System for measuring shear wave travel times

A system for ascertaining with certainty that detected return acoustic cycles are shear wave cycles for the calculation of shear wave travel times using the known travel time relationship that shear wave travel times are equal to 1.55 to 1.9 times the compressional wave travel time for the same acoustic wave through most geological formations. The system determines the compressional wave travel time and determines after what point following the compression wave return that shear wave cycles are occurring. The validity of the shear wave travel time determinations using shear wave cycles after such time and that no cycles used for such measurement are skipped is assured by determining that the shear wave travel time is within the acceptable limits of 1.55 to 1.9 times the compressional wave travel time.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention pertains to measurement of acoustic wave travel time and 
more particularly to the discrimination of shear waves in a return 
acoustical logging signal and the determination of shear wave travel times 
without using an erroneously selected return cycle for such measurement. 
2. Description of the Prior Art 
It is well known that an acoustic wave induced into a geological formation 
produces a plurality of wave propagation modes which can be received and 
detected to give information about the formation. Two of the most useful 
wave mode components of such an acoustic wave are the compression wave and 
the shear wave. The compression wave is the strongest and most rapidly 
travelling of the wavefronts and results from a compression-type impact on 
a geological interface. The shear wave, on the other hand, is a slower 
moving wavefront and is a result of lateral propagation along an 
interface. 
Although the compression wave characteristics provide much valuable 
information by itself, it does not provide some of the information that is 
revealed by the shear wave returns or by a comparison of the compression 
wave returns with the shear wave returns. For example, fissures that are 
approximately normal to an acoustic wave would not cause an appreciable 
compression wave attenuation but would cause appreciable shear wave 
attenuation. Also, fluids have a different effect on the appearance of a 
compression wave as compared with the appearance of a shear wave, both 
with respect to amplitude and travel time. 
One of the important values that is provided is the velocity or travel time 
values of the various components of the overall acoustic wave. In the 
above discussion for example, it is true that different valuable 
information is revealed about the formation from the velocity or travel 
time of the compression wavefront and from the information provided by the 
velocity or travel time of the shear wavefront. The compression wavefront 
is fairly easy to recognize and to measure the speed of by the use of 
spaced-apart receivers since, as mentioned above, the compression wave 
returns are the first returns received following an acoustic impulse event 
induced into the formation. 
It is not always so easy to recognize the shear wavefront arrival at a 
receiver and to take its measurement. If a wrong cycle of a return is 
selected as detected by the second of spaced-apart receivers compared with 
the cycle detected by the first receiver, erroneous and obvious misleading 
results are indicated. 
Pickett, et al., U.S. Pat. No. 3,276,533, is directed to a method of 
identifying the arrival of shear wave components in two received acoustic 
signals in well logging operations. The method there disclosed is based on 
the recognition of wave components having different velocities in each of 
the received signals. The arrival times of the beginning of the 
compressional waves in the first and second received signals are detected 
(T.sub.11 and T.sub.12 in FIG. 4). The time delay between the time of the 
arrival of the compressional wave and each successive peak in the signal 
is computed for each signal. The ratio of the time delays for 
corresponding peaks (i.e., first, second, etc.) in the two received 
signals is observed after each successive peak detection, and the first 
peaks for which the ratio is significantly different from unity are 
labelled as the first shear wave peaks for their respective signals. A 
cycle skipping, of course, would cause this same result, and go 
undetected, in the Pickett, et al. method. Further, there is no 
recognition of shear waves with respect to a standard, such as with 
respect to the arrival of the compressional waves. As will be explained 
hereinafter, there is a relationship which the Pickett, et al. method does 
not utilize at all. 
Engle, U.S. Pat. No. 3,467,875, discloses a method and apparatus for 
eliminating cycle skipping in acoustic well logging. A value 
representative of the maximum acceptable time change which can occur 
between successive sample time values in successive received acoustic 
waves is stored in a maximum delta circuit 24. The time difference between 
transmission and receipt of an incoming signal is compared with that of a 
previously validated signal stored in a digital-to-analog converter 20 to 
determine if the incoming signal falls within the acceptable range. If it 
does, it is established as the new valid signal and is recorded for 
logging. The Engle Patent fails to disclose the use of a predetermined 
relationship betweeen compressional and shear components of acoustic waves 
as a basis of validation of received signals. 
Trouiller, et al, U.S. Pat. No. 3,900,824, discloses a method for the 
elimination of cycle skipping which is similar to that of the Engle patent 
method. In the Trouiller patent method, the maximum acceptable difference 
between time values of successive measurement signals is computed as a 
given fraction of the average period of the acoustic waves transmitted by 
the transmitter in the logging tool. The Trouiller patent method does not 
employ utilization of a predetermined relationship between compressional 
and shear components of acoustic waves for shear wave identification. 
