Method for acoustic signal detection

Claim 1

A method of determining acoustic transit times in an earth formation traversed by a borehole, comprising: (a) disposing in a borehole a transmitter and first and second receivers spaced at different distances from the transmitter; (b) transmitting a pulse of acoustic waves from the transmitter; (c) receiving, at each receiver, a signal wave corresponding to the pulse of acoustic waves transmitted by the transmitter; (d) detecting, at the first receiver, the times of occurrence of an early positive and an early negative zero crossing in the received signal wave, said early positive and early negative zero crossings being selected responsive to the amplitude of the received signal wave; (e) detecting, at the second receiver, the times of occurrence of an early positive and an early negative zero crossing in the received signal wave, said early positive and early negative zero crossings being selected responsive to the amplitude of the received signal wave; and (f) determining the acoustic transit time from the times of occurrence of the earliest detected zero crossings at each receiver having the same polarity.

Claim 2

A method of determining acoustic transit times in an earth formation traversed by a borehole, comprising: (a) disposing in a borehole a transmitter and first and second receivers spaced at different distances from the transmitter; (b) transmitting a pulse of acoustic waves from the transmitter; (c) receiving, at each receiver, a signal wave corresponding to the pulse of acoustic waves transmitted by the transmitter; (d) detecting, at the first receiver, the times of occurrence of an early positive and an early negative zero crossing in the received signal wave, said early positive and early negative zero crossings being selected responsive to the amplitude of the received signal wave; (e) detecting, at the second receiver, the times of occurrence of an early positive and an early negative zero crossing in the received signal wave, said early positive and early negative zero crossings being selected responsive to the amplitude of the received signal wave; and (f) determining the acoustic transit time from the times of occurrence of the earliest detected zero crossings at each receiver having the same polarity.

Claim 3

A method of measuring transit times between spaced points in an earth formation traversed by a borehole, comprising: (a) lowering into the borehole an acoustic probe including a transmitter and first and second receivers spaced at different distances from the transmitter; (b) emitting successive pulses of acoustic waves from said transmitter to produce signal waves in said first and second receivers; (c) detecting when each successive signal wave in each receiver first attains a first amplitude and polarity selected for at least frequent attainment in an early half-cycle of the signal waves and producing signals representative of times of the next zero crossing in the signal received at each receiver; (d) detecting when each successive signal wave in each receiver first attains a second amplitude of greater magnitude than said first amplitude and of opposite polarity, selected for attainment in another early half-cycle and producing signals representative of the times of the next zero crossing in the signal received at each receiver; (e) comparing the zero crossing signals of step (c) with at least one previous measurement to determine whether the zero crossing signals lie within a range of acceptable values; (f) determining whether the zero crossings detected in step (c) at the first receiver occurred before the zero crossing detected in step (d) at the first receiver; (g) determining whether the zero crossings detected in step (c) at the second receiver occurred before the zero crossing detected in step (d) at the second receiver; and (h) recording a quantity representative of the difference in the zero crossing signals of step (c) as a function of the depth of the probe in the borehole if said signals are determined to lie within the range of acceptable values and both were determined in steps (f) and (g) to have occurred before the zero crossings detected in step (d) at the respective receivers.

Claim 4

A method of measuring transit times between spaced points in an earth formation traversed by a borehole, comprising: (a) lowering into the borehole an acoustic probe including a transmitter and first and second receivers spaced at different distances from the transmitter; (b) emitting successive pulses of acoustic waves from said transmitter to produce signal waves in said first and second receivers; (c) detecting when each successive signal wave in each receiver first attains a first amplitude and polarity selected for at least frequent attainment in an early half-cycle of the signal waves and producing signals representative of times of the next zero crossing in the signal received at each receiver; (d) detecting when each successive signal wave in each receiver first attains a second amplitude of greater magnitude than said first amplitude and of opposite polarity, selected for attainment in another early half-cycle and producing signals representative of the times of the next zero crossing in the signal received at each receiver; (e) comparing the zero crossing signals of step (c) with at least one previous measurement to determine whether the zero crossing signals lie within a range of acceptable values; (f) determining whether the zero crossings detected in step (c) at the first receiver occurred before the zero crossing detected in step (d) at the first receiver; (g) determining whether the zero crossings detected in step (c) at the second receiver occurred before the zero crossing detected in step (d) at the second receiver; and (h) recording a quantity representative of the difference in the zero crossing signals of step (c) as a function of the depth of the probe in the borehole if said signals are determined to lie within the range of acceptable values and both were determined in steps (f) and (g) to have occurred before the zero crossings detected in step (d) at the respective receivers.

