Method for using electromagnetic grounded antennas as directional geophones

Abstract

A method of seismic prospecting using electromagnetic grounded antennas to detect electromagnetic waves that are produced from acoustic waves in the earth's formation. Seismic waves reflected by a formation in the earth are converted into electromagnetic waves in the vicinity of the antenna according to the streaming potential theory. The antenna has two electrodes which detect the horizontal component of the electromagnetic waves, thus providing additional seismic information that is not readily available using standard geophones. Antennas are also not subject to coupling problems and thus provide more accurate information than traditional geophones. For example, using multicomponent detection, all three components of the seismic pressure gradient can be detected. In addition, using geophones and antennas directly provides a method of separating source and receiver-generated static corrections for more effective stacking of seismic data and allows computation of seismic signal velocity through the low-velocity layer.

What is claimed is: 
     
       1. A method of seismic prospecting using electromagnetic grounded antennas, comprising the steps of: placing an antenna comprising at least two electrodes at or below the earth's surface;   initiating a seismic wave into the earth from a source location such that the downgoing seismic wavefront encounters a reflector and is reflected back to the surface as reflected seismic waves;   detecting electromagnetic waves generated by the reflected seismic waves in the vicinity of the antenna;   measuring the induced voltage across the antenna; and   analyzing the induced voltage to determine the horizontal component of the reflected seismic wavefront.   
     
     
       2. The method of claim 1, wherein said step of placing includes placing said two electrodes horizontally relative to the surface. 
     
     
       3. The method of claim 1, wherein said step of placing includes placing an array of antennas at or below the earth's surface. 
     
     
       4. The method of claim 1, wherein said antenna is placed so as to be spaced apart from said seismic source. 
     
     
       5. The method of claim 1, wherein said step of placing includes placing said antenna below the earth's surface. 
     
     
       6. The method of claim 1, wherein said source location is located below the earth's surface. 
     
     
       7. The method of claim 1, wherein said step of placing includes placing said antenna below the earth's surface; and wherein said source location is located below the earth's surface.   
     
     
       8. The method of claim 1, wherein said step of placing includes placing said antenna on the earth's surface. 
     
     
       9. The method of claim 1, wherein said source location is located on the earth's surface. 
     
     
       10. The method of claim 1, wherein said step of placing includes placing said antenna on the earth's surface; and wherein said source location is located on the earth's surface.   
     
     
       11. The method of claim 1, wherein said step of analyzing the induced voltage includes separating the voltage resulting from compressional and shear wave components of the reflected seismic wavefront by using the time of arrival of the electromagnetic waves. 
     
     
       12. The method of claim 1, wherein said step of analyzing the induced voltage includes separating the voltage resulting from electromagnetic waves generated by the reflected seismic wavefront in the vicinity of the antenna from the voltage resulting from electromagnetic waves generated by the reflector by using the time of arrival of the electromagnetic waves. 
     
     
       13. A method of determining seismic wave velocity, comprising the steps of: placing an antenna comprising first and second electrodes at or below the earth's surface, wherein said electrodes are positioned horizontally and said second electrode is placed at a selected distance from said first electrode;   initiating a seismic wave into the earth from a source location such that the downgoing seismic wavefront encounters a reflector and is reflected back to the surface as reflected seismic waves, wherein said reflected seismic waves generate electromagnetic waves in the vicinity of the antenna;   determining the time of arrival of said electromagnetic waves at said second electrode; and   determining the velocity of said reflected seismic waves by calculating the ratio of said distance between said first and second electrodes to the difference in times of arrival of said electromagnetic waves at said electrodes.   
     
     
       14. The method of claim 13, wherein said step of placing includes placing said antenna below the earth's surface. 
     
     
       15. The method of claim 13, wherein said source location is located below the earth's surface. 
     
     
       16. A method of seismic prospecting using electromagnetic grounded antennas, comprising the steps of: placing three antennas at or below the earth's surface, wherein said antennas are placed orthogonally relative to each other;   initiating a seismic wave into the earth from a source location such that the downgoing seismic wavefront encounters a reflector and is reflected back to the surface as reflected seismic waves;   said three antennas detecting electromagnetic waves generated by the reflected seismic waves in the vicinity of said antennas;   measuring the induced voltage across each of said antennas; and   analyzing the induced voltage to determine the X,Y, and Z components of the reflected seismic waves.   
     
     
       17. A method of seismic prospecting using electromagnetic grounded antennas, comprising the steps of: placing an antenna comprising first and second electrodes in the borehole of a well, the electrodes being spaced apart;   initiating a seismic wave into the earth from a source location;   detecting electromagnetic waves generated by the seismic wave by measuring an induced voltage across the antenna; and   analyzing the induced voltage to determine the component of the seismic wave which is parallel to the antenna.   
     
     
       18. The method of claim 17 wherein the first and second electrodes are vertically deployed in the well. 
     
     
       19. The method of claim 17 wherein the first and second electrodes are deployed in a horizontal array in the well. 
     
     
       20. The method of claim 17 wherein the first electrode is deployed in a first borehole and the second electrode is deployed in a second borehole.