[0001] The present invention relates to a method and apparatus for digitizing the maximum
amplitude and time occurrence of an acoustic signal and, more particularly, of an
acoustic signal produced in a borehole televiewing apparatus.
[0002] Borehole televiewing apparatus is commonly used to produce an image representing
the physical characteristics of the inside surface of a borehole. Typically, a borehole
transducer positioned within a borehole tool transmits an acoustic pulse into the
borehole in a scanning period. The acoustic pulse is reflected by the borehole wall
and received by a receiver/detector in the tool, which, in turn, transmits the envelope
of the reflected signal to a recording and/or imaging apparatus located at a surface
location. Incremental rotative movement of the transducer about the borehole for each
of a plurality of successive scanning periods produces reflection signals which can
be used to produce a representative image of the borehole wall structure, which may
be recorded and/or displayed on a cathode ray tube. Representative borehole televiewer
devices are described and illustrated in U. S. Patent Nos. 3,668,619 and 3,369,626.
[0003] In a borehole televiewing apparatus, an analog signal path is provided from the envelope
detector in the borehole tool to the surface recording and/or image display devices.
Analog signal transmission presents problems in that the attenuation characteristics
of the transmission lines limit the dynamic range of, and thus distort, the transmitted
signal, making it difficult to obtain high quality raw data at the surface equipment.
One way of improving the fidelity of the transmitted signal is to digitize it at the
borehole tool and transmit only digital data to the surface equipment. However, the
scanning frequency of the transmitter/receiving apparatus is typically quite high,
e.g., 2,000 scanning periods per second, and a high sampling frequency is also required,
e.g., 500 sampling pulses per scanning period, to faithfully reproduce a high quality
image at the surface. If 8 oits of data are required for each sample, then (8 bits)
x (
50
0 samples/period) x (2,000 periods/sec) = 8M bits/sec of data must be transmitted to
the surface, which considerably exceeds the capability of present digital wire pair
telemetry systems, making digitization of the detected reflection pulse signal impractical.
An object of the present invention is therefore to obviate or minimize this proolem.
[0004] Accordingly, tne invention resides in one aspect in a method for determining the
maximum amplitude and its time of occurrence of an analog signal comprising:
defining at least one time window within which said analog signal is expected to occur;
generating a timing signal representing elapsed time from a predetermined start time;
analog-to-digital converting and sampling said analog signal during said time window
to produce successive digital sample values representing the amplitude of said analog
signal;
comparing during said time window a present digital sample value S with a previously
stored digital sample value sn-x;
storing value Sn-x when said present sample value Sn is greater than said previously stored sample value; and,
storing the.present value of said timing-signal when a said present sample value Sn is stored as said previously stored sample value Sn-x.
[0005] In a further aspect, the invention resides in apparatus for determining the maximum
amplitude and its time of occurrence of an analog signal comprising:
means for defining at least one time window within which said analog signal is expected
to occur;
means for generating a digital timing signal representing elapsed time from a predetermined
start time;
means for analog-to-digitally converting and sampling said analog signal during said
timing window, to produce successive digital sample values representing the amplitude
of said analog signal;
means for comparing a present sample Sn value with a previously stored sample value Sn-x;
means responsive to said comparing means for storing a present sample value Sn as said previously stored sample value Sn-x when said present sample value Sn is greater than said previously stored sample value; and,
means responsive to said comparing means for storing the present value of said timing
signal when said present sample S is stored as said previously stored sample value.
[0006] In the accompanying drawings in which:
Fig. 1 is a signal amplitude vs. time diagram useful in explaining the invention;
Fig. 2 is a block diagram representing digitizing apparatus according to one example
of the invention;
Fig. 3 is a signal amplitude vs. time diagram useful in explaining a modified digitizing
apparatus;
Fig. 4 is a block diagram illustrating a borehole televiewing apparatus;
Fig. 5 is a block diagram representation of a first embodiment of surface apparatus
for reconstructing a properly timed analog amplitude signal;
Fig. 6 is an amplitude vs. time diagram of a reconstructed analog signal;
Figs. 7a, 7b and 7c are timing diagrams useful in explaining a second embodiment of
surface apparatus for reconstructing a properly timed analog amplitude signal;
Figs. 8a, 8b and 8c are signal formal diagrams useful in explaining the second embodiment
of the surface reconstruction apparatus;
Fig. 9 is a block diagram showing a modification of the Fig. 2 digitizing apparatus
for use with the second embodiment of the surface reconstruction apparatus; and
Fig. 10 is a block diagram representation of the second embodiment of the surface
reconstruction-apparatus.
