Narrow band acoustic source

Abstract

A seismic source for downhole use is disclosed which is formed by an outer tubular member having weighted ends which carries within a coaxially mounted tubularly shaped piezoelectric element. The piezoelectric element is cyclically driven to produce a standing wave having a frequency in the range of 0.25 to 5 kHz and a narrow bandwidth in the range of 5-50 Hz.

We claim: 
     
       1. An acoustic source comprising: a. a tubular member having a first end and a second end and a length L as measured between the first end and the second end;   b. a tubularly shaped piezoelectric element having a generally cylindrical outer surface, said tubularly shaped piezoelectric element being located within the tubular member such that the generally cylindrical outer surface of the tubularly shaped piezoelectric element is in contact with an inner surface of the tubular member along an entire circumference of the generally cylindrical outer surface of the tubularly shaped piezoelectric element and said tubularly shaped piezoelectric element is securely fastened to the inner surface of the tubular member, said tubularly shaped piezoelectric element having a length L which is in the range of 0.05 L to 0.5 L;   c. a means for electrically exciting the piezoelectric element operatively associated with said piezoelectric element;   d. a first end mass fastened rigidly to the first end of the tubular member; and   e. a second end mass fastened rigidly to a second end of the tubular member.   
     
     
       2. An acoustic source, as recited in claim 1, wherein the tubular member has a length, L, measured in feet between the first end and the second end which is related to a frequency of an acoustic signal to be produced, F R , measured in cycles per second by the relation:   L=v.sub.e /2F.sub.R     in which v e  is the velocity of an extensional wave in the tubular member, v e  being measured in feet per second.   
     
     
       3. An acoustic source, as recited in claim 2, wherein each of the first end mass and the second end mass comprises a cylinder having a mass in the range of 3-30 kg. 
     
     
       4. An acoustic source, as recited in claim 1, wherein the tubular member is comprised of steel and has a length of between approximately 7 ft. (2.1 m.) and approximately 10 ft. (3.05 m.), as measured between the first end and the second end of the tubular member. 
     
     
       5. An acoustic source, as recited in claim 1, wherein the tubular member is comprised of brass and has a length of between approximately 5 ft. (1.5 m.) and approximately 7 ft. (2.1 m.), as measured between the first end and the second end of the tubular member. 
     
     
       6. An acoustic source, as recited claim 1, wherein the tubular member is comprised of Lucite plastic and has a length of between approximately 2.5 ft. (0.76 m.) and approximately 4 ft. (1.2 m.), as measured between the first end and the second end of the tubular member. 
     
     
       7. An acoustic source, as recited in claim 1, wherein the tubularly shaped piezoelectric element is securely fastened to the inner surface of the tubular member at a location spaced apart from and equidistant from the first end and from the second end of the tubular member. 
     
     
       8. A method of generating an acoustic signal, said method comprising: a. positioning a tubular member in a fluid, said tubular member having a first end and a second end;   b. cyclically exciting a standing extensional wave having a resonance frequency F R  in the tubular member, the resonance frequency of the standing extensional wave being related to a length of the tubular member by the relation:   L=v.sub.e /2F.sub.R        where L is the length of the tubular member in feet, as measured between the first end and the second end, v e  is a velocity of the standing extensional wave in the tubular member, v e  being measured in feet per second, and F R  is the resonance frequency in Hz; and   c. emitting an acoustic signal from the tubular member.   
     
     
       9. A method as recited in claim 8, wherein the acoustic signal has a frequency between approximately 250 Hz and approximately 5 khz and a bandwidth between approximately 5 Hz and approximately 50 Hz, said acoustic signal having a radiation pattern controlled by a factor f(φ), wherein:   f(φ)=L sin φ/φ     L being the length of the tubular member measured in feet and φ being defined by the relation:     φ=p v.sub.e cos φ/2 v.sub.p     v e  being the velocity in feet per second of the standing extensional wave in the tubular member, v p  being the velocity of P-waves in a formation surrounding the fluid in which the tubular member is positioned and through which the acoustic signal will travel, and φ is a colatitude angle between the longitudinal axis of the tubular member and the direction of the radiation.   
     
     
       10. A method of generating an acoustic signal, as recited in claim 9, wherein the standing extensional wave is cyclically excited in the tubular member by cyclically driving a generally cylindrical outer surface of a tubularly shaped piezoelectric element radially outward against a generally cylindrical inner surface of the tubular member, said outer surface of the tubularly shaped piezoelectric element being securely fastened to an inner surface of the tubular member, a first node of the standing extensional wave being fixed by a first mass affixed to the first end of the tubular member and a second node of the standing extensional wave being fixed by a second mass affixed to the second end of the tubular member. 
     
     
       11. A method of generating an acoustic signal, as recited in claim 9, wherein the outer surface of the tubularly shaped piezoelectric element is cyclically driven radially outward against the inner surface of the tubular member a period of approximately 0.5 ms approximately twenty times per second. 
     
     
       12. A method of acquiring seismic data, said method comprising: a. positioning a tubular member in a first fluid-filled wellbore, said tubular member having a first end and a second end;   b. driving a generally cylindrical outer surface of a tubularly shaped piezoelectric element against an inner surface of the tubular member, said generally cylindrical outer surface of the tubularly shaped piezoelectric element being securely fastened to an inner surface of the tubular member;   c. exciting a standing wave in the tubular member, a first node of the standing wave being fixed by a first mass affixed to the first end of the tubular member and a second node of the standing wave being fixed by a second mass affixed to the second end of the tubular member, thereby emitting a high frequency, narrow band acoustic signal from the tubular member;   d. reflecting said high frequency, narrow band acoustic signal off an interface in a body of rock surrounding the first fluid-filled wellbore; and   e. receiving the high frequency, narrow band acoustic signal at a plurality of seismic receivers.   
     
     
       13. A method of acquiring seismic data, as recited in claim 12, further comprising recording one phase shift at each of said plurality of seismic receivers. 
     
     
       14. A method of acquiring seismic data, as recited in claim 12, wherein the plurality of seismic receivers are positioned in a second fluid-filled wellbore. 
     
     
       15. A method of acquiring seismic data, as recited in claim 12, wherein the plurality of seismic receivers are positioned in the first fluid-filled wellbore. 
     
     
       16. A method of acquiring high resolution seismic data, as recited in claim 12, wherein the high frequency, narrow bandwidth acoustic signal emitted is a signal whose bandwidth is between approximately 5 Hz and approximately 50 Hz and whose frequency is centered between approximately 250 Hz and approximately 5 khz.