GB2096318A - Borehole logging system - Google Patents

Abstract

An acoustic borehole logging tool includes a transmitter and a plurality of receivers. Successive signals from each receiver are digitized in ADC 41, and routed to memory system 42 by muitiplexer 44. As further signals are presented the signal stored for the greatest length of time may be dropped. Multiplexers 45 and Adders 46-48 are used to average the signals in order to provide an improved signal to noise ratio. The averaged signals may be recorded (57) or converted to analog form (50) for further processing. <IMAGE>

Description

SPECIFICATION Borehole logging system This invention relates to a recording system for use with an acoustic logging tool.

In petroleum exploration, well logging techniques are used to determine the character of subsurface formations penetrated by a borehole.

One technique of logging employs a logging tool including a transmitter of acoustic pulses and a receiver, or receivers, for detecting the acoustic pulses after they have traversed the subsurface formations adjacent the tool. By measuring the traveltime, the amplitude change or the frequency change of an acoustic pulse from the transmitter through the formation to the receiver, an indication of the character of the subsurface formation is obtained.

There are three types of acoustic logging. The first type of acoustic velocity logging in which the time interval between the generation of each acoustic pulse and the detection of that pulse at a particular receiver is recorded. Successive time intervals are subtracted to provide an indication of the acoustic velocity of formations adjacent the logging tool. The variations in the acoustic velocity which are caused by the borehole medium as the acoustic pulses travel to and from the borehole tool are cancelled so that the measurements are dependent solely upon the character of the subsurface formations between successive pulse receiving points.

The second type of logging is acoustic amplitude logging in which the amplitude of the received pulse is measured. The change in amplitude of successive pulses, either from successive receivers or the same receiver at different depths gives an indication of the formation characteristics between the successive receivers or between levels for the same receiver.

The third type of logging is acoustic frequency logging in which the frequency changes in the pulses are measured. As with velocity and amplitude logging, the differences between successive pulses give an indication of the formation characteristics between the receiving points of the pulses.

Although these methods have been adequate for known logging systems in which the transmitter and receivers are closely spaced, problems are added when the receivers are separated by about 5 metres or more from the transmitter. The closely spaced transmitter/receiver unit provides excellent resolution when the acoustic pulses are not required to penetrate deeply into the formation surrounding the borehole, but as the borehole ages, the problems of mudcake and invasion tend to disguise readings based on shallow acoustic pulse penetration. To achieve deeper formation penetration, the spacing between transmitter and receiver is increased. Unfortunately, the increased spacing attenuates the pulses to the point where the signal-to-noise ratio becomes unfavorable.

The increased spacing between the transmitter and receiver may also add additional sources of acoustic noise as will be explained below.

We have now devised a system for improving the performance of acoustic logging systems, particularly long-spaced acoustic logging systems (LSAL systems). According to the present invention the received signals are digitized and a predetermined number of the successive digital signals are stored in a memory. The stored signals are than averaged to provide an output signal having an improved signal-to-noise ratio. The averaged signal is preferably updated by adding each new signal from the receiver or receivers to the memory and dropping the oldest signal from the memory.

Further features of the invention are described below with reference to the accompanying drawings in which: Fig. 1A illustrates a known borehole logging tool with which the recording system of the present invention may be utilized.

Fig. 1 B illustrates a long spaced acoustic logging tool.

Fig. 1 C illustrates the long spaced acoustic logging tool of Fig. 1 B in a borehole deviated 150 from vertical.

Figs. 2A-2G illustrate waveforms of signals generated by the logging tool of Fig. 1.

Fig. 3 is an electrical schematic of the recording system of the present invention.

Fig. 4 is an electrical schematic of a clocking circuit.

Fig. 1 A shows an acoustic logging system of the type described in U.S. Patent No. 3,302,166, in which, a downhole tool comprises a plurality of tranducers including a transmitter and multiple receivers. The borehole logging tool 10 is suspended within borehole 11 by means of logging cable 12. The receivers R1 and R2 are spaced at different distances from a transmitter T so that an acoustic pulse from the transmitter arrives at the receivers by way of different travel paths through the formation surrounding the borehole. A pulser circuit 13 energizes transmitter T to transmit high frequency acoustic pulses into the surrounding earth formation 14. For each acoustic cycle, the pulser 13 sends a transmitter trigger pulse uphole through cable 12. These acoustic pulses are detected by receivers R1 and R2.The signals produced by the receivers in response to the arrival of the acoustic pulse are also transmitted uphole by way of conductors within the logging cable. The logging system is provided with a downhole gating circuit 1 5 which permits the first acoustic pulse to be detected by receiver R1 and sent uphole through cable 12, the second acoustic pulse to be detected by receiver R2 and sent uphole through cable 12, and so on until each receiver has detected and sent an acoustic pulse uphole. The cycle is then repeated with successive acoustic pulses being detected and sent uphole by the receivers, alternately. The receiver gating circuit 1 5 also sends a receiver select signal uphole by way of cable 12, to indicate which receiver output is being gated at any given time.