[0007] Referring to Figs. 1 and 4, in a borehole televiewing apparatus an acoustic pulse
signal P (Fig. 1) is transmitted from a transducer 5 located in a borehole tool 1
(Fig. 4) by means of a pulse generator 3 at a time t
0, which defines the beginning of a scanning period t
0...t
0+1. The transmitted signal P is reflected by the borehole peripheral surface back to
the borehole tool as a reflected acoustic pulse signal R during the scanning period
and is detected by transducer 5. An additional echo signal R' generated from reflections
beyond the borehole wall also typically occur during the scanning period and it too
is detected by transducer 5. Other reduced amplitude miscellaneous reflection signals
and noise components R" may also be present in the signal received and detected by
transducer 5.
[0008] Both the maximum amplitude of the reflected pulse signal R, as well as the time of
its occurrence during each scanning period t
0...t
0+1, are digitized and transmitted to surface equipment 1
0 by a digitizing apparatus 7. The maximum amplitude and time of occurrence data are
sequentially transmitted to surface equipment 10 over conventional well logging transmission
lines 9 by means of a multiplexer 6 and data encoder 8, e.g., a manchester data encoder.
Apparatus at the surface receives the maximum amplitude and time of occurrence data
and reconstructs an analog signal therefrom, which can be used in a conventional borehole
televiewer display apparatus to construct an image of.:the borehole interior wall
structure.
[0009] Fig. 2 illustrates in greater detail the digitizing apparatus 7 for digitizing the
maximum amplitude and time of occurrence of the reflected acoustic pulse signal R.
The reflected acoustic pulse signal R detected by acoustic transducer 5, is applied
to the analog input of a high speed sampling analog-to-digital converter 11. A high
frequency sampling input signal consisting of typically 100ns pulses, is applied to
the sampling input of converter 11, while a window control signal is applied to an
enabling input of converter 11 from a programmable decoder 33. The window control
signal enables converter 11 and establishes a time window W (from time t
1 to time t
2 in fig. 1), during which the apparatus of Fig. 2 operates to "look for" the reflected
acoustic pulse signal R. The time window W is established where the reflected signal
R is expected to occur. The time of its beginning, as well as its duration, are empirically
determined to ensure that it encompasses the reflected acoustic pulse signal R. The
window W may be as wide as a scanning period, i.e., t
0...t
0+1, but as a practical matter, it is typically shorter, as shown in Fig. 1.
[0010] The window control signal is generated automatically a predetermined period of time
after an acoustic pulse P is transmitted into the borehole by the borehole tool, i.e.,
a predetermined period of time after t
0, for example, by a counter 31 counting a clock frequency f
c and feeding the programmable decoder 33 which decodes the output of counter 31 and
generates, at the appropriate time, the window control signal. The timing cycle for
operation of counter 31 and decoder 33 is the scanning period t
0...t
0+1. Accordingly, counter 31 is reset at the beginning of each scanning period by a signal
to generated thereat by the pulse generator 3. The programmable decoder 33 may be,
for example, a Harris HM-7616 PROM, or a programmable logic array. Decoder 33 produces
the window control signal and other output signals described below at predetermined
times, in accordance with program selection inputs thereto and the monitored output
of counter 31. To change the timing of the'window control signal and the other output
signals, the program selection inputs are changed. For convenience, the program selection
inputs to decoder 33 are controlled by the surface equipment 10 which interconnects
with decoder 33 by transmission lines 9 (Fig. 4) of a well logging cable. The timing
signals generated by decoder 33 include the window control signal, as well as latch
control and reset control signals discussed in greater detail below.
[0011] The digitizing apparatus also includes a digital comparator 15 which continually
compares a current amplitude sample value S from converter 11 (the A input to comparator
15) with a previously stored amplitude value S
n-x (the 8 input to comparator 15) stored in a latch 17. Wnenever the current sample
value S
n exceeds the previously stored value S
n-x (input A I input 6), the comparator applies an enabling signal to latches 17 and
19 which causes latcn 17 to store the current sample value S
n as the stored sample value, which is then used for comparison with subsequent current
sample values S occurring during the time window W.
[0012] As a result of the continued and combined operation of converter 11, comparator 15
and latcn 17, latch 17 will, at the end of time window W, contain an amplitude value
representative of the peak amplitude of reflected signal R. The time of occurrence
of the peak amplitude is determined by counter 21 which counts the output pulses from
an oscillator 23. Counter 21 is reset at the beginning of a scanning period by means
of the signal t
0, so that the counter 21 contains a value representing the instantaneous time (or
running time) following the time t
0. The output of counter 21 is applied to latch 19 and, whenever comparator 15 determines
that a present amplitude value sample S
n is to be stored in latch 17, it causes latch 19 to store the time of occurrence of
that sample value in latch 19. Accordingly, at the end of time window W, latch 19
contains time of occurrence information for the maximum amplitude sample value then
stored in latch 17.
