Circumferential acoustic device

A method and apparatus for acoustical logging wherein a plurality of transducers are spaced circumferentially in a plane perpendicular to the borehole and in close proximity to the wall of the borehole. Numerous circumferential acoustic transmission paths are defined by using some of the transducers as transmitters and others as receivers. Ultrasound is transmitted and received separately along each of the paths and amplitudes of the received waves are separately recorded to provide a side-by-side comparison of the signals. The received waves may be comprised of shear waves and compressional waves, and the shear wave amplitude may be recorded to the exclusion of later arriving waves. Proper spacing of the transmitters and receivers will produce a complete circumferential acoustic log of the borehole that can be used to detect the presence of vertical fractures.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and apparatus for acoustical 
logging, and more particularly, to a method and apparatus utilizing 
ultrasonic energy to detect the presence of vertical fractures and 
anomalies in the earth formations surrounding a borehole. It is of 
considerable importance in the petroleum industry to detect vertical 
fractures, since many formations produce hydrocarbons through fractured 
reservoirs. Most well logging devices are designed for responding to 
horizontally extending anomalies and therefore are not suitable for 
locating these vertical fractures. 
Prior attempts at detecting the presence of vertical fractures generally 
have resorted to acoustic logging means. Typically, attempts have been 
made to produce acoustic waves that would travel circumferentially around 
the borehole to detectors where the acoustic waves would be received. Two 
types of acoustic waves are normally generated by the acoustic logging 
device, shear waves and compressional waves. Compressional waves will pass 
through a fluid filled fracture with little or no attenuation, while the 
shear waves, which are readily propagated only in solid materials, are 
significantly attenuated by the fluid filled fracture. Hence, in the prior 
art attempts, if a compressional wave, but no shear wave, was present, it 
was an indication that a vertical fracture was present in the formation. 
Prior attempts at detecting fractures in acoustic logging have failed in 
several respects. One approach, described in U.S. Pat. No. 2,934,694 
sought to direct shear waves along the borehole wall. Variations of the 
received shear wave were observed as an indication of the presence of open 
fractures. The logging device utilized continuous acoustic waves, which 
gave rise to special transmission and separation problems. Physical 
separation of the transmitter and receiver was used to separate the shear 
wave from the compressional wave, it being recognized that the shear waves 
travel at a lesser velocity than does an associated compressional wave. 
Since different formation materials may significantly alter the speeds of 
propagation of the two types of waves, the approach was not entirely 
successful. U.S. Pat. No. 3,585,580 describes a device which sought to 
utilize the shear wave energy by projecting the acoustic wave into the 
formation at such an angle of incidence that the shear wave energy would 
be maximized while the compressional wave energy would be minimized. 
Rather than using a continuous acoustic signal, a narrow ultrasonic beam 
was used to generate the acoustic waves in the formation. Under the 
assumption that the compressional waves were essentially nonexistent, 
gating circuitry was used to initiate the recording of the received 
acoustic wave shortly after its arrival at a detector and to continue 
recording the wave for a period of predetermined duration based on the 
average velocity of the shear wave through the transmission medium. While 
this approach made improvements over the prior art, it was not altogether 
successful, possibly due to the arrival of compressional waves 
notwithstanding the particular angle of incidence used, or possibly due to 
the recordation of waves other than the shear wave during the fixed period 
of recordation. Another approach, described in U.S. Pat. No. 3,794,976 
utilized substantially omni-directional transmitters to produce acoustical 
waves which travel through the formation about the circumference of the 
borehole. This device was directed particularly toward the detection of 
vertical fractues, using a particular angle of orientation of the 
transmitters with respect to the receivers to minimize the compressional 
wave effect. Thus, it was assumed that all waves received were shear 
waves. This approach suffered from some disadvantages, possibly due to the 
fact that the transmitting transducers were located a considerable 
distance from the surface of the borehole wall. In addition, some 
compressional wave energy was possibly received by the receiving 
transducers, resulting in erroneous interpretations. Still another 
approach utilized two sets of transmitting transducers in an attempt to 
detect vertical fractures. In the device described by U.S. Pat. No. 
3,775,739, one set of transmitting transducers was oriented to provide 
substantially compressional waves in the formation while another set of 
transmitting transducers was oriented to provide substantially shear waves 
in the formation. This approach was a considerable improvement over the 
single omnidirectional transducer in that the shear waves and 
compressional waves were separately produced and recorded. While this 
approach improved the results, it also included the additional 
complication of having two sets of transmitting transducers and two sets 
of separate receiving transducers in place of a single-transmitting and 
single-receiving device. 
Accordingly, it is an object of the present invention to overcome the 
difficulties of conventional systems for detecting vertical fractures in 
formations. 
Another object of the invention is to provide a method and apparatus for 
detecting vertical fractures from electrical signals produced solely by 
shear waves transmitted through the formations adjacent the borehole. 
Another object of the invention is to provide improved electrical circuitry 
for separating the shear waves from any later arriving compressional 
waves. 
Another object of the invention is to provide improved electrical circuitry 
for detecting and recording guided fluid waves transmitted 
circumferentially about the borehole wall. 
Another object of the invention is to provide novel electrical circuitry by 
which only one transmitter may become operative during any one time 
period. 
Still another object of the invention is to provide novel electrical 
circuitry whereby parts of each or all of the shear waves and 
compressional waves resulting from each acoustic wave produced may be 
recorded for interpretation. 
SUMMARY OF THE INVENTION 
The present invention attains the above objectives by providing a 
circumferential acoustic logging system which utilizes a plurality of 
transducers functioning as transmitters and receivers to transmit short 
bursts of acoustic energy into the well fluid and the formation 
surrounding a wellbore and to receive the resultant acoustic waves after 
they have travelled in the formation and well fluid in the form of shear 
waves and compressional waves. The transducers are mounted on long, 
extendable arms of the logging tool and are arranged circumferentially in 
a plane which is perpendicular to the borehole. The arms may be 
mechanically extended so that the transducers are placed in close 
proximity with the wall of the borehole to insure proper transmission and 
reception of the acoustic waves. The transducers are so constructed that 
circumferentially travelling waves are propagated which allow the 
detection of vertical fractures and anomalies in the underground 
formations. The mechanical design of the novel acoustic logging device and 
the design of the transducers has been fully set forth in U.S. Pat. No. 
4,130,816, issued to Charles B. Vogel, et al. 
At least two transmitters and two receivers mounted on the logging tool are 
arranged alternately about the circumference of the borehole so that four 
transmission paths are formed which define the circumference of the 
borehole. During one measurement cycle, acoustic energy is transmitted 
over each of these transmission paths in a predetermined order. A wave 
generator is utilized to synchronize the operation of the transmitters and 
receivers, and four distinct measurement intervals result during each 
measurement cycle. For instance, during the period of operation of the 
first receiver, the first and second transmitters are alternately actuated 
in succession, thus transmitting acoustic energy over two of the 
transmission paths, and during the period of operation of the second 
receiver, the first and second transmitters again are alternately actuated 
in succession, thus transmitting acoustic energy over the remaining two 
transmission paths. Hence, the acoustic waves received and transmitted to 
the surface substantially completely traverse the circumference of the 
borehole at a particular depth. Actuation of a transmitter out of sequence 
due to cross-coupling of electrical signals is prevented by a novel 
latching circuit associated with each transmitter which insures that, 
while one transmitter is actuated and during a short time interval 
immediately following its actuation, no other transmitter may become 
actuated. The acoustic waves that arrive at the receivers are converted to 
their electrical equivalents and transmitted to the surface where the 
signals are processed. Improved electronic circuitry at the surface allows 
selective recording and display of the shear waves and compressional waves 
comprising the acoustic waves. A novel delay circuit is provided to allow 
selective recording of the entire shear wave to the complete exclusion of 
any later arriving wave, since the shear wave is particularly sensitive to 
vertical fractures and anomalies in the formation through which the wave 
is propagated. 