The long spaced (LSAL) system 1 OA shown in Fig. 1 B has a transmitter T and receivers R1 and R2, each having its own compartment connected by a cable 12, with weights w and bottom weight W. These compartments are held in the center of the borehole 1 A by solid rubber centering arms R connected to weights w and W. The transmitter T is preferably separated from the receivers R1 and R2 by about 5 metres, and the receivers from each other by about 1.5 metres, however, other long spacings may be used. Two receivers are illustrated but a single receiver or more than two receivers separated by distances other than about 1.5 metres may be used.

In a perfectly smooth vertical borehole the LSAL tool will remain centered and the centering arms will slide noiselessly along the surface of the borehole. However, the borehole is never a perfectly smooth cylinder for its entire length due to washouts, loose formations etc. as illustrated in Fig. 1 B. An absolute vertical borehole is rare and may have a deviation from vertical as illustrated in Fig. 1 C. Thus, the centering arms will snap against the side of the borehole when sudden changes in the diameter occur. Furthermore, when a borehole deviates from vertical, the LSAL tool will lean toward the lower edge as illustrated in Fig. 1 C and occasionally come in contact with the side of the borehole creating spikes of acoustic noise at collision.The rubber centering arms generate acoustic noise which can be of such a magnitude that they are often read as acoustic signals generated by the transmitter. An additional source of noise in the recordings may come from asynchronous random electrical signals and will occur at various portions of the detected waveform. These types of noise cannot be filtered (highpass or lowpass) by conventional methods because much of the noise occurs in the same frequency band as the desired signal and filtering would mean the removal of information along with the noise.

As indicated in the description of Fig. 1 A, a pulser circuit 13 energizes transmitter T to transmit high frequency acoustic pulses into the surrounding earth formation 14. For each acoustic cycle, the pulse 1 3 sends a transmitter trigger pulse uphole through the cable 12. The acoustic pulses are detected by receivers R1 and R2.

Normally, the arrival of an acoustic pulse is measured when the amplitude of the signal is greater than a predetermined level such as a voltage level ET. During the first acoustic cycle, the receiver gating circuit 1 5 sends the detected signal from receiver R1 uphole by means of cable 12. During the next acoustic cycle, the receiver gating circuit 1 5 sends the detected signal from receiver R2 uphole by means of cable 1 2. These cycles are repeated with successive acoustic pulses being detected by the two receivers and the receiver outputs being selectively gated for sending the detected pulses uphole. The receiver gating circuit 1 5 also sends a receiver select signal uphole by way of cable 12, indicating which receiver output is being gated at any given time.For a more detailed description and operation of such a borehole logging tool, reference may be made to U.S. Patent No.

3,302,166. Typical examples of the signals received from a logging tool as described in U.S.

3,302,166, are illustrated in Figs. 2A-2C.

Fig. 2A illustrates the transmitter trigger pulse produced by the pulser 13 each time the transmitter T is fired which is sent uphole as a reference signal.

Fig. 2B illustrates the receiver select signal from the receiver gating circuit 1 5 which is sent uphole and identifies the receiver whose output is being sent uphole. Fig. 2C illustrates the selected receiver output which is being sent uphole.

These transmitter trigger pulses, receiver signals and receiver select signals are recorded by the recording system 20 as a function of the depth of the tool within the borehole. Driven by the logging cable 12 is a depth converter including a sheave 21 a and a shaft encoder 21 b that converts the mechanical rotation of the sheave to depth pulses. These depth pulses are recorded by the recording system 20 along with the transmitter trigger pulses, receiver signals and receiver select signals.

Figs. 2D through 2G illustrate the quality of receiver signals such as those from the long spaced acoustic logging system of Fig. 1.

Superimposed on these figures is a dashed line Et to symbolize the point at which the arrival of the pulse is triggered, where Et is a predetermined level of the signal.

Fig. 2D illustrates an ideai receiver output containing information concerning formation characteristic surrounding the wellhole with no noise content.

Fig. 2E illustrates a typical receiver output where a loud burst of acoustic noise has been generated prior to the receipt of the acoustic signal transmitted from the transmitter to the receiver.