[0013] At the end of time window W, the window control signal from decoder 33 is removed
from the enable input to converter 11, which effectively stops operation of the Fig.
2 circuit as no new sample values can be generated.
[0014] The data stored in latches 17 and 19 is stored for transmission to surface equipment
10 after the end of time window W by a latch control signal applied to latches 25,
27 which respectively receive and store for transmission the contents of latches 17
and 19. The latch control signal is generated by decoder . 33 after the transmission
of time window W.
[0015] Although Fig. 2 illustrates an apparatus in which a single time window W is employed,
the illustrated apparatus can also be used with a plurality of time windows W
1...W
n of, for example, the same duration within a scanning period t
0...t
0+1, as shown in Fig. 3. For operation with a plurality of time windows W
1...W
n, a different programming of decoder 33 is selected by the programming inputs thereto
so that suitable time window control, latch control and reset control signals are
generated for each timing window. Counter 21 need not be reset for each time window,
but is reset once for each scanning period t
0...t
0+1, at time t
0, as described earlier.
[0016] The use of successive time windows for a scanning time period t
0...t
0+1 allows for isolation and digitization of not only the maximum amplitude and time
of occurrence of the principle reflected signal R, but also of additional reflected
signal components, such as the echo signal R' and other later reflections R''' (Fig.
3), should digitization and transmission of these signals be desired.
- For transmission to surface equipment 10, the paired digital amplitude and time
of occurrence data stored in latches 25 and 27 are multiplexed by multiplexer 6 (Fig.
4) and encoded by an encoder 8, e.g., a manchester encoder, and then sent over transmission
lines 9. In the borehole tool, one bit of an encoded data word is encoded by encoder
8 as a flag bit in response to operation of multiplexer 6 to indicate whether the
encoded word contains amplitude data, e.g., a "1" flag bit encoding, or time of occurrence
data, e.g., a "0" flag bit encoding.
[0017] Fig. 5 illustrates the apparatus employed in surface equipment 10 for reconstructing
an analog signal having an amplitude and time of occurrence corresponding to the digital
amplitude and time of occurrence data produced by the Fig. 2 apparatus. The surface
apparatus includes a decoder 41, e.g., a manchester decoder which receives the encoded
data from the apparatus and which produces a conventional "valid word" output whenever
it recognizes that a valid encoded data word (either amplitude or time of occurrence
data) has been received. The "valid word" output of decoder 41 enables a flag detector
43, which examines a flag bit in the decoded word, to determine whether it contains
amplitude or time of occurrence information. The flag detector 43 loads the decooed
data word into a down counter 45 by enabling the counter 45 if the data word represents
time of occurrence data, and loads the data word into a latch 47 by enabling latch
47 if the data word represents amplitude data.
[0018] When loaded with a new data word, down counter 45 immediately begins deincrementing
tne loaoed count value by means of a clocking signal input thereto until a predetermined
count value, e.g., zero, is reached, at which time a one-shot multivibrator 49 is
enabled. The output of one-shot multivibrator 49 is used to enable a digital-to-analog
converter 51, which converts the digital amplitude data stored in latch 47 into an
analog signal value on output line 53. The reconstructed analog signal is shown in
Fig. 6. The analog signal generated on output line 53 has an amplitude value corresponding
to the maximum analog signal amplitude value digitized in the borehole tool, with
time of occurrence from a predetermined start time, e.g., t
o' corresponding to the time of occurrence of the maximum analog signal amplitude digitized
in the borehole tool. A characteristic of the Fig. 5 reconstruction apparatus is that
it is not necessary separately to transmit or receive information representing the
start time of a scanning period, e.g., t
0, as this information is implicitly contained in the time of occurrence data. That
is, whenever down counter 45 is loaded with new timing data it restarts a time period
which expires when counter 45 counts t
d a preset value, which represents the time of occurrence of the maximum amplitude
data signal from the scanning period start time t
0. The analog output on line 51 may be used oy a conventional borehole televiewing
apparatus to record and reconstruct an image representing the physical characteristics
of the inside surface of a borehole.
[0019] The reconstruction apparatus illustrated in Fig. 5 is intended to operate with one'time
window in a scanning period t
0...t
0+1, and cannot easily be adapted for use with a plurality of time windows. An alternative
reconstruction apparatus for use with one, two or four time windows during a scanning
period is shown in Fig. 10. The Fig. 10 apparatus is used in conjunction with the
data transmission format shown in Fig. 8, which will be described first.