To facilitate the location of vertical fractures and anomalies, electronic 
circuitry may further process the shear waves for each of the four 
transmission paths, so that a single indication of presence or absence of 
a vertical fracture may be computed for each particular measurement cycle.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is shown a cross-sectional view of a typical 
borehole with the transmitters and receivers of the invention in place 
adjacent the borehole wall. The transmitters radiate compressional wave 
acoustic energy 146 of an onmidirectional character in the azimuthal 
direction into the well fluid filling the borehole. Thus, appreciable 
energy is radiated in the directions of the critical angles of refraction 
so that the compressional wave in the fluid impinges on the borehole wall 
so as to produce shear waves in the formation 145. The transmitters and 
receivers alternately placed about the circumference of the borehole 
define four quadrant paths Q1, Q2, Q3 and Q4 along which the acoustic 
energy is transmitted. The transmitters and receivers are sequentially 
switched so that each transmitter-receiver pair becomes operative during 
each measurement cycle. The acoustic waves that are developed in the 
formation 145 by the propagation of acoustic energy from transmitters T1 
or T2 are broadly classified as shear waves and compressional waves. 
However, experimentation has indicated that these waves may be further 
classified by their order of appearance at the receiver of the 
transmitter-receiver pair which is operative at the particular time. The 
first-arriving wave is a compressional wave which has been propagated 
through the formation. This compressional rock wave, as it is referred to, 
is of such small amplitude that it contributes negligibly to the amplitude 
of the received signal. The next-arriving wave is a wave which has 
travelled as a compressional wave from the transmitter through the well 
fluid, thence along the borehole wall as a refracted, diffracted shear 
wave 141, thence to the receiver as a compressional wave in the fluid. The 
path 147 indicates the direction of travel of shear wave 141 as dictated 
by Snell's Law. However, the arcuate nature of the borehole wall 143 
causes a circumferential propagation. Because the wave length of the 
refracted shear wave is relatively long, for example one inch, the wave 
spreads sufficiently to provide appreciable detectable energy along this 
circumferential path. The third wave to arrive is a compressional wave 
that is thought to travel directly through the borehole fluid to the 
receiver, which is designated a direct fluid wave. The fourth wave 
identified at the receiver is a compressional wave 140 propagated along 
the wall 143 with a velocity approximately equal to the propagation 
velocity of the well fluid. Transmission of this compressional "guided 
fluid" wave 140 and the shear wave 141 are impaired by an open fracture 
142. In the case of shear wave 141, the fact that the fracture is filled 
with wellbore fluids does not significantly affect the degree of 
attenuation. However, the guided fluid wave 140 exhibits a high degree of 
attenuation when travelling along the wall past a fluid filled fracture 
only if the fracture is of sufficient width to cause "leakage" of the wave 
pressure into the open fracture. Hence, the shear wave 141 and the guided 
fluid wave 140 are of particular significance in determining the presence 
of vertical fractures, while the intermediately arriving direct fluid wave 
is not essential to this objective. 
Referring now to FIGS. 2a and 2b, the surface electronic equipment and 
subsurface electronic equipment of the invention are shown in block form, 
separated by the dashed line designating the earth's surface and 
interconnected by cable 1 and the plurality of conductors 151 and 152. The 
logging tool 45 is suspended in borehole 74 by way of suspension cable 1. 
The logging tool includes two acoustic energy transmitters T1 and T2 and 
two acoustic wave receivers R1 and R2 disposed in a path concentric with 
the wall of borehole 74. The transmitters and receivers are spaced 
alternately at 90-degree intervals about the circumference so as to form 
four separate transmission paths, as previously mentioned in FIG. 1. In 
accordance with the invention, one or more of a plurality of measurements 
between transmitters and receivers in the logging tool are taken during a 
complete measurement cycle. With the transmitter-receiver arrangement 
shown, four separate measurement intervals are possible during the 
measurement cycle, one for each quadrant Q1-Q4 as indicated in FIG. 1. 
Quadrant Q1 of FIG. 1 represents the path taken during actuation of T1-R1, 
Q2 represents the path taken during actuation of T2-R1, Q3, the path 
during actuation of T1-R2, and Q4, the path during actuation of T2-R2. It 
is therefore apparent that, during any measurement cycle, measurements are 
made which may completely define the circumference of the borehole. 
Because of the attenuation of the circumferentially propagated shear waves 
by the presence of vertical fractures in the surrounding formations, such 
a vertical fracture at any depth at which the circumferential acoustic 
logging system is operative may be detected. 
The cable 1 is of the armored type, extending from the upper end of the 
tool to the surface of the earth. The cable is spooled on a winch 100 
known in the art, operation of which serves to raise and lower the tool 
through the wellbore. Known depth measuring means, not shown, are provided 
to indicate tool depth. Cable 1 may contain a plurality of conductors 151 
and 152 for providing conducting paths for electrical signals between the 
surface equipment and the subsurface apparatus, as well as to supply 
electrical power from a power source at the earth's surface to the 
subsurface apparatus. 
Overall system synchronization is provided by a wave generating source 
comprised of an alternating wave generator 20 and a synchronizing pulse 
generator 21. The power for the wave generating source may be commercially 
supplied or from separate generators. Power is conducted from its source 
at the surface to the surface equipment, and via suitable conductors 152 
in cable 1 to the subsurface equipment. 
Alternating wave generator 20 provides a reference frequency for selecting 
the proper transmitter-receiver pairs during each measurement interval of 
a measurement cycle. The alternating wave frequency is chosen to provide 
measurement intervals of sufficient duration to allow adequate time for 
transmission to the surface of the received acoustic waves. For instance, 
a 50 c.p.s. sine wave provides a measurement cycle of 20 ms., the duration 
of one period of the sine wave. For the transmitter-receiver arrangement 
shown, this measurement cycle is further subdivided so that four 5 ms. 
time periods result, a single measurement interval occurring during each 5 
ms. period. Synchronizing pulse generator 21 takes the sine wave from 
generator 20 as its input to produce at its output a series of pulses in 
synchronization with the 50 c.p.s. sine wave. If receiver R1 is actuated 
during the positive half cycle of the sine wave, and R2 is actuated during 
the negative half cycle, pulse generator 21 may then be used to 
synchronize the operation of the transmitters with the operation of the 
receivers by producing a train of alternating positive and negative-going 
pulses. Two pulses, a positive and a negative-going pulse, are produced 
each half-cycle of the sine wave. The positive-going pulse may be used to 
actuate transmitter T1 and the negative-going pulse may be used to actuate 
transmitter T2. The synchronizing pulses are spaced at 5 ms. intervals so 
that each pulse initiates a measurement interval, resulting in four 
measurement intervals per measurement cycle. The transmitter-receiver pair 
for each quadrant is therefore operated once during a complete measurement 
cycle. The center frequency for the synchronizing pulses is chosen 
sufficiently high so that several cycles of the shear wave may be received 
before the arrival of the guided fluid wave. For borehole diameters 
greater than eight inches and for rock p-wave velocities exceeding 20,000 
ft/sec., 129 kHz is a good frequency. For larger borehole diameters, 
better results will be obtained at a lower frequency. For small diameters 
or lower rock velocities, a higher frequency would be desirable. 
