Method and apparatus for determining acoustic wave parameters from acoustic well logging waveforms

Claim 3

The method of claim 1 wherein the composite wave incudes compressional wave components and shear wave components and a parameter being estimated is the slowness of said compressional wave and shear wave components.

Claim 4

The method of claim 2 wherein the windowing index begins at the earliest estimated arrival of the compressional wave and ends at the latest estimated arrival of sonic energy.

Claim 5

The method of claim 1 wherein the initial window moveout is 40 microseconds per foot and succeeding moveouts at each index position are at 20 microsecond per foot increments.

Claim 6

The method of claim 4 wherein the window is a Hanning function and said incrementing takes place at fifty microsecond increments along the length of the received energy.

Claim 7

The method of claim 1 wherein the parameter being estimated is slowness and those estimates obtained in step (g) and accepted are those falling within a predetermined range of the value of slowness represented by the window moveout associated with the estimate.

Claim 8

The method of claim 6 wherein the range is plus of minus ten microseconds per foot.

Claim 9

The method of claim 6 wherein the accepted estimates of slowness are clustered for each wave of interest and a single value of each cluster of estimates is obtained.

Claim 10

The method of claim 8 wherein said single values are plotted as a function of depth along the borehole to produce a slowness log.

Claim 11

The method of claim 1 wherein the estimates of parameters in accordance with step (g) are performed with a Prony estimation algorithm said algorithm yielding complex roots.

Claim 12

The method of claim 10 wherein the slowness is derived from the argument of said complex roots.

Claim 13

The method of claim 10 wherein the parameter estimated is signal attenuation and said attenuation is obtained from the absolute values of said complex roots.

Claim 16

The method of claim 13 in which step (j) is performed by clustering the estimates of step (i) as a function of slowness, frequency and time, and assigning to each cluster a value of slowness based on a weighted combination of the members of each cluster to obtain a best estimate of slowness.

Claim 17

An acoustic logging system for determining the acoustic slowness characteristics of waves propagated through an earth formation, said waves including compressional and shear waves, comprising: (a) an acoustic borehole logging tool having at least eight serially spaced acoustic receivers and at least one acoustic transmitter, (b) means for generating waveform outputs representative of the acoustic waves propagated through different lengths of substantially the same formation intervals and between different transmitter receiver pairs in response to each burst of acoustic energy from said transmitter, the waveforms being displaced in time according to their differential propagation times and rates through the formation interval, (c) means for establishing a window of predetermined length and moveout, (d) means for multiplying the waveform outputs by said window along a plurality of moveouts of said window taken at each of a plurality of spaced index points along the waveforms to produce windowed segments of said waveforms outputs, (e) means for generating a Fourier transform of said window segments to produce complex signals in the frequency domain, (f) means for computing from said complex signals for each window moveout and each index position values of slowness of propagation of said waveforms said means for computing including a model for wave propagation defining a superposition of acoustic wave propagating at different velocities, (g) means for accepting only those computed values of slowness falling within a predetermined range of the value of slowness represented by the window moveout associated with the computation, (h) means for selecting from said accepted values of slowness a single value of slowness for each mode of wave propagation, and (i) means for plotting each of the single values of slowness as a function of depth of said logging tool.

Claim 18

An acoustic logging system for determining the acoustic slowness characteristics of waves propagated through an earth formation, said waves including compressional and shear waves, comprising: (a) an acoustic borehole logging tool having at least eight serially spaced acoustic receivers and at least one acoustic transmitter, (b) means for generating waveform outputs representative of the acoustic waves propagated through different lengths of substantially the same formation intervals and between different transmitter receiver pairs in response to each burst of acoustic energy from said transmitter, the waveforms being displaced in time according to their differential propagation times and rates through the formation interval, (c) means for establishing a window of predetermined length and moveout, (d) means for multiplying the waveform outputs by said window along a plurality of moveouts of said window taken at each of a plurality of spaced index points along the waveforms to produce windowed segments of said waveforms outputs, (e) means for generating a Fourier transform of said window segments to produce complex signals in the frequency domain, (f) means for computing from said complex signals for each window moveout and each index position values of slowness of propagation of said waveforms said means for computing including a model for wave propagation defining a superposition of acoustic wave propagating at different velocities, (g) means for accepting only those computed values of slowness falling within a predetermined range of the value of slowness represented by the window moveout associated with the computation, (h) means for selecting from said accepted values of slowness a single value of slowness for each mode of wave propagation, and (i) means for plotting each of the single values of slowness as a function of depth of said logging tool.

Claim 19

The system of claim 15 wherein said receivers are spaced six inches apart to provide an unambiguous spatial frequency range of two cycles per foot.

Claim 20

The system of claim 15 wherein said window has a length of 500 microseconds, the index points are spaced 50 microseconds and said moveouts begin at 40 microseconds per foot.

Claim 21

The system of claim 15 in which said selecting means including means for clustering said accepted values of slowness, and means for assigning to each cluster a value of slowness based on a weighted combination of the members of each cluster to obtain said single values of slowness.

Claim 22

The system of claim 15 in said means for computing includes a model for wave propagation defining a superposition of acoustic waves propagating at different velocities.