Laser geophone

United States Patent [i«>]

Coon et al.

[ii] 4,284,350 [45] Aug. 18, 1981

[54] LASER GEOPHONE

[75] Inventors: Julian B. Coon; Bobby J. Thomas;

William L. Chapman, all of Ponca
City, Okla.; James C. Fowler, Burke,
Va.

[73] Assignee: Conoco, Inc., Ponca City, Okla.

[21] Appl. No.: 94,141

[22] Filed: Nov. 14, 1979

[51 ] Int. Q.3 G01P 3/36; G01B 11/02;

G01B 9/02

[52] U.S. CI 356/28.5; 73/657;

356/4.5; 356/356

[58] Field of Search 356/28.5, 356, 4.5;

73/653, 655, 657

[56] References Cited

U.S. PATENT DOCUMENTS

3,373,401 3/1968 Bayer 73/653

3,523,735 8/1970 Taylor 356/356

3,601,491 8/1971 Vaniz 356/4.5

3,863,064 1/1975 Doyle et al 455/605

4,026,655 5/1977 Gunler, Jr 356/28.5

4,185,503 1/1980 Saito 73/653

Primary Examiner—S. C. Buczinski
Attorney, Agent, or Firm—William J. Miller

[57] ABSTRACT

A method for measuring the velocity of earth surface motion utilizing coherent light energy which comprises steps of generating and directing a coherent light beam toward a selected earth surface position, reflecting said beam from a stationary reflector located at the earth surface position, and simultaneously reflecting said beam from a reflector at said earth surface position which moves with earth surface motion; and, detecting the reflected beam, including both the reflection path components, to derive an instantaneous difference frequency that is proportional to the velocity of the earth surface motion.

9 Claims, 6 Drawing Figures

LASER GEOPHONE

BACKGROUND OF THE INVENTION

1. Field of the Invention 5 The invention relates generally to seismic energy

detection devices and, more particularly, but not by way of limitation, it relates to a high sensitivity detection apparatus that utilizes laser beam energy for detection of the velocity of earth surface movement. 10

2. Description of the Prior Art

To the knowledge of this source the seismic art has not yet utilized light energy techniques for isolation of earth surface movement as caused by seismic energy vibrations. High frequency light energy, specifically the 15 laser and related devices, has been utilized for various velocimetric purposes where the particular application lends itself to the surrounding physical conditions and exigencies of the particular application. For example, Doppler frequency measurement techniques are well- 20 known in a number of radar-type applications as well as in certain measurement methods for fluid streams, turbulent layers in fluids and other forms of small amplitude vibration that occur in the various phases of nondestructive testing. In all of the applications known to 25 the present Applicant, such measurement schemes take the form of beam transmission and transition patterns wherein the transmitted beam traverses a different space or path than the reflected beam or beams prior to detection and readout such that variations in the tra- 30 versed matter will greatly affect the readout results as it relates to frequency and the attendant distance variations. That is, variations as to the travel paths of the high frequency light energy, e.g. as regards differences in the physical paths through air, constitution of the air 35 and heat effects, will more often than not result in a total degredation of the finite measurement qualitites of the energy. This would be especially true as regards the minute variations to be encountered relative to seismic energy disturbances. 40

The closest prior art encountered today is believed to be the U.S. Pat. No. 3,409,369 in the name of Bickel, a laser radar application wherein the source-target-detector path is nearly along the same traverse; however, this application utilizes a still further differential frequency 45 energy input in order to ascertain the velocity of moving objects under detection surveillance.

SUMMARY OF THE INVENTION

The present invention contemplates a method and 50 apparatus wherein laser generated light energy is utilized to more accurately ascertain seismic energy vibrations as detected in the earth surface at a selected point of detection. The method utilizes a different mode of detection than has heretofore been used with higher 55 frequency applications, and the method is particularly effective because of the fact that seismic energy detection responses are in a much lower frequency range than the known prior art applications of laser energy velocimetry. 60

The invention utilizes selected laser output as beamed from a source point to a selected earth site wherein it is desired to detect earth surface movement. Thereat, novel reflection equipment rigidly secured to the earth surface is capable of reflecting two components of en- 65 ergy, a first component indicative of earth movement and a second component indicative of comparison or reference energy that is not affected by earth surface

movement. Both energy paths are very nearly identical as they travel in juxtaposition from source to reflection to detection such that the external interfering components tend to cancel out, and the final detection is able to produce a true surface movement indication free of alt extraneous noise and destructive effects.

Therefore, it is an object of the present invention to provide a geophone structure utilizing coherent light energy that is extremely accurate and free from temporal variations due to the index of refraction of air such as may be caused by wind, temperature fluctuations, inhomogeneities in composition, etc.; thus, it eliminates apparent frequency shifts that affect the final movement readout.

It is another object of the invention to provide a seismic energy vibration detector that is extremely sensitive and accurate without undue attention to alignment and adjustment.

Finally, it is an object of the present invention to provide a seismic geophone utilizing laser energy that offers readout sensitivity and freedom from environmental interference far greater than present known laser velocimeter systems.

Other objects and advantages of the invention will be evident from the following detailed description when read in conjunction with the accompanying drawings which illustrate the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a general form of application of the seismic detector-laser source apparatus;

FIG. 2 is a side elevation in section of the geophone reflector apparatus of the present invention;

FIG. 3 is a front view in elevation of the seismic reflector of the present invention;

FIG. 4 is a top view of the rectangular spring member.

FIG. 5 is a top view section taken along line 4—4 of FIG. 2; and

FIG. 6 is a block diagram of one form of energy detection and processing circuitry.

DETAILED DESCRIPTION OF THE
INVENTION

As previously discussed, laser velocimetric systems are old in the art but these have generally taken the form of devices for use in very high frequency measurement applications resulting from velocities which are a factor of !06 to 10s larger than seismic velocities with which the present invention is concerned. Thus, these seismic velocities are generally two orders of magnitude smaller than the smallest measured velocities measured by the prior art systems.

One form of prior art system utilizes a laser beam source operating through a half-silvered beam splitter reflector to a stationary reflector with positioning of the test specimen perpendicular to the reference beam adjacent the beam splitter, and photo detection of energy change is effected on the opposite side of the beam splitter. Another form of prior art device utilizes a linear reference beam and an angularly deflected signal beam as deflected off the test specimen, both of the reference beam and deflected beam then being read by photo detectors to ascertain the Doppler shift and. subsequently, the speed of the movement of the vibrating object or specimen. In each case, the reference beams