Electrical equivalents of the acoustic waves received at receivers R1 and 
R2 are transmitted to the surface via one of the conductors in the group 
of conductors 151. These electrical equivalents are supplied to an 
acoustic signal processor 50 for processing and to an analog tape 
transport 85. The tape transport records the received electrical signals, 
for instance on magnetic tape, for later analysis of the data received. 
The first stage of the acoustic signal processor 50, the acoustic signal 
amplifier and pre-amplifier 28 is continuously responsive to the 
electrical equivalents received from the sub-surface equipment. The 
electrical equivalents are subsequently passed through a delay circuit 30 
switchedly connected to the amplifier and pre-amplifier 28, the delay 
circuit being utilized to process the shear wave signal to the exclusion 
of later arriving waves during a single measurement interval. The delay 
circuit of the invention is utilized in a novel way to allow selection of 
the complete shear wave signal in each instance. Prior art systems have 
used delay circuits or similar concepts to interpose a fixed time delay 
usually based on the average velocity of an acoustic wave through the 
medium of interest to either initiate or terminate the recording of a 
particular acoustic wave. For instance, in U.S. Pat. No. 2,691,422, means 
were used to block the acoustic wave from further processing after a fixed 
period of time based on the average velocity of a compressional wave in 
the borehole media. In general, the prior art systems have relied on this 
average velocity concept which often introduces erroneous data due to 
inherent variations in the acoustic wave velocities. As will be explained 
more fully in the detailed circuit description following, the concept 
being used in the present invention is a different one altogether. 
The next component of interest in the acoustic signal processor 50 is 
window switch 41, made operative by window, caliper, and velocity signal 
generator 80 which insures that only the signals of interest for detection 
of vertical fractures during the particular measurement interval are 
further processed by processor 50. Window, caliper, and velocity signal 
generator 80 operates in synchronization with the alternating wave from 
generator 20 and the synchronizing pulses from pulse generator 21 to 
provide the proper period of operation for switch 41. The portion of the 
signal that is passed by switch 41 is further processed by rectifier 46, 
peak detector 49, and sampling switches 51 so that chart recorder 60 
receives a single interpretable signal for each measurement interval of a 
measurement cycle. 
Demultiplexer pulse generator 65 and window trigger generator 75 provide 
outputs in the form of electrical pulses of predetermined duration which 
are supplied to window, caliper, and velocity signal generator 80 which 
provides "gating" pulses for the operation of switch 41. Generator 65 uses 
as its input the sine wave from alternating wave generator 20 to divide 
the half-cycle of the sine wave into two parts of pre-determined duration. 
Trigger generator 75 uses as its input the series of pulses from pulse 
generator 21 to provide a train of electrical pulses of longer duration, 
which pulses are used for the initiation of the measurement intervals 
which occur during a measurement cycle. 
After the electrical equivalents of the received acoustic waves are fully 
processed, the resultant electrical response may be displayed in various 
ways. Multichannel chart recorder 60 allows separate display of the 
processed electrical response of each measurement interval, so that each 
quadrant of the wall of the borehole which was traversed during the 
measurement cycle is represented. In addition, since all unprocessed 
signals have been recorded by the analog tape transport 85, visual display 
of the complete waveform received during each measurement interval is 
possible by playback of the recorded signals over the appropriate 
equipment, such as multichannel chart recorder. Various conventional means 
may also be used to compute a single signal such as a minimum peak signal 
from the four measurement intervals of a measurement cycle to be displayed 
versus depth on a conventional well log, the peak signal being indicative 
of the presence or absence of a vertical fracture at the depth recorded. 
Likewise, an average of the peak signals of the four measurement intervals 
may be computed and displayed on a conventional well log. A particularly 
useful means of displaying the processed electrical responses involves the 
production of a well log for each quadrant being investigated. FIG. 13 
indicates the responses from quadrants Q1, Q2, Q3 and Q4 of a typical well 
over approximately 100 feet of the borehole depth. Parts a, b, c and d 
represent the peak amplitude of the shear wave received for quadrants Q1, 
Q2, Q3 and Q4, respectively, displayed versus depth. Parts e, f, g and h 
represent the peak amplitude of the guided fluid wave received for 
quadrants Q1, Q2, Q3 and Q4, respectively, displayed versus depth for the 
same 100-foot interval. Each measurement cycle would correspond to a 
single point, such as is shown at P in parts a through h. A succession of 
the amplitudes at each such point results in the well logs as shown. When 
the amplitude decreases at a certain point, such as at A, B, C, D or E, 
vertical fracturing at that point is indicated. These deflectons appear 
differently in quadrants Q1 through Q4 and exhibit sufficient vertical 
continuity not to be produced by noise. Sharpness of the deflections 
indicates a high degree of vertical resolution. It may be noted that both 
the guided fluid wave response and the shear wave response exhibit 
deflections of a decreasing amplitude at the same points of the respective 
quadrants. This is further indication of the system's ability to detect 
vertical fractures accurately. 
Referring again to FIGS. 2a and 2b, the 50 c.p.s. sine wave and the series 
of alternating positive and negative-going pulses supplied by generators 
20 and 21 may be conducted via a single conductor in the group of 
conductors 152 to operate the sub-surface equipment located below the 
dashed line, which subsurface equipment is housed within the logging tool 
45. Alternatively, the since wave and series of pulses may be supplied via 
separate conductors. If supplied by a single conductor, separating means 
such as a separator circuit 3 may be used to separate the signals downhole 
so that the sine wave may be supplied to the receiver circuitry and the 
series of synchronizing pulses may be supplied to the transmitter 
circuitry. This separator circuit may be a combination of well known low 
pass filter and high pass filter circuits whose function is to filter out 
any high frequency components of the sine wave signal which is sent to 
receivers R1 and R2 and to filter out any low frequency components of the 
train of pulses sent to transmitters T1 and T2. Because of crossfeed 
normally encountered in multiconductor cables, even if the respective 
signals are transmitted downhole by separate conductors, a separator 
circuit 3 is still necessary. Controlling means for selectively actuating 
the transmitters and receivers comprises separator circuit 3 and the 
following transmitter and receiver circuitry. 
The transmitter circuitry is comprised of a transmitter trigger 5 and a 
transmitter drive 6. Trigger circuit 5 receives and separates the 
positive-and negative-going pulses thus controlling the selective 
actuation of transmitters T1 and T2. The positive-going pulses are 
utilized to actuate transmitter T1 and the negative-going pulses are 
utilized to actuate transmitter T2. Drive 6 is comprised of two 
cross-coupled circuits, one for each transmitter, each of which acts to 
fire the respective transmitter upon receipt of the proper synchronizing 
pulse. A novel latching circuit is incorporated into the transmitter drive 
6 to prevent the erroneous firing, such as by signals cross-coupled from 
one conductor to another, of any other transmitter while one is actuated. 
This inoperative period begins at the instant of firing of a first 
transmitter and continues for a pre-determined period approximately the 
duration of a measurement interval. 
