Indirect shearwave determination

Claim 1

A method of estimating parameters of shear waves included in acoustic waves arriving sequentially at a plurality of points spaced vertically along the length of a borehole comprising the steps of: (a) generating acoustic energy in the borehole, (b) receiving the acoustic energy at each of the points after refraction, reflection and direct transmission through and along a formation adjacent the borehole, (c) establishing a window of known length with a known moveout between adjacent receiving points, (d) positioning said window along the composite wave relative to the estimated arrival of a selected guided acoustic wave, (e) multiplying the energy received at each point by the window, (f) generating the Fourier transform of that portion of received guided wave energy multiplied by the window, to produce a plurality of complex signals in the frequency domain, (g) determining the cross-spectral magnitude and phase for pairs of signals from adjacent receiving points, (h) computing the phase velocity α from the phase at each frequency, (i) scanning the cross-spectral magnitudes for a peak in a selected frequency range and identifying the frequency at which the peak occurs, (j) selecting from the phase velocity α versus frequency a value of guided wave phase velocity α s at the identified frequency, (k) estimating the ratio of Lame constants μ 2 and λ 1 in accordance with the expression ##EQU5## where A and γ are values found by matching two points on known dispersion curves for said guided wave where α, represents mud velocity, (l) obtaining an estimate of shear wave velocity β 2 in accordance with the expression ##EQU6## where ρ 2 is formation density and ρ 1 is mud density, and (m) plotting the estimate of β 2 as a function of depth.

Claim 2

A method of estimating parameters of shear waves included in acoustic waves arriving sequentially at a plurality of points spaced vertically along the length of a borehole comprising the steps of: (a) generating acoustic energy in the borehole, (b) receiving the acoustic energy at each of the points after refraction, reflection and direct transmission through and along a formation adjacent the borehole, (c) establishing a window of known length with a known moveout between adjacent receiving points, (d) positioning said window along the composite wave relative to the estimated arrival of a selected guided acoustic wave, (e) multiplying the energy received at each point by the window, (f) generating the Fourier transform of that portion of received guided wave energy multiplied by the window, to produce a plurality of complex signals in the frequency domain, (g) determining the cross-spectral magnitude and phase for pairs of signals from adjacent receiving points, (h) computing the phase velocity α from the phase at each frequency, (i) scanning the cross-spectral magnitudes for a peak in a selected frequency range and identifying the frequency at which the peak occurs, (j) selecting from the phase velocity α versus frequency a value of guided wave phase velocity α s at the identified frequency, (k) estimating the ratio of Lame constants μ 2 and λ 1 in accordance with the expression ##EQU5## where A and γ are values found by matching two points on known dispersion curves for said guided wave where α, represents mud velocity, (l) obtaining an estimate of shear wave velocity β 2 in accordance with the expression ##EQU6## where ρ 2 is formation density and ρ 1 is mud density, and (m) plotting the estimate of β 2 as a function of depth.

Claim 16

A system for estimating parameters of shear waves included in acoustic waves arriving sequentially at a plurality of points spaced vertically along the length of a borehole comprising the steps of: (a) means for generating acoustic energy in the borehole, (b) means for receiving the acoustic energy at each of the points after refraction, reflection and direct transmission through and along a formation adjacent the borehole, (c) means for establishing a window of known length with a known moveout between adjacent points and for positioning said window along the composite wave relative to the estimated arrival of Stoneley waves, (d) means for multiplying the energy received at each point by the window, (e) means for generating the Fourier transform of that portion of received Stoneley wave energy multiplied by the window, to produce a plurality of complex signals in the frequency domain, (f) means to determine the cross-spectral magnitude and phase for pairs of signals from adjacent points, (g) means to compute the phase velocity α from the phase at each frequency, (h) means to scan the cross-spectral magnitudes for a peak in a selected frequency range and identify the frequency at which the peak occurs, (i) means to select from the phase velocity α versus frequency a value of Stoneley phase velocity α s at the identified frequency, (j) means for estimating the ratio of Lame constant μ 2 and λ 1 in accordance with the expression ##EQU7## where α, represents mud velocity, and A and γ are value found by matching two points on known Stoneley wave dispersion curves, (k) means for obtaining an estimate of shear wave velocity β 2 in accordance with the expression ##EQU8## where ρ 2 is formation density and ρ 1 is mud density, and (l) means for plotting the estimate of ρ 2 as a function of depth.

Claim 17

A system for estimating parameters of shear waves included in acoustic waves arriving sequentially at a plurality of points spaced vertically along the length of a borehole comprising the steps of: (a) means for generating acoustic energy in the borehole, (b) means for receiving the acoustic energy at each of the points after refraction, reflection and direct transmission through and along a formation adjacent the borehole, (c) means for establishing a window of known length with a known moveout between adjacent points and for positioning said window along the composite wave relative to the estimated arrival of Stoneley waves, (d) means for multiplying the energy received at each point by the window, (e) means for generating the Fourier transform of that portion of received Stoneley wave energy multiplied by the window, to produce a plurality of complex signals in the frequency domain, (f) means to determine the cross-spectral magnitude and phase for pairs of signals from adjacent points, (g) means to compute the phase velocity α from the phase at each frequency, (h) means to scan the cross-spectral magnitudes for a peak in a selected frequency range and identify the frequency at which the peak occurs, (i) means to select from the phase velocity α versus frequency a value of Stoneley phase velocity α s at the identified frequency, (j) means for estimating the ratio of Lame constant μ 2 and λ 1 in accordance with the expression ##EQU7## where α, represents mud velocity, and A and γ are value found by matching two points on known Stoneley wave dispersion curves, (k) means for obtaining an estimate of shear wave velocity β 2 in accordance with the expression ##EQU8## where ρ 2 is formation density and ρ 1 is mud density, and (l) means for plotting the estimate of ρ 2 as a function of depth.