Claim 6

A method of measuring transit times of acoustic waves between spaced points in an earth formation traversed by a borehole, comprising: (a) lowering into the borehole an acoustic probe including transmitter means, and first and second receivers spaced at different distances from the transmitter means; (b) transmitting a pulse of acoustic waves from said transmitter means; (c) receiving a signal wave corresponding to the acoustic pulse at each of the first and second receivers. (d) detecting the receipt of a half cycle of the signal wave at each receiver, having an amplitude greater than a predetermined threshold value and providing a time signal representative of the times of occurrence of the next zero crossing point of the signal wave received at each receiver following said detection of the receipts of the half cycles; (e) detecting a subsequent half cycle at each receiver having an amplitude greater than a second predetermined threshold value and providing a time signal representative of the times of occurrence of the next zero-crossing point of the signal wave received at each receiver; (f) comparing the time signal provided in step (d) with a previous measurement to determine whether the time signals provided in step (d) is a valid representation of the transit time between the receivers; (g) recording said time signal provided in step (d) as a function of the depth of the probe in the borehole only if step (f) determines that the time signal is a valid representation of the transit time between the receivers; and (h) recording the time signal provided in step (e) as a function of said depth of the probe in the borehole if the performance of step (f) does not determine that the time signal provided in step (d) is a valid representation of the difference in time between corresponding zero-crossing points.

Claim 7

A method of measuring transit times of acoustic waves between spaced points in an earth formation traversed by a borehole, comprising: (a) lowering into the borehole an acoustic probe including transmitter means, and first and second receivers spaced at different distances from the transmitter means; (b) transmitting a pulse of acoustic waves from said transmitter means; (c) receiving a signal wave corresponding to the acoustic pulse at each of the first and second receivers. (d) detecting the receipt of a half cycle of the signal wave at each receiver, having an amplitude greater than a predetermined threshold value and providing a time signal representative of the times of occurrence of the next zero crossing point of the signal wave received at each receiver following said detection of the receipts of the half cycles; (e) detecting a subsequent half cycle at each receiver having an amplitude greater than a second predetermined threshold value and providing a time signal representative of the times of occurrence of the next zero-crossing point of the signal wave received at each receiver; (f) comparing the time signal provided in step (d) with a previous measurement to determine whether the time signals provided in step (d) is a valid representation of the transit time between the receivers; (g) recording said time signal provided in step (d) as a function of the depth of the probe in the borehole only if step (f) determines that the time signal is a valid representation of the transit time between the receivers; and (h) recording the time signal provided in step (e) as a function of said depth of the probe in the borehole if the performance of step (f) does not determine that the time signal provided in step (d) is a valid representation of the difference in time between corresponding zero-crossing points.

Claim 8

The method of measuring transit times of 4 wherein the first and second predetermined threshold values are of different magnitudes, the second predetermined value having a greater magnitude than the first, and wherein the zero-crossing points of step (d) are positive zero-crossing points and the zero-crossing points of step (e) are negative zero-crossing points.

Claim 9

The method of measuring transit times of 4 wherein the first and second predetermined threshold values are of different magnitudes, the second predetermined value having a greater magnitude than the first, and wherein the zero-crossing points of step (d) are positive zero-crossing points and the zero-crossing points of step (e) are negative zero-crossing points.

Claim 10

An apparatus for measuring transit times of acoustic waves between spaced points in an earth formation traversed by a borehole, comprising: an acoustic probe including means for transmitting a pulse of acoustic waves and first and second receivers spaced at different distances from the acoustic transmitting means for receiving a signal wave corresponding to the acoustic pulses transmitted by the transmitting means; means for raising and lowering the acoustic probe within the borehole; means, connected to the first receiver, for detecting when the amplitude of the signal waves received by the first receiver attains a first preselected value during a negative half wave, for providing a time signal (A) representative of the next zero crossing point in the signal wave, for detecting when the amplitude of the signal waves received by the first receiver attains a second preselected value, higher than the first, during a positive half wave, and for providing a time signal representative of the next zero crossing point (B) in the signal waves; means, connected to the second receiver, for detecting when the amplitude of the signal waves received by the second receiver attains a third preselected value during a negative half wave, for providing a time signal (A') representative of the next zero crossing point in the signal wave; for detecting when the amplitude of the signal waves received by the second receiver attains a fourth preselected value, higher than the third during a positive half wave, and for providing a time signal representative of the next zero crossing point (B') in the signal waves; means for testing the validity of the zero crossing time signals following the first detected negative half waves (A and A') to determine their suitability for calculating transit time between the receivers; means for recording, as a function of the depth of the probe in the borehole a quantity representative of the difference between the zero crossing time signals following the first detected negative half waves (A'-A) when the testing means determines that said zero crossing signals are suitable; and for recording a quantity representative of the difference between the zero crossing time signals following the first detected positive half waves (B'-B) when the testing means determines that the zero crossing signals A' and A are unsuitable.