The receiver circuitry is comprised of a multiplexer drive 4, a multiplexer 
7, a receiver amplifier and pre-amplifier 8, and a cable driver 9. The 
receivers R1 and R2 each are provided with separate amplifying means 
incorporated in amplifier and preamplifier 8, whereby the acoustic wave 
received by the receiver and connected to its electrical equivalent is 
amplified for transmission to the surface equipment. Multiplexer 7 acts as 
a selector to select the proper receiver for the respective measurements 
intervals. As previously discussed, receiver R1 is actuated during the 
positive half-cycle of the sine wave and receiver R2 is actuated during 
the negative half-cycle. The sine wave is supplied to multiplexer drive 4, 
which in turn controls multiplexer 7 in selectively actuating one of 
receivers R1 or R2. The respective electrical equivalents of the acoustic 
waves are transmitted by multiplexer 7 to a cable driver 9, which further 
amplifies the signals and transmits them to the surface equipment. It is 
understood, of course, that the selected receiver in logging tool 45 
converts the incident acoustic wave into electrical signals having 
waveforms representative of such acoustic waves in a conventional manner. 
To illustrate the operation of the overall system, assume that it is 
desired to make a recording of the wall of the borehole at depth D in FIG. 
2b. Four individual measurements will be taken, a measurement interval for 
each quadrant Q1-Q4 as shown in FIG. 1. 
Upon receipt of the first half-cycle of the sine wave supplied by generator 
20 at multiplexer drive 4, multiplexer 7 becomes operative for receipt of 
an electrical signal from receiver R1 and remains operative throughout the 
first half-cycle. After a pre-determined time interval, a positive-going 
pulse supplied by synchronization pulse generator 21 is transmitted to 
transmitter trigger 5 which senses the positive-going pulse and transmits 
it to the circuit associated with transmitter T1 in transmitter trigger 6. 
The transmitter is selectively and momentarily actuated by trigger 6 so 
that a pulse of acoustic energy is transmitted about an arcuate path 
surrounding the well bore. At the same time, the latching circuit prevents 
the actuation of any other transmitter for a pre-determined time interval. 
As the acoustic wave propagated about the circumferential path over 
quadrant Q1 is received at receiver R1, the wave is converted into its 
electrical equivalent and the variations in acoustic energy amplitude are 
transmitted by cable driver 9 to the surface equipment. There, a waveform 
of the type shown in FIG. 3a appears at the input to acoustic signal 
processor 50 for further processing. A marking pulse 160 is included in 
the waveform for the purpose of indicating the time of production of the 
synchronization pulse. Indirectly, this pulse also indicates the time of 
firing of the transmitter, which occurs after a predetermined and known 
time interval about 100 microseconds following the production of the 
synchronization pulse. This time interval is a system delay comprising the 
travel time of the synchronization pulse from the surface down the cable 
to the transmitter trigger, plus the response time of the trigger. This 
complete operation occurs prior to the receipt of the first negative-going 
pulse from generator 21. 
The operation is repeated upon receipt by transmitter trigger 5 of the 
first negative-going pulse from generator 21, which occurs approximately 5 
ms. after the positive-going pulse. Transmitter T2 becomes actuated by 
transmitter trigger 5 and transmitter drive 6 to transmit a pulse of 
acoustic energy into the formation, and transmitter T1 is prevented from 
becoming actuated by the latching circuit. Receiver R1 is still in an 
operative state, since the second synchronizing pulse occurs during the 
positive half-cycle of the sine wave. The acoustic wave received at 
receiver R1 from transmitter T2 is converted to its electrical equivalent 
and transmitted to the surface, providing the sigal from quadrant Q2. 
Upon receipt by the multiplexer drive 4 of the negative half-cycle of the 
sine wave, receiver R1 is rendered inopertive and R2 becomes actuated. In 
a like manner as described above, transmitters T1 and T2 are momentarily 
and successively actuated by the positive-and negative-going pulses from 
generator 21, so that receiver R2 provides signals from quadrants Q3 and 
Q4. 
At the surface, as each respective signal is received for the particular 
measurement interval, it is processed by acoustic signal processor 50 as 
controlled by demultiplexer pulse generator 65, window trigger generator 
75, and window, caliper, and velocity signal generator 80. During this 
time, the logging tool has moved uphole to a different position along the 
wall of the borehole. It will be seen then that as the tool moves through 
the borehole, successive measurement cycles each with four separate 
measurement intervals are completed, thus resulting in a recording of any 
vertical fractures or anomalies present in the logged portion of the 
borehole. 
The details of the individual components of the system shown broadly in 
FIGS. 2a and 2b will now be described. 
Alternating Wave Generator 20 and Synchronizing Pulse Generator 21 
A wave generating source comprised of alternating wave generator 20 and 
synchronizing pulse generator 21 is used to synchronize overall system 
operation. The alternating wave generator may be any of several types well 
known in the art, such as a 50 c.p.s. sine wave generator which derives 
its power from a commercial source or a power generator. Pulse generator 
21 is shown in schematic form in FIG. 4. Input is provided by a 50 c.p.s. 
generator 20. Referring to FIG. 4, slicer 22, comprised of zener diodes 
165 and 166 and diodes 163 and 164, provides waveform A as shown in FIG. 
11. As the input sine wave increases in a positive direction, diode 163 
prevents passage of the signal to the output of slicer 22. However, at the 
point the sine wave signal reaches a level equal to the breadkdown voltage 
of zener diode 166, diode 164 allows transmission of the remaining portion 
of the positive half-cycle of the sine wave signal, at least until the 
signal level again falls below the zener breakdown voltage. As the sine 
wave begins its negative half-cycle, diode 164 blocks passage of the 
signal, but zener diode 165 allows transmission of the negative half-cycle 
of the sine wave signal after it has become sufficiently negative to 
exceed the zener breakdown voltage. When the negative half-cycle 
approaches a zero level, the zener breakdown threshold is again reached 
and transmission by diode 163 is prevented. The negative portion of this 
`sliced` sine wave A of FIG. 11 is separated by diode 161 and transmitted 
through clipper 15 where that portion of the sine wave is converted into a 
square pulse in a conventional fashion. Waveform A in its entirety is 
passed through inverter 14 where an exact inverse of waveform A is 
produced. Diode 162 allows transmission of only the negative portion which 
was prevented from being transmitted by diode 161. This negative portion 
is converted into a series of square pulses by clipper 25 in a 
conventional manner, and the outputs of clipper 15 and clipper 25 are 
added at point 18 of pulse generator 21, resulting in waveform B of FIG. 
11. Amplifier and differentiator 16 reduces the series of square pulses to 
a series of synchronizing pulses (waveform C, FIG. 11) by differentiating 
the square pulse train. This series of pulses is then amplified and 
converted into pulse train D of FIG. 11 by line driver 17 for transmission 
to the surface and subsurface equipment of the invention. The zener diodes 
are chosen so that the breakdown voltages effectively separate the 
resultant positive-and negative-going pulses of the series of pulses by 
approximately 5 ms. for a sine wave period of 20 ms. Thus, approximately 
2.5 ms. after the initiation of the positive half-cycle of the 20 ms. sine 
wave, a positive-going pulse appears at the output of pulse generator 21. 
Approximately 5 ms. later, a negative-going pulse appears. After another 5 
ms., another positive-going pulse appears, and so on. The pulses derived 
from generator 21 serve to synchronize the operation of the transmitters 
with the operation of the receivers, insuring continuous and accurate 
timing. 