[0020] The signal transmitted from the borehole tool-to-surface receiving equipment 10 for
use with the one, two or four window reconstruction of Fig. 10 has the data format
illustrated in Fig. 8. Each scanning period t
0...t
0+1 of, e.g., 500us, is divided into a frame of the same time period, e.g., 500us, containing
four equal duration data block D
1...D
4. Each data block contains paired maximum amplitude (A) data and corresponding time
of occurrence (T) data.
[0021] For a one window system, illustrated in Fig. 7a, the maximum amplitude and time of
occurrence data (A,, T
l) is repeated during transmission to the surface in each of the data blocks D
1...D
4. For a two window system, illustrated in Fig. 7b, the paired amplitude and time of
occurrence data (A
1, T
1) for the first window is repeated in the first two data blocks D
1 and D
2, while the same data for the second window (A
2, T
2) is repeated in the last two data blocks D
3 and D
4. For a four window system, illustrated in Fig. 7c, each data block D
1...D
4 contains pair amplitude and time values corresponding to respective windows, e.g.,
A
1,
Tl;
A2,
T2; A
3,
T3;
A41 T
4.
[0022] The data frames illustrated in Fig. 8 are formed by the apparatus illustrated in
Fig. 9, which is a modification of the Fig. 4 borehole tool apparatus 1. The data
frame is formed in a parallel to serial frame register 63. Sequential paired amplitude
and time data from encoder 8 are loaded into the various stages of register 63 by
way of loading gates 61. The program selection inputs to decoder 33, which select
the number-of timing windows in the Fig. 2 digitizing apparatus are used to operate
loading gates 61 so that one of the data formats illustrated in Fig. 8a, 8b or 8c
is created in frame register 63, corresponding to a surface selection of one, two
or four timing windows. The serial output of frame register 63 is applied to transmission
lines 9 for transmission to the surface apparatus illustrated in Fig. 10.
[0023] The Fig. 10 apparatus is designed to receive any one of the selected data formats
shown in Figs. 8a, 8b or 8c and reconstruct therefrom, the maximum amplitude and time
of occurrence data for each of the time windows used in the system. Incoming encoded
data is decoded by decoder 83 and then loaded frame-by-frame serially into a frame
receiver register 67. To simplify further discussion, it will oe assumed, for the
moment, that four windows have been selected by the programming inputs to decoder
33 and loading gates 61. The causes register 67 to have a loaded data format, wnerein
each data block D
1...D
4 contains paired amplitude and time data for a respective time window W
1...W
4. The parallel outputs of register 67 are applied to a data select circuit 69 having
ganged switches 71a and 71b. Switch 71a sequentially connects each of the amplitude
values A
1...A
4 to an output line 0
1, while switch 71b sequentially connects each of the timing values T
1...T
4 to an output line 0
2. Consequently, as switches 71a, 71b sequentially move, paired amplitude A and timing
T data are applied to output lines 0
1 and 0
2 of data select circuit 69. Switches 71a and 71b move in unison in response to a command
signal from sequencer 73, which is generated with a period of to/4, that is, four
times in a scanning period t
0...t
0+1. Assuming a scanning period of 500us, the command signals occur every 125µs. Thus,
during the frame time of 0-500µs data is available at the output lines 0
1' 0
2 of selector'69 as follows:

[0024] The command signals begin when sequencer 73 is informed by the output of frame detector
81 that a frame of data is present in register 67. To reconstruct both the maximum
amplitude and its time of occurrence for each of the four windows W
1...W
4, a comparator 71, time base counter 73, oscillator 85, coincidence gate 75 and digital-to-analog
converter 77 are employed. Time oase counter 73 is reset by a signal t
6 at the beginning of a scanning period of the signal reconstruction apparatus. This
scanning period is equal in duration to that used in the borehole tool, i.e., t
0...t
0+1, but its initiation need not coincide with that of the scanning period' in the borehole
tool. Once reset by signal t
B, time base counter 73 begins counting the output of oscillator 85. Comparator 71
receives the count value from counter 73 and compares it witn a timing value then
present at the 0
2 output line of data select circuit 69. Whenever comparator 71 detects a coincidence
between a timing value and the output of counter 73, it supplies an enable signal
to gate 75 which allows a maximum digital amplitude value A corresponding to the timing
value to pass to digital-to-analog converter 77, where the maximum amplitude value
is reconstructed at a proper time with respect to the beginning t
6 of a scanning frame in the reconstruction apparatus. For a four window system, the
paired digital values A
1, T
l; A
2, T
2; A
3, T
3; and A4, T
4 will be sequentially connected to the data selector 69 outputs by the command signal
and the maximum amplitude values A
1...A
4 will be reconstructed, in properly timed sequence in a scanning period by the timing
values T
1...T
4.
