Apparatus and method for determining the position of a gas-saturated porous rock in the vicinity of a deep borehole in the earth

Abstract

Apparatus for determining the position of a gas-saturated porous rock in the vicinity of a deep borehole in the earth, drilled to a selected depth below the expected position of the porous rock. The apparatus includes a source of seismic waves of selected characteristic, positioned at a selected point in the borehole, with an array of geophones positioned on the earth in known positions around the borehole. These geophones are adapted to receive the seismic waves generated by the source in the borehole, after traveling upwardly through the earth, and to produce corresponding facsimile electrical signals. If the seismic waves from the source, in their upward movement to the surface, pass through a gas-saturated porous rock they will be attenuated, and therefore the signals from the geophones which are in the shadow of this gas-saturated rock will show a change of character and/or an attenuation. If the shadow is observed while drilling at two or more known depths, the depth, azimuth, and radial position, from the borehole, of the gas-saturated rock can be determined. Using the principle of reciprocity, the same measurements can be made by interchanging the positions of seismic source and receiving sensor. A geophone lowered into an open borehole on a wire line will receive signals from a surface seismic source. Attenuations along certain downward ray paths indicate the shadow zones from which the depth, size, and position of a gas-saturated reservoir can be deduced.

Dependencies are inferred from the claim text.