Fig. 2F illustrates a typical prior art receiver output having a low level of noise content due to acoustic noise and non-synchronous random electrical noise.

Fig. 2G illustrates a typical prior art receiver output having a high level of acoustic noise.

In acoustic logging, the waveform from the receiver may be used in a variety of ways. In acoustic velocity logging, the time of arrival of the waveform is marked. The arrival time of two waveforms is compared to obtain the difference in arrival times. This may be done by either comparing the waveforms of two receivers that are spaced-apart on the well logging tool or by comparing the arrival times of two successive pulses from the transmitter to the same receiver at two different levels in the borehole. Waveforms such as those in Fig. 2E or 2G would indicate a false or early arrival time and give an erroneous indication of the formation properties of the well since the arrival time of the pulse would be triggered prior to the actual arrival of the signal.

In acoustic amplitude logging, the decrease in the amplitude of the transmitted acoustic pulse as indicated by the output of the receiver or receivers gives an indication of the formation properties of the well. In acoustic amplitude logging, the waveforms illustrated in Figs. 2E, 2F and 2G would give erroneous indications of the formation properties since Fig. 2E illustrates one peak and then the waveform apparently is reduced in amplitude to 0. Figs. 2F and 2G would give erroneous indications since in one case the noise reduces the amplitude of the waveform and in the second case the noise increases the amplitude of the waveform, both giving erroneous indication of the formation properties of the well.

Previously, when waveforms such as those illustrated in Figs. 2E, 2F and 2G were encountered, the data which was based upon these waveforms would not be used. Under normal circumstances where the oil well is deep and acoustic pulses were taken frequently, the neglected data would constitute a very small portion of the total data taken in the well log.

However, if there were a high incidence of the erroneous data, the well would either have to be logged a second time at great expense and effort, or the data would be essentially useless.

Fig. 3 illustrates, in block diagram form, the system of the present invention. In this illustration, receivers are used and each receiver signal is digitized and stored in memory. However, as indicated previously, a single receiver may be used and data at two different successive levels may be compared. A predetermined number of signals such as four or eight from each receiver is averaged. This average is updated by adding each new signal from the receiver and dropping the oldest signal within the predetermined number of signals. Statistical anaiysis for random noise occurring throughout the waveform indicates that signal-to-noise ratio is improved by vi; where n is the predetermined number of signals averaged.

The signal-to-noise ratio for non-synchronous random noise, such as acoustical noise typically encountered in a well and isolated to one of the nsignals or electrical noise encountered in the electrical cirCuits is improved by n where n is the predetermined number.

More particularly, the analog signal from the receivers R1 and R2, such as c shown in Fig. 2C, is applied by way of the sample and hold circuit 40 to the analog-to-digital converter 41. The converter digitizes each signal for 1,024 fivemicrosecond samples. The memory system 42 of the preferred embodiment stored the digital signal in the form of 8 bits per word, however, memories of 15 bits per word etc. may be used. Eight separate memories are illustrated within the memory system 42 of the preferred embodiment, four for the R1 digital signal and four for the R2 digital signal. However, it is to be understood that any number of memories may be used, such as eight or more memories per receiver.Thus, the total memory capability may be as small as four memories for a single receiver with improved signal-to-noise ratio or as many as thirty or more for three receivers with greatly improved signalto-noise ratios. In the illustrated embodiment, memories 1, 3, 5 and 7 store the R1 signals; and memories 2, 4, 6 and 8 store the R2 signals.

Strobing of the R1 and R2 signals into the appropriate memories is provided by the flip flop 39, divider 43 and multiplexer 44. The receiver select signal b of Fig. 2B is divided by four in the divider 43. Multiplexer 44, in response to the divider 43, permits the R1 and R2 digital signals to be successively strobed down the memory 42, with the R1 signals going successively into memories 1, 3, 5 and 7 and the R2 signals going successively into memories 2, 4, 6 and 8.

The four R1 signals from memories 1,3,5 and 7 are then simultaneously strobed out of memory by the multiplexer system 45 which includes a separate multiplexer for each of the four signals to be averaged. Such strobing by the multiplexer is under control of the receiver select signal t through conductor 55. The R1 signals from each of the four multiplexers are combined by way of adders 46-48 to provide for the R1 averaged digital signal on conductor 49. This may be accomplished by methods well known in the art, such as adding two digital numbers and dropping the least significant bit of information, adding four digital numbers and dropping the two least significant bits of information, etc.The averaged digital signal may be obtained at terminal 56 on conductor 49 to be recorded on digital recorder 57 for further processing at a later time. However, the averaged digital signal is normally converted to analog signal by the digital-to-analog converter 50 for further field recording and processing.