Claim 11

An apparatus for measuring transit times of acoustic waves between spaced points in an earth formation traversed by a borehole, comprising: an acoustic probe including means for transmitting a pulse of acoustic waves and first and second receivers spaced at different distances from the acoustic transmitting means for receiving a signal wave corresponding to the acoustic pulses transmitted by the transmitting means; means for raising and lowering the acoustic probe within the borehole; means, connected to the first receiver, for detecting when the amplitude of the signal waves received by the first receiver attains a first preselected value during a negative half wave, for providing a time signal (A) representative of the next zero crossing point in the signal wave, for detecting when the amplitude of the signal waves received by the first receiver attains a second preselected value, higher than the first, during a positive half wave, and for providing a time signal representative of the next zero crossing point (B) in the signal waves; means, connected to the second receiver, for detecting when the amplitude of the signal waves received by the second receiver attains a third preselected value during a negative half wave, for providing a time signal (A') representative of the next zero crossing point in the signal wave; for detecting when the amplitude of the signal waves received by the second receiver attains a fourth preselected value, higher than the third during a positive half wave, and for providing a time signal representative of the next zero crossing point (B') in the signal waves; means for testing the validity of the zero crossing time signals following the first detected negative half waves (A and A') to determine their suitability for calculating transit time between the receivers; means for recording, as a function of the depth of the probe in the borehole a quantity representative of the difference between the zero crossing time signals following the first detected negative half waves (A'-A) when the testing means determines that said zero crossing signals are suitable; and for recording a quantity representative of the difference between the zero crossing time signals following the first detected positive half waves (B'-B) when the testing means determines that the zero crossing signals A' and A are unsuitable.

Claim 16

A method of measuring transit times of acoustic waves between spaced points in an earth formation traversed by a borehole, comprising: (a) lowering into the borehole an acoustic probe including a transmitter and first and second receivers spaced at different distances from the transmitter; (b) generating successive pulses of acoustic waves in said transmitter to produce successive signal waves in said first and second receivers; (c) detecting when each successive signal wave in each receiver first attains a preselected condition and providing a time signal representative of the first zero crossing point thereafter in the signal wave; (d) providing an indication of the difference in times between the time signals representative of said first zero crossing points in the signal waves from the first and second receivers; (e) determining whether said indication of the difference in times provided in step (d) differs by less than a predetermined amount from a comparison quantity representative of a possible valid value for said indication of the difference in times; (f) recording a quantity representative of the indication of difference in times provided in step (d) as a function of the depth of the probe in the borehole if step (e) determines that said indication of difference in times differs by less than said predetermined amount from said comparison quantity; and (g) recording a second quantity representative of a possible valid value for said indication of difference in times as a function of the depth of said probe in the borehole if (e) determines that said indication of difference in times does not differ by less than said predetermined amount from said comparison quantity.

Claim 17

A method of measuring transit times of acoustic waves between spaced points in an earth formation traversed by a borehole, comprising: (a) lowering into the borehole an acoustic probe including a transmitter and first and second receivers spaced at different distances from the transmitter; (b) generating successive pulses of acoustic waves in said transmitter to produce successive signal waves in said first and second receivers; (c) detecting when each successive signal wave in each receiver first attains a preselected condition and providing a time signal representative of the first zero crossing point thereafter in the signal wave; (d) providing an indication of the difference in times between the time signals representative of said first zero crossing points in the signal waves from the first and second receivers; (e) determining whether said indication of the difference in times provided in step (d) differs by less than a predetermined amount from a comparison quantity representative of a possible valid value for said indication of the difference in times; (f) recording a quantity representative of the indication of difference in times provided in step (d) as a function of the depth of the probe in the borehole if step (e) determines that said indication of difference in times differs by less than said predetermined amount from said comparison quantity; and (g) recording a second quantity representative of a possible valid value for said indication of difference in times as a function of the depth of said probe in the borehole if (e) determines that said indication of difference in times does not differ by less than said predetermined amount from said comparison quantity.