The pulse separation times may be controlled by proper choice of zener 
diodes 165 and 166. Choice of the zener diodes based on the breakdown 
voltages will effectively change the duration elapsing between the pulses 
to provide the desired firing times for the transmitters. For the 
waveforms of FIG. 11, the pulse rate frequency is approximately 200 pulses 
per second. 
Demultiplexer Pulse Generator 65 
Demultiplexer pulse generator 65, shown in block form for its relationship 
with the system of the invention in FIG. 2, is shown in greater detail in 
FIG. 5. As illustrated therein, generator 65 comprises a low pass filter 
66, a comparator 67, an inverter 68, monostable multivibrators or single 
shots 70 and 72, and flip flops 71 and 73. All waveforms to which 
reference is hereafter made will appear in FIG. 12, unless otherwise 
noted. 
Low pass filter 66 receives as its input a sine wave A, for instance of 50 
c.p.s., and removes any spurious high frequency signals superimposed on 
the sine wave. Comparator 67 generates alternating square waveform B by 
providing a positive potential of a predetermined level when the sine wave 
is positive and a potential that is negative relative to the positive 
potential when the sine wave signal is negative. Square wave signal B 
becomes the "CL" input for flip-flop 71, which is an asynchronous input 
not dependent on the clock, or "CK", input state of the flip-flop. 
Inverter 68 inverts the positive and negative square pulses shown in 
waveform B so that the exact opposite of waveform B is supplied as the 
input to single shot 70 and the "CL" input to flip-flop 73. It is, of 
course, understood that although the signals are referred to as positive 
and negative in the following discussion, neither signal becomes negative 
with respect to ground potential, since the particular binary devices used 
would cease to function if a potential less than ground potential appeared 
at an input. Hence, a signal is negative only relative to the 
corresponding positive signal. Single shot 70 functions in the 
conventional manner, providing a pulse output of predetermined duration at 
its Q output and the inverse of that pulse at its Q output, which inverted 
pulse becomes the "CK" input for flip-flop 71. Single shot 70 triggers to 
provide its pulse output on the negative-going edge of the square output 
waveform C of inverter 68. Thus, at each point X of waveform C, a pulse of 
predetermined width (waveform D) appears at output Q of single shot 70. 
The Q output is connected to window trigger generator 75 for purposes 
which will be explained later. Flip-flop 71 operates to change its state 
via its own Q output from a negative sense to a positive sense upon 
receipt at its "CK" input of the positive-going edge of the Q output from 
a single shot 70. Since the Q output of single shot 70 is the inverse of 
waveform D, each time waveform D goes negative (its inverse goes positive) 
the flip-flop 71 generates a positive waveform E at its own Q output. This 
positive signal continues at the Q output until a negative-going signal 
appears at the "CL" input of flip-flop 71. Since the signal at the "CL" 
input is output waveform B of comparator 67, a negative-going edge occurs 
at each point Y. Thus, the output Q of flip-flop 71 reverts to a low 
voltage level at each point Y of waveform B. It is apparent then, that the 
joint actions of single shot 70 and flip-flop 71 divide the time period of 
the positive half-cycle of sine wave A into two portions, the first being 
while the Q output of single shot 70 is at a high voltage level, the 
second being while the Q output of the flip-flop 71 is at a high level. It 
should be understood that the time of termination of the square pulse from 
single shot 70, and hence the time of initiation of the square pulse from 
flip-flop 71 may be varied by choosing a single shot 70 with a different 
period of operation. However, the time of initiation of single shot 70 and 
the time of termination of flip-flop 71 are controlled by the time period 
of the first half-cycle of the sine wave. 
The Q output of flip-flop 71 is connected to the input of single shot 72 
and to the window trigger generator 75, the function of which will be 
explained later. Single shot 72 is preferably chosen to have the same 
period of operation as single shot 70, for reasons which will become 
apparent. Upon sensing the negative-going edge of waveform E at its input, 
single shot 72 generates positive signals in the form of waveform F at its 
Q output which is transmitted to window trigger generator 75. The inverse 
of waveform F appears at its Q output which is connected to the "CK" input 
of flip-flop 73. Since flip-flop 73 changes state from a negative level to 
a positive level upon sensing the positive-going edge of a signal at its 
"CK" input, this occurs when waveform F has a negative-going edge. Thus, 
when output Q of a single shot 72 drops to a negative level, its Q output 
rises to a positive level, and flip-flop 73 changes state to a positive 
level upon receipt of the positive-going edge. In the same manner as 
explained above regarding flip-flop 71, when a negative-going edge of a 
signal appears at the "CL" input of flip-flop 73, output Q drops to its 
negative state. Since the "CL" input receives its signal from the output 
of inverter 68 (waveform C), flip-flop 73 drops its negative level at each 
point X of the waveform, resulting in a signal such as waveform G at its Q 
output. 
Hence, the effect of single shot 72 and flip-flop 73 is the same as to the 
time period of the negative-going half cycle of sine wave A as the effect 
of single shot 70 and flip-flop 71 is to the positive half-cycle. If the 
periods of operation at a positive level of single shots 70 and 72 are 
chosen to be equal, the periods of operation of flip-flops 71 and 73 will 
be equal since each of the flip-flops drops back to a low level at the end 
of the respective half-cycle of sine wave A. The total effect of 
demultiplexer 65 is to divide one full cycle of sine wave A into four not 
necessarily equal parts, as exemplified in FIG. 12, waveforms D, E, F, G. 
These output waveforms, in addition to being transmitted to window, 
caliper, and velocity signal generator 80 for purposes which will be later 
explained. 
Window Trigger Generator 75 
Window trigger generator 75 is shown in detailed schematic form in FIG. 5. 
Generator 75 comprises a high pass filter 76, a full wave detector 90, an 
inverter 113, four flip-flops 105, 106, 107 and 108, and four inverters 
109, 110, 111 and 112. 
Synchronization pulse generator 21 supplies the series of alternating 
positive-and negative-going pulses to the input of generator 75. High pass 
filter 76 receives this series of pulses at its input, each pulse having a 
center frequency of approximately 120 kHz as previously explained, and 
filters out any lower frequency signals before transmitting the pulses to 
full wave detector 90. Full wave detector 90, operation of which will be 
explained in greater detail following this discussion, generates a series 
of pulses of short duration, each pulse being initiated by a positive or 
negative pulse at its input, and each pulse of detector 90 being in the 
positive sense. Hence, a series of positive-going pulses separated by 
approximately 5 ms. results at the output of detector 90. These pulses are 
in turn inverted by inverter 113 so that at point 75A of FIG. 5, the 
output of inverter 113, waveform H results. This signal is connected to 
each of the asynchronous "CL" inputs of flip-flops 105, 106, 107 and 108, 
which flip-flops function in the same manner as the flip-flops of 
demultiplexer pulse generator 65. In other words, when a negative-going 
signal appears at any of the "CL" inputs, a positive state on output Q of 
any of the flip-flops changes to a negative state. As indicated in FIG. 5, 
the "CK" input of each flip-flop of window trigger generator 75 is 
supplied by one of the Q outputs of the single shot and flip-flop circuits 
of demultiplexer pulse generator 65. Single shot 70 supplies the "CK" 
input (wave-form D) to flip-flop 105, flip-flop 71 supplies the "CK" input 
(waveform E) to flip-flop 106, single shot 72 supplies the "CK" input 
(waveform F) to flip-flop 107, and flip-flop 73 supplies the "CK" input 
(waveform G) to flip-flop 108. 