Elliott, et al, U.S. Pat. No. 3,390,377, utilizes at least a pair of 
receivers spaced apart in a borehole for receiving formation compressional 
and shear wave returns, adjusting the amplitude and time of the second 
receiver to correspond with that of the first receiver and cancelling the 
first returns by the second returns, the remaining returns presumably 
being those other than compressional waves. Although such technique may 
enhance the presence of shear waves, it is not the technique employed 
herein by Applicant and does not assure against false data being 
interpreted as true data because of cycle skipping. 
Waters, et al, U.S. Pat. No. 3,302,164, shows the development of 
compressional wave induced returns using a particular type of transmitter 
as well as the development of shear wave induced returns using a different 
acoustical generator for comparison purposes. The technique may give some 
information about shear waves, but it does not employ the technique 
utilized as set forth herein. 
The technique described herein employs a relationship that is known to 
exist in most geological formations between the travel time of 
compressional waves and the travel time of shear waves produced for a 
common impulse source of less than 15 kc. A simple sine wave impulse can 
be employed as the acoustical transmitted signal, but different types of 
such signals, and even complex signals, can be employed with the method 
herein described, with equal validity of result. The travel time of a 
compressional wave is readily determinable by observing the onset of the 
wave at two spaced apart receivers and by dividing the time difference 
results by the distance there between in terms of appropriate linear units 
of measurement, such as feet. 
Because it is known that the range of shear wave to compression wave travel 
time ratios that exists for almost all geological formations, the 
approximate arrival of shear waves can be determined. In fact, after the 
compression wave velocity or travel time is known and by picking the 
largest number of the relationship range, it is possible to determine for 
a given return received sequence of cycles that no more compression wave 
cycles are detected after a predetermined amount of time after the initial 
onset. Therefore, the cycles that then occur are assumed to be shear wave 
cycles. By subtracting the time of arrival of such such detected shear 
wave cycle detected by a first receiver with the time of arrival of a 
corresponding detected shear wave cycle detected by a second receiver, and 
corresponding for the respective distances the receivers are from the 
transmitter that produces the impulse event causing the wave onsets, the 
travel time of the shear wave is determined. 
This measurement is assured to be the shear wave travel time provided that 
its value fits within the window or limits of 1.55 to 1.9 times the 
compression wave travel time. If it does not fit the window, a cycle has 
been skipped somewhere, probably by the second receiver. 
Therefore, it is a feature of the present invention to provide an improved 
shear wave velocity or travel time measurement of an acoustic signal by 
validating it with respect to the readily determinable velocity or travel 
time of the compression wave component thereof. 
It is another feature of the present invention to provide an improved 
measurement of shear wave velocity or travel time in an acoustic signal 
that ensures against false data being employed because of signal cycle 
skipping. 
SUMMARY OF THE INVENTION 
The present invention employs two receivers and a transmitter for operating 
in conjunction with an acoustic signal, such as employed in a well-logging 
tool. The receivers detect the acoustical return signal in a plurality of 
cycles, the first several of which are compression wave cycles and, after 
a delay, the next several are shear wave cycles. The compression wave or 
travel time is determined by respectively detecting the corresponding 
zero-level crossings of the first cycles received of the compression wave 
onset and subtracting the time therebetween. The compression wave travel 
time is found by dividing the time difference into the distance in length 
measurement between the receivers. 
Next, it is determined where it is safe to make a shear wave travel time 
measurement. This is done by first determining for the respective 
receivers the maximum time where compression wave cycles can still be 
occurring. After that time, it is assumed that the next cycle received is 
a shear wave cycle. This maximum time is determined by multiplying the 
compression time travel time by the distance from the transmitter to a 
receiver and further by a factor of 0.9, which is then added to the time 
of arrival of the first compression wave cycle. 
The time differences from the respectively detected next cycles is then a 
measure of the travel time of the shear wave component provided that the 
shear wave travel time is in the range between 1.55 to 1.9 times the 
compression wave travel time. If not, then a cycle has been skipped by the 
second receiver. 
Alternately, after the maximum period has been determined for the closest 
receiver to the transmitter, cycles can be selected for the second 
receiver until a result is obtained which gives a shear wave travel time 
in the 1.55-1.9-times-the-compression-wave-travel time result.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now referring to the drawings and first to FIG. 1, a schematic 
representation of a typical acoustic well logging tool 10 is shown 
suspended by cable 12 via a support and lowering/raising system 
schematically represented by winch 14. Cable 12 not only provides the 
necessary support for tool 10, but also comprises signalling and power 
lines for communicating appropriate signals down and up the cable, as will 
be explained more fully hereafter, and for providing power to the 
electronic components of the system housed in the tool. 
