earth for seismic surveying. It comprises an elongated, hollow, substantially vertical cylinder having an earth-contacting pad pivotally mounted to its lower end by a horizontal pivot shaft extending parallel with the front-to-back centerline of a transport vehicle. The lower surface of this large rigid pad contains multiple, blade-like teeth extending parallel with the pivot shaft for gripping the earth's surface for transmitting powerful horizontally polarized shear stress impulses into the earth. The cylinder contains a slidable piston-like reaction mass. The bottom of the cylinder is a strong upwardly diverging conical surface serving to focus and direct upwardly the discharge blast from each port of an air gun extending horizontally and with the axis of the air gun being oriented parallel with the pivot axis and being located a short distance above the pivot shaft. Thus, the cylinder bottom is defined by the intersection of an elongated cylinder with a truncated cone and each air gun port is located near the center of the upwardly facing cone for focusing the blast upwardly toward the reaction mass. A small amount of water below the reaction piston mass and above the conical bottom surrounds the air gun ports. The air gun is selectively discharged to forcibly drive the earth-contacting pad down. By inclining the cylinder to the left of vertical or to the right of vertical while the pad seats on the earth with its parallel blade-like teeth gripping the earth's surface, the resulting sudden downward angled thrust is transmitted into the earth to generate horizontally polarized shear waves in addition to compression waves. The entire assembly is mounted on a transporter, such as a truck or log-skidder from which it may be lowered against the earth's surface as desired. The transporter carries a prime mover, an air compressor and receiver tank, and an air/water separator for the operation of the shear wave generator. A seismic survey method includes progressing from shot point to shot point along a seismic survey line, and at each shot point the cylinder is stood upright, is inclined to the left and is inclined to the right, in any desired sequence, and the air gun is fired with the cylinder in each of these attitudes for generating predetermined sequences and mixtures of compressional (P) waves, vertically polarized shear (SV) waves, left-handed horizontally polarized shear (SH) waves, and right-handed horizontally polarized shear (SH) waves thereby enhancing the ability to employ seismic energy transmitted into the earth from its surface for acquiring information about geological features in the earth.