Since the operation of each of the flip-flops of generator 75 is identical, 
only the operation of flip-flop 105 will be explained in detail. Flip-flop 
105 changes from a negative to a positive state at the instant a 
positive-going signal appears at its "CK" input. Referring to waveform D, 
this occurs on the leading edge of each pulse of the square pulse train 
shown. However, upon receipt of the negative-going edge of a signal 
represented by waveform H at its "CL" input, flip-flop 105 reverts to a 
negative state, resulting in the appearance of waveform K at its Q output. 
In like manner, waveform L, M and N result at the Q outputs of flip-flops 
106, 107 and 108, respectively. Each of these outputs is initiated by the 
leading edge of the Q outputs from demultiplexer pulse generator 65 and is 
terminated by the negative-going edge of the series of pulses of waveform 
H. The Q outputs of flip-flops 105, 106, 107 and 108 are inverted by 
inverters 109, 110, 111 and 112, respectively, and combined at point 112A 
of FIG. 5. The resultant waveform O, a series of square pulses each 
beginning at the same instant as one of the pulses of waveform H, appears 
in FIG. 12. It should be noted that, although the initiation of each of 
the pulses of waveform O is coincident with one of the synchronizing 
pulses, the time of termination at least of the first pulse to appear in 
each half-cycle of waveform A may be varied by varying the periods of 
operation of single shots 70 and 72 of generator 65. From this point, for 
purposes of explanation, the pulses appearing in waveform D during the 
positive half-cycle of waveform A will be designated pulses 1 and 2, 
respectively, and those appearing during the negative half-cycle will be 
designated pulses 3 and 4, respectively. 
Full wave detector 90 is shown in FIG. 7. In that figure, operational 
amplifier 114 serves as an inverting stage with low gain, and acts as a 
precision full wave rectifier because of the presence of the two diodes 98 
and 99. Diode 99 causes the gain of amplifier 114 to be zero for negative 
input at the inverting input point. Diode 98 and resistors 92 and 94 cause 
a positive input to be amplified with a gain of approximately unity. The 
output of amplifier 114 is applied to the inverting input of amplifier 115 
in which the output is clipped to a value equal to the zener voltage of 
zener diode 97. Resistor 96 causes the gain of amplifier 115 to be a 
constant value below the clipping level. At the clipping level the gain 
becomes unity. The resistor 93 improves the waveform. Thus, the output of 
amplifier 115 is a series of constant amplitude positive pulses, even if 
the input varies. 
The signal at point 112A of FIG. 5 consisting of the cycle of pulses 1-4 is 
transmitted to window, caliper, and velocity signal generator 80, which 
will be explained now, and wherein the importance of waveform O will 
become apparent. 
Window, Caliper and Velocity Signal Generator 80 
Referring to FIGS. 6a and 6b, window, caliper, and velocity signal 
generator 80 is shown in detailed form. Generator 80 comprises a window 
delay single shot 81, a window width single shot 82, window generator 
flip-flop 83, AND gates 101, 102, 103 and 104, adjustable comparator 120 
and travel time flip-flop 86. 
Generator 80 takes as its input a signal which is represented by waveform O 
from window trigger generator 75. This series of pulses causes delay 
single shot 81 to generate a second series of pulses, the duration of each 
pulse being variable by varying a controlling potentimeter associated with 
single shot 81. The leading edge of each pulse of the cycle of pulses 1-4 
as shown in waveform O trigger single shot 81 so that the initiation of a 
pulse at output Q of single shot 81 occurs simultaneously. For purposes of 
the following discussion, output Q of single shot 81 will be represented 
by waveform P of FIG. 12, keeping in mind that the duration of each pulse 
is variable. Output of single shot 81 is connected to the input of window 
width single shot 82, another single shot whose pulse width may be varied 
by varying a potentimeter connected thereto. However, single shot 82 
triggers to produce a pulse upon sensing a negative-going edge at its 
input. Hence, when one of the pulses of waveform P drops to a low level, 
single shot 82 generates a pulse at its Q output. At the same time, the Q 
output of single shot 82, which was previously at a high level, drops to a 
low level. This Q output is inverted and connected to the "CK" input of 
window generator flip-flop 83. At this point, it becomes necessary to 
discuss two modes of system operation; manual and automatic. Switch S6, 
which has a "Man" and an "Auto" setting, determines which signal will be 
connected to the "CL" asynchronous input of flip-flop 83. In the "Man" 
setting, a pulse appearing at the Q output of flip-flop 83 is terminated 
at the instant a negative-going signal appears at the "CL" input, which 
occurs when the Q output of single shot 82 drops to a low level. It is 
apparent then, that in the manual switching mode, window generator 
flip-flop 83 merely follows single shot 82. At the instant a 
positive-going pulse appears at its "CK" input, flip-flop 83 generates a 
positive pulse at its Q output. This occurs when a negative-going signal 
appears at the Q output of single shot 82, which is the same instant that 
a positive-going signal appears at the Q output of single shot 82. In the 
"Auto" switching mode, the "CL" input of flip-flop 83 is supplied with a 
signal from adjustable comparator 120. The adjustable comparator compares 
the electrical equivalent of the acoustic wave as received from the 
subsurface equipment with a predetermined level, and upon the electrical 
equivalent reaching that level, gives an output which terminates the pulse 
at the Q output of flip-flop 83. Thus, the initiation of the pulse at the 
Q output of flip-flop 83 remains in synchronization with the beginning of 
the pulse at the Q output of single shot 82, but the time of termination 
of the pulse may vary depending on the time at which the electrical 
equivalent reaches the predetermined level as set in adjustable comparator 
120. 
Adjustable comparator 120 may be explained in greater detail as follows, 
with reference to FIG. 8: This subcircuit consists of a high gain 
differential operational amplifier 121 which is connected without feedback 
resistors. The non-inverting input is connected to an adjustable voltage 
source provided by resistors 122 and 124 and by a potentimeter 123. The 
adjustable voltage is a reference voltage. The input signal is applied to 
the inverting input. Whenever the input signal exceeds the reference 
voltage, the output voltage goes high, at other times being saturated and 
low. Conversely, if the potentimeter 123 is set to a voltage higher than 
the input the state of the comparator input will suddenly switch from high 
to low when the input voltage drops below the reference voltage. The dual 
single shot 125 has a 100 ms. pulse output duration and prevents the 
production of rapidly repeating outputs from the comparator and possible 
resulting erratic operation of other circuits. Output of the dual single 
shots is taken through a logic or gate comprising the two NAND gates 126 
and 127. The output of the OR gate subcircuit feeds the travel time 
flip-flop 86 and resets it if it has been set by a pulse from the window 
delay single shot. 