The tool itself houses a transmitter 16 near the top of the tool and two 
receivers 18 (R1) and 20 (R2) near the bottom of the tool. These 
components of the acoustic welllogging system are typical 
electro-acoustical components and operate typically to produce a 
transmitted signal at a frequency in the range up to about 15 KHz. The 
transmitted signal is normally a pulse having a frequency response which 
would be in that frequency range, but it could be a sine wave, a pulse 
train or a complex pulse. The typical spacing of the components in the 
tool is such that receiver 18 is located four feet vertically below the 
transmitter and receiver 20 is located six feet vertically below the 
transmitter. 
The acoustic impulse from the transmitter is produced on command as a 
result of a control signal from the surface and results in the generation 
of an acoustical wavefront into the adjacent formation. After propagating 
through the formation, a return wavefront is detected by both of the 
receivers. A return wavefront that corresponds to a transmitted wavefront 
is naturally received at receiver 20 at a time later than received at 
receiver 18, as shown in FIGS. 2a and 2b. Further, the returning cyclical 
wavefronts include within the first several cycles information concerning 
the compressional wave propagation and then the shear wave component 
propagation, the shear wave being known to arrive at the same receiver 
within the limits of 1.55 to 1.9 times the time of arrival of the 
corresponding compression waves. 
Referring again to FIGS. 2a and 2b, please note that the first ten 
positive-going zero crossing of the return signal detected by receiver 18 
are illustrated in FIGS. 2a and the first ten positive-going zero crossing 
of the return signal detected by receiver 20 are illustrated in FIG. 2b. 
The positive-going crossing are selected for illustrative purposes, but is 
equally valid to operate in accordance with the method herein described 
with respect to the respective negative-going zero crossings. 
In order to determine the travel time of the compressional wave in travel 
time per foot (or other linear measurement indicia), the time of arrival 
of the first zero-crossing of the wave detected by the first receiver 
(receiver 18) is subtracted from the time of arrival of the corresponding 
first zero-crossing of the wave detected by the second receiver (receiver 
20), and then the time difference is divided by the distance between the 
two receivers. In the illustrated example, if the time difference between 
the first zero crossing detected by the first receiver and the 
corresponding first zero crossing detected by the second receiver is 
0.0001 seconds and the distance between the receivers is two feet, then 
the compressional wave travel time is 0.0001/2 =0.00005 seconds per foot 
(50 microseconds per foot). 
With this information about compression wave travel time known, it can be 
determined that only shear wave returns are going to be detected after a 
predetermined time later. It is known that shear waves travel slower than 
compression waves. By experimentation, it has been discovered that the 
shear waves are somewhere between 1.55 and 1.9 times as slow as 
compression waves through virtually all geological formations. Therefore, 
to determine the maximum time in which some other kind of wave can exist 
in a detected series of cycles, the distance from the transmitter to the 
receiver in distance units (e.g., feet) is multiplied times the 
compression wave travel time as determined above. This product is then 
further multiplied by 0.9, which is then added to the time of arrival of 
the first zero level crossing after the impulse from the transmitter. 
Therefore, by way of example with respect to receiver 18, assuring that the 
first zero-crossing occurs 0.0004 seconds after the production of the 
corresponding transmitted impulse and that receiver 18 is located 4 feet 
below transmitter 16, then the maximum time beyond which the next cycle is 
assumed to be a shear wave cycle is 
EQU 4.times.0.00005.times.0.9+0.0004=0.00058 seconds. 
For receiver 20 located 6 feet below the transmitter and whose initial 
compressional wave zero-crossing cycle occurred 0.0005 seconds after the 
transmitter impulse occurrence, the maximum time beyond which the next 
cycle is assumed to be a shear wave cycle is 
EQU 6.times.0.00005.times.0.55+0.0005=0.000665 seconds. 
It should be noted in the above that the first shear wave cycle zero 
crossing can occur respectively before the times indicated, but, in each 
instance, the next cycle zero crossing after those respective times are 
assuredly shear wave cycles. 
In other words, for the second receiver, the quickest possible shear wave 
arrival is at the distance-corrected 1.55 times the compressional wave 
arrival. The next cycle after this time is compared to see if it fits 
within the required limits. If it does not fall within the proper limits, 
then the next cycle is used until the proper cycle is selected or until 
the distance-corrected 1.9 times the