Similarly, the four R2 signals in memories, 2, 4, 6 and 8 are combined through the multiplexers 45 and adders 46-48 under control of the receiver select signal b to provide the R2 averaged digital signal on line 49. This signal is likewise converted to an analog signal for further recording and processing.

Each subsequent R1 and R2 signal received at the recording system is strobed into one of the memories 42 to replace the oldest R1 or R2 signal. In this manner, running averages of the four latest R1 and four latest R2 signals are provided. As previously pointed out, signal-to-noise ratio is improved by . Consequently, in the above-described embodiment, n equals 4 and the signal-to-noise ratio is improved by a factor of 2. Higher factors can be achieved by increasing the predetermined number n so that more R1 and R2 signals are averaged. Thus, by averaging four or more input signals received at close-spaced stations in the borehole a receiver output signal will approach that of the ideal signal of Fig. 2D. The random noise of Figs. 2F and 2G are reduced by a factor of v (a factor of two for the illustrated embodiment) as previously described. The asynchronous noise of Fig. 2E will be reduced by a factor of n (four for the illustrated embodiment) also as previously described. By reducing the value of noise, both electrical and acoustical, its effect on the data gathered is also reduced: the value of the noise is reduced to a level where it no longer triggers a time measurement nor increases or decreases the amplitude of the signal of interest nor adds signal frequencies disguising the frequency of the signal of interest.

Clock control for the recording system is provided by the circuitry illustrated in Fig. 4. This circuitry includes primarily the 4-megahertz oscillator 51 and the three clock dividers 52-54 which provide the clock pulses shown for control of the recording system of Fig. 3.

Various types and values of circuit components may be utilized. In accordance with the embodiment illustrated in Figs. 3 and 4, the following Table 1 sets forth specific types and values of circuit components.

Table 1 Reference Designation Description Sample and hold ' MN 346 (Micro Networks 40 Corp.) A/D converter 41 MN 5140 (Micro Networks Corp.) Memory 42 2533 (Signetics) Multiplexer 44 4051 (RCA) Multiplexer 45 741 57 (Signetics) Adders 46,47 4008 (RCA) and 48 D/A converter 50 7522 (Analog Devices) Oscillator 51 CMO-76 (Bliley Electric Co.) Divider 52 741 60 (Texas Instruments) Divider 53 7474 (Texas Instruments) Divider 54 4040 (RCA)

Claims (7)

Claims

1. A method for acoustic logging in which acoustic pulses are generated by a transmitter within a borehole and received by a receiver which is axialiy spaced from the transmitter within the borehole, characterized in that the signals from the receiver are digitized and a predetermined number of successive digitized signals are stored and averaged, at least one of the signals being dropped from storage and replaced by at least the next successive received signal for that receiver.

2. A method according to claim 1 in which the signal dropped from storage for each receiver is the signal which had been in storage the greatest length of time.

3. A method according to claim 1 or 2 in which a pair of receivers located at axially-spaced positions in the borehole are used, the signals from each receiver being separately digitized, stored, averaged, and dropped from storage.

4. An acoustic logging system having an acoustic transmitter and at least one receiver for producing successive analog electric signals representing the acoustic pulse detected by means of the receiver, characterized by a converter (41) connected to the receiver (R1, R2) for converting the analog signals to digital signals; a memory (42) connected to the converter (41) means for storing a predetermined number of digital signals; a memory controller (39, 43, 44) associated with the converter (41) and the memory (42) for controlling storage of the digital signals and selectively replacing the stored digital signals with additional digital signals; a multiplexer (45) connected to the memory (42) for reading a predetermined number of said digital signals; and an averaging circuit (46, 47, 48) connected to the multiplexer (45) for averaging the values of the predetermined number of said digital signals.

5. An acoustic logging system according to claim 4 characterized by a converter (50) connected to the averaging circuit (46, 47, 48) for converting said digital average value to an analog average value.

6. An acoustic logging system according to claim 4 or 5 characterized by a pair of memories (42-1, 3, 5, 7 and 42-2, 4, 6, 8) for each of a pair of separate receivers (R1, R2) and a storage selector (43, 44) for storing the signals for each receiver (R1, R2) separately in each of the pair of memories (42-1, 3, 5, 7 and 42-2, 4, 6, 8).

7. An acoustic logging system according to any of claims 4 to 6 characterized in that the memory controller (39, 43, 44) is adapted to replace the least recent one of the predetermined number of digital signals with a more recent digital signal.