Method and apparatus for acoustic impedance and P-wave anisotropy measurements
View Patent ↗Measurements of impedance are made using a piezoelectric transducer oriented at different angles to a formation bedding plane. The impedance measurements are then used to estimate the anisotropic velocity of the formation.
1. A method of evaluating an earth formation, the method comprising:
conveying a logging tool into a borehole;
exciting at least one transducer on the logging tool at a first orientation and a second orientation to the wall of the borehole;
using a first measured impedance of the transducer at a first resonant frequency at the first orientation and a second measured impedance at a second resonant frequency at the second orientation for estimating a ratio of a compressional velocity of the formation at the first orientation and a compressional velocity of the formation at the second orientation; and
using the estimated ratio of the compressional velocities for conducting further exploration of the formation.
2. The method of claim 1 further comprising measuring an impedance at an additional frequency close to at least one of: (i) the first resonant frequency, and (ii) the second resonant frequency to confirm an absence of any other resonance.
3. The method of claim 1 further comprising making a first calibration measurement and a second calibration measurement using the at least one transducer in a first medium having a known acoustic impedance and a second medium having a known acoustic impedance.
4. The method of claim 3 further comprising using the first calibration measurement and the second calibration measurement to estimate at least one of: (i) an acoustic impedance of the formation at the first orientation, and (ii) an acoustic impedance of the formation at the second orientation.
5. The method of claim 1 wherein making a measurement at a resonant frequency further comprises making a measurement at a plurality of frequencies and identifying the resonant frequency therefrom.
6. The method of claim 1 further comprising conveying the logging tool into the borehole on a wireline.
7. The method of claim 1 wherein the first orientation is in a vertical direction and the second orientation is in a circumferential direction, the method further comprising:
(i) measuring an impedance of the transducer at an additional angle between the first orientation and the second orientation,
(ii) measuring a shear velocity of the formation in the vertical direction and a circumferential direction; and
(iii) using the measured impedance at the additional angle, the shear velocity at the first orientation and the second orientation to derive a compressional wave velocity field in the earth formation and perform an anisotropic migration of a line of surface seismic data.
8. An apparatus configured to evaluate an earth formation, the apparatus comprising:
a logging tool configured to be conveyed into a borehole;
at least one transducer on the logging tool configured to be excited at a first orientation and a second orientation to the wall of the borehole;
a processor configured to:
use a first measured impedance of the transducer at a first resonant frequency at the first orientation and a second measured impedance at a second resonant frequency at the second orientation to estimate a ratio of a compressional velocity of the formation at the first orientation and a compressional velocity of the formation at the second orientation; and
use the estimated ratio of the compressional velocities for conducting further exploration of the formation.
9. The apparatus of claim 7 wherein the at least one transducer is further configured to make a first calibration measurement and a second calibration measurement in a first medium having a known acoustic impedance and a second medium having a known acoustic impedance.
10. The apparatus of claim 9 wherein the processor is further configured to use the first calibration measurement and the second calibration measurement to estimate at least one of: (i) an acoustic impedance of the formation at the first orientation, and (ii) an acoustic impedance of the formation at the second orientation.
11. The apparatus of claim 7 further comprising a wireline configured to convey the logging tool into the borehole.
12. A computer-readable medium having instructions thereon that when read by at least one processor cause the at least one processor to execute a method, the method comprising:
using a first measurement of an impedance at a first resonant frequency of at least one transducer in contact with a wall of a borehole at a first orientation and a second measurement of an impedance at a second resonant frequency of the at least one transducer at a second orientation to estimate a ratio of the first impedance and the second impedance; and
using the estimated ratio of the compressional velocities for conducting further exploration of the formation.
13. The computer-readable medium of claim 12 further comprising at least one of: (i) a ROM, (ii) an EPROM, (iii) an EEPROM, (iv) a flash memory, and (v) an optical disk.