Output Q of flip-flop 83, here represented by waveform R, is connected to 
switch 41 of acoustic signal processor 50 for reasons which will become 
apparent. It is also connected in common to a first input of a series of 
dual-input AND gates, one AND gate being associated with each quadrant 
being investigated by the logging tool. Hereafter, the four quadrants 
being investigated will each be assigned a channel, so that Channel 1 
corresponds to quadrant 1, Channel 2 to quadrant 2, and so on. Thus, AND 
gate 101 corresponds to Channel 1, AND gate 102 to Channel 2, AND gate 103 
to Channel 3, and AND gate 104 to Channel 4. Before the AND gates will 
generate positive signals at their outputs, both inputs must receive 
positive signals. Since the pulses represented by waveform R are connected 
in common to the first input, the state of the second input of each will 
determine when the respective AND gate exhibits a high output. The pulse 
trains represented by waveforms D, E, F and G of FIG. 12, which are the 
outputs of the single shots and flip-flops of demultiplexer pulse 
generator 65, appear at the second inputs of AND gates 101, 102, 103 and 
104, respectively. It is apparent that only one period of time, a pair of 
AND gates exhibiting high levels at their outputs during each half cycle 
of sine wave A of FIG. 12. The overall effect of AND gates 101-104 on 
system operation may be best illustrated by referring to waveforms S-1, 
S-2, S-3 and S-4 of FIG. 12. These represent the outputs of AND gates 
101-104, respectively. This collection of "gating" pulses at the outputs 
of the AND gates which define the measurement intervals associated with 
each quadrant being investigated, is connected to accoustic signal 
processor 50 for reasons which will be explained. 
Travel time flip-flop 86 is interposed between adjustable comparator 120 
and the "CL" input of flip-flop 83 to provide a signal which is indicative 
of the time of travel of that portion of the acoustical wavetrain which 
first produces an electrical equivalent equal the reference voltage 
selected by potentiometer 123 for the adjustable comparator 120. Thus, the 
duration of the output pulse of this travel time flip-flop and 
particularly variations thereof, will be a meassure of travel time about a 
portion of the circumference of the borehole. This output of flip-flop 83 
is averaged and connected to chart recorder 60 for appropriate display, 
resulting in a convenient caliper measurement of the borehole. The output 
Q of flip-flop 81 is connected to the "CK" input of flip-flop 86, and 
assuming that the delay period as set by flip-flop 81 marks the beginning 
of the arrival of the shear wave, output Q of flip-flop 86 initiates a 
high level pulse at the termination of the pulse at output Q of window 
delay flip-flop 81. The pulse at output Q of flip-flop 86 is terminated at 
the instant a negative-going signal is sensed at its "CL" input, which 
occurs when adjustable comparator 120 senses the predetermined level of 
the electrical equivalent received from the subsurface equipment as 
previously discussed. Since this level is chosen to be slightly greater 
than the largest amplitude associated with the shear wave, output Q of 
flip-flop 86 will display a series of pulses each of which represents the 
travel time of the shear wave through one quadrant of the formation 
surrounding the well bore. 
To summarize, the overall function of the window, caliper, and velocity 
signal generator is to provide a number of gating pulses representing 
measurement intervals which will allow processing of certain parts of the 
received acoustic waves to the exclusion of all other parts. 
Acoustic Signal Processor 50 
Up to the present, the primary purpose of the system circuitry has been to 
provide the gating pulses for selecting the particular parts of the 
electrical equivalent received from the well bore that is most useful for 
analysis. Acoustic signal processor 50 utilizes this "gating" pulse to 
process the electrical equivalent of the acoustic wave and provide a 
processed electrical response for each quadrant being investigated in the 
well bore. 
Referring now to FIGS. 6a and 6b, the signal received at point 29 is the 
electrical equivalent of the acoustic wave as received at one of receivers 
R1 or R2 which is amplified and transmitted to the surface. The 
composition of a typical acoustic wave is shown in FIG. 3(a), where a time 
break, or marking pulse produced by one of the transmitters, is the first 
signal received. A short time later, after noise due to the time break has 
subsided, a compressional rock wave of very small amplitude is recorded. 
Next, a shear wave of larger amplitude appears, then a compressional 
direct fluid wave of still larger amplitude, and finally a compressional 
guided fluid wave appears. This cycle is repeated each time a receiver 
receives a signal from an associated transmitter, or once for each 
quadrant being investigated. The resultant wavetrain is amplified by 
preamplifier 53 and amplifier 52 of processor 50, resulting in a wavetrain 
resembling waveform T of FIG. 12. The output of amplifier 52 is switchedly 
connected via switches S4 and S5 to a delay circuit 30 so that bypass of 
the delay to the next processing circuit may be accomplished if desired. 
The output of preamplifier 53 is connected to an input of analog tape 
transport 85, which may be any of several types of magnetic tape recorders 
well known in the art. Switch S3 allows "re-logging" of the well via the 
recording from tape transport 85 at any time after the logging tool has 
been passed through the well bore. Relay 30 causes the complete electrical 
equivalent as received from downhole and amplified to be delayed 
approximately 1/4 cycle of its frequency of oscillation so that the 
non-delayed signal may be transmitted to adjustable comparator 120 where 
it is used to allow automatic operation of the system by proper setting of 
switch S6. Thus, delay 30 may be used to permit a particular part of the 
electrical equivalent of the acoustic wave to be further processed to the 
exclusion of any later arriving parts. Particularly, according to the 
invention, the direct fluid wave, shown in FIG. 3(a) to occur immediately 
after receipt of the shear wave, is utilized to cause comparator 120 to 
terminate the square pulse being generated at the output of flip-flop 83 
of window, caliper, and velocity signal generator 80 which in turn 
controls the outputs of AND gates 101-104. Referring to FIG. 3(b) and 
FIGS. 6a and 6b, comparator 120 senses the first negative-going peak of 
the direct fluid wave by comparing the electrical equivalent at one input 
with a potential level at the other input which has been chosen from 
experimental data. This level is chosen so that it will be greater than 
peak amplitude received from the shear wave but less than the initial 
direct fluid wave signal. At point X of the wavetrain of FIG. 3(b) this 
threshold level is sensed by the comparator, which transmits a signal to 
flip-flop 83 to terminate the pulse at its Q output. This, in turn, 
terminates the gating pulse for the channel then operative at the output 
of one of AND gates 101-104. In the meantime, the electrical equivalent 
which will be recorded and processed further by acoustic signal processor 
50 has been delayed 1/4 cycle by delay 30. 
In the manual mode of operation which was previously discussed, any portion 
of the electrical equivalent of the acoustic wave received may be further 
processed, at least to the extent that it occurs within the time frame as 
defined by single shots 70 and 72 and flip-flops 71 and 73 of 
demultiplexer pulse generator 65. However, assuming for purposes of 
discussion that delay 30 is utilized, the delayed electrical equivalent is 
transmitted to window switch 41 whose operation is controlled by output Q 
of flip-flop 83 of window, caliper, and velocity signal generator 80. 
Since flip-flop 83 is controlled by comparator 120, only the shortened 
part of the signal, the shear wave, is transmitted for further processing, 
as indicated in FIG. 3(c). It should be noted that the intermittent 
operation of switch 41 is in synchronization with the channel 1-4 
operation of AND gates 101-104, the Q output of flip-flop 83 controlling 
both points. The truncated electrical equivalent of the acoustic wave for 
each quadrant representing only the shear wave signal is illustrated by 
waveform V of FIG. 12. This truncated signal is transmitted to germanium 
rectifier 46 where the positive portion of the signal is rejected, which 
insures that in no event will a part of the direct fluid wave be 
processed. This occurs since the first negative-going cycle of the direct 
fluid wave was used to operate comparator 120, and delay 30 delays the 
acoustic wave sufficiently to eliminate this negative peak. Any positive 
direct fluid wave signal is removed by rectifier 46. Integrator 49 
processes the rectified signal by charging a capacitor so that a single 
peak signal for each of quadrant 1-4 results as illustrated by waveform X. 
The output of integrator 49 is connected to inputs of sampling switches 
54, 55, 56 and 57, operation of which are controlled by the pulses at the 
outputs of AND gates 101-104 represented by waveforms S-1, S-2, S-3 and 
S-4, respectively. Hence, these sampling switches are intermittently 
closed to allow transmission of the processed electrical response to the 
proper channel of chart recorder 60. Switch 54 connects the electrical 
response from quadrant Q1 to channel 1, switch 65 connects the electrical 
response from quadrant Q2 to channel 2, and so on for switches 56 and 57. 
In an alternative mode of operation, circuit 49 may be used to compute the 
average of the signals received from rectifier 41 for each quadrant. In 
like manner as before, this signal may be connected to sampling switches 
51 for transmission to the appropriate channel of chart recorder 60 which 
may include a low pass filter to average signals vs. depth. These 
processed electrical responses may be suitably displayed on a chart versus 
depth for each channel, resulting in one chart for each quadrant, or if 
desired, the average or peak of all four quadrants may be computed so that 
a single processed electrical response for four quadrants may be 
displayed. Another mode of display particularly helpful in locating 
vertical fractures invokes utilizing the unprocessed electrical 
equivalents recorded on the analog tape transport and displaying such 
unprocessed signals for each quadrant simultaneously or means such as an 
oscilloscope. In this manner, indications of vertical fractures due to 
shear wave attenuation may be quickly and easily located. 
Receiver Circuitry 
Referring to FIG. 9, the subsurface electronics associated with receivers 
R1 and R2 will now be explained in greater detail. A set of preamplifiers 
58 receive the electrical equivalents of the acoustic waves as supplied by 
receiving transducers R1 and R2 and provide primary amplification. The 
preamplifier circuit for receiver R1, comprised of a typical transistor 
amplifier made up of transistor 27, resistor 116, and resistor 117, 
transmits the preamplified signal to amplifier 43. Amplifier 43, 
associated with receiver R1, amplifies the input signal and transmits it 
to switch 78 of multiplexer 7, whose operation is controlled by 
multiplexer drive 4. Multiplexer drive 4 receives as its input the sine 
wave from generator 20 at the surface and generates a square wave which 
alternates in synchronization with the sine wave. Receiver R2 has 
associated its operative period the electrical equivalents are transmitted 
to switch 79 of multiplexer 7. NAND gates 88 and 89 act as inverters so 
that upon receipt of a high signal at their respective inputs, their 
outputs generate a low signal and the switch 78 or 79 connected to the 
respective output opens. It can be seen that the operation of the switches 
proceeds in an alternating fashion in synchronization with the sine wave 
from generator 20 so that multiplexing of signals from receivers R1 and R2 
take place. The electrical equivalents transmitted by either of switches 
78 or 79 is thereafter secondarily amplified by cable driver 9 and 
transmitted to the surface for further processing by acoustic signal 
processor 50. 
Transmitter Circuitry 
Referring to FIG. 10, the transmitter circuitry for each transmitter T1 or 
T2 comprises a pulse detector 23 or 24 and a transmitter drive circuit 6 
comprised of a latching circuit. The operation of the circuitry associated 
with transmitter T1 will now be described in detail, and reference to 
transmitter T2 will be made where necessary for proper explanation. 
Positive pulse detector 23 receives at its input the series of alternating 
positive- and negative-going pulses from pulse generator 21, generating a 
signal at its output only on receipt of the positive-going pulses. The 
latching circuit associated with transmitter T1 may be further 
sub-divided, for purposes of explanation, into a charging circuit and a 
triggering circuit. The latching circuit serves two functions; it provides 
the triggering of transmitter T1, and it prevents any transmitter from 
firing during, or in an interval of predetermined duration immediately 
following, the actuation of transmitter T1. The last-mentioned period 
immediately following the actuation of transmitter T1 may be approximately 
of the same duration as the duration of large acoustical signals following 
the actuation of a transmitter. This period is determined by the values of 
capacitors 129 and 136 and resistors 130 and 200, proper selection of 
which will be explained in the following discussion of the latching 
circuit operation. Upon receipt of a pulse from positive pulse detector 23 
at diode 128, capacitor 129 charges very rapidly, causing a positive pulse 
to appear at point 201 because of charging current flowing through 
resistor 131. As a result, a positive pulse appears at the gate electrode 
of SCR 132, causing capacitor 136 to discharge through resistor 137. This 
discharge current produces a voltage at point 202 which is applied to the 
piezoelectric transducer T1 through resistor 135, causing production of a 
sound pulse. When capacitor 136 is discharged, capacitor 136' also 
discharges through resistor 138', so that SCR 132' can in no way fire with 
SCR 132 simultaneously. Thus, the SCRs are in succession, when triggered, 
latched by three mechanisms; first, the anodes of the SCRs are tied 
together by resistor 138, which is relatively small, such as 1000 ohms. 
Thus, as one fires, the voltage of the other is reduced to too low a value 
for immediate mistriggering. Second, the time constant of resistor 136 and 
resistor 200 is made about 5 ms., so that once either SCR fires, neither 
can again fire for about 5 ms. Also, current flowing through resistor 137 
momentarily biases the cathode of the SCR positive with respect to its 
gate, preventing immediate re-triggering of the SCR. Third, the diode 128 
causes the capacitor 129 to remain blocked for about 2.4 ms. because of 
the time constant of resistor 130, resistor 131, and capacitor 129, 
preventing re-triggering during this time period. 
When a negative firing pulses arrives in the downhole instrument, 
transmitter T2 is actuated in a manner identical with that described above 
for T1. 
From the foregoing, it will be seen that a novel acoustic logging technique 
has been provided which allows complete and accurate detection of vertical 
fractures in a wellbore. The versatility of the system allows processing 
and recording of only the shear waves received to the complete exclusion 
of later arriving waves at a receiver, or processing and recording of any 
portion of the complete acoustic wavetrain received. If it is desired to 
utilize the guided fluid waves received after the shear waves at a 
particular receiver, this may be accomplished by appropriate adjustments. 
Recordation of only the shear waves does not depend on the average wave 
velocity in the formation, since the processing circuitry may be rendered 
inoperative only after the shear wave has been received. Other novel 
electronic circuitry has been provided so that the aforementioned 
advantages may be achieved. Of course, it is apparent that the concept of 
the invention may be extended to the use of multiple receivers and 
transmitters by merely duplicating the relevant parts of the component 
circuitry. 
Data obtained from experimental use of the above described system has shown 
that a particularly useful method of operating the system to identify 
fracture zones comprises the following steps: Producing a chart recording 
displaying, as in FIG. 3, the peak amplitude of shear waves propagating 
along the respective quadrants of a borehole at each depth; producing a 
chart recording displaying the variations of travel time averaged at each 
depth for the four respective quadrants for the direct fluid wave; 
producing a chart recording displaying the amplitude for the guided fluid 
wave along the respective quadrants at each depth; and, comparing the 
traces of the pen recordings to determine at what depths there are 
correlatable reductions in amplitude for both shear waves and guided fluid 
waves and simultaneously an indication of near normal travel time for the 
direct fluid wave. 
It will be understood that various modifications of the circuit details 
will occur to those skilled in the art, and it is intended that the 
invention be limited only by the scope of the appended claims.