[0025] As noted, the Fig. 10 apparatus can also be used with a one or two window system
as well. For a one window system, each of the four data blocks loaded into register
67 will have the same data A
1, T
1, as shown in Fig. 8a. As a result, although the data select circuit 69 sequentially
applies the A
1 and T
1 data from each of the data blocks to output line 0
1 and 0
2, comparator 71 will still only provide one output signal when it determines coincidence
between T
1 and the output of counter 73, no matter which data block D
1...D
4 switch 71 is then examining, so that a maximum amplitude output properly timed with
respect to the beginning t
6 of a reconstruction scanning frame is produced by digital-to-analog converter 77.
[0026] Similarly, when two windows are employed and the transmitted data format for the
four data blocks D
1...D
4 is as shown in Fig. 8b, comparator 71 will recognize only two timing values T
1 and T
2 and will appropriately enable gate 75 so that corresponding maximum amplitude values
A
1 and A
2 will be applied at the proper time to converter 77 and reconstructed.
[0027] For proper operation of the Fig. 10 apparatus, counter 21 in the digitizing apparatus
is reset only at the beginning of a new scanning period, i.e., at t
0, so that each timing value T in a scanning period t
0...t
0+1 has a different value.
[0028] As is apparent, the Fig. 10 reconstruction apparatus is capaole of use in a system
employing one, two or four timing windows during a scanning period t
0...t
0+1. It should be apparent that other reconstruction apparatus could be easily devised
for other numbers of timing windows.
1. A method for determining tne maximum amplitude and its time of occurrence of an
analog signal comprising:
defining at least one time window within wnich saiJ analog signal is expected to occur;
generating a timing signal representing elapsea time from a predetermined start time;
analog-to-digital converting and sampling said analog signal during said time window
to produce successive digital sample values representing the amplitude of said analog
signal;
comparing during said time window a present digital sample value Sn with a previously stored digital sample value Sn-x;
storing value Sn-x when said present sample value Sn is greater than said previously stored sample value; and,
storing the present value of said timing signal when a said present sample value Sn is stored as said previously stored sample value Sn-x.
2. The method of.Claim 1, wherein said analog signal is a reflection signal caused
by the generation of an acoustic pulse scanning signal within a borehole and said
time window is defined at a time subsequent to generation of said scanning signal.
3. A method for reconstructing the maximum amplitude and time of occurrence of an
analog signal wnerein said maximum amplitude and time of occurrence are determined
by the method of Claim 1, comprising:
receiving said stored sample value representing tne maximum amplitude of the analog
signal occurring during said at least one time window;
receiving said stored present value of the timing signal representing the time of
occurrence of said maximum amplitude during said time window; and,
digital-to-analog converting the received sample value a predetermined period of time
after a reference start time, said predetermined period of time being determined by
the value of the received timing signal.
4. The method of.Claim 3, further comprising the steps of storing said sample value
after its receipt, loading said timing signal as preset data into a counter after
its receipt, said counter counting a clocking signal, and operating a digital-to-analog
converter which converts said stored sample value into an analog signal when.said
counter reaches a predetermined count value.
5. The method of Claim 3, further comprising the steps of storing said sample value
and said timing signal after their receipt, comparing said stored timing signal with
the output of a counter counting a clocking signal and generating a control signal
when the output of the counter has a predetermined relationship with respect to the
stored timing signal, and digital-to-analog converting the stored sample value when
said control signal is generated.
6. An apparatus for determining the maximum amplitude and its time of occurrence of
an analog signal comprising:
means for defining at least one time window within which said analog signal is expected
to occur;
means for generating a-digital timing signal representing elapsed time from a predetermined
start time;
means for analog-to-digitally converting and sampling said analog signal during said
timing window, to produce successive digital sample values representing the amplitude
of said analog signal;
means for comparing a present sample S value with a previously stored sample value
Sn-x;
means responsive to said comparing means for storing a present sample value Sn as said previously stored sample value Sn-x when said.present sample value Sn is greater than said previously stored sample value; and,
means responsive to said comparing means for storing the present value of said timing
signal when said present sample Sn is stored as said previously stored sample value.
7. The apparatus of claim 6 and further comprising:
means for digital-to-analog converting said stored sample value a predetermined period
of time after a reference start time, said predetermined period of time being determined
by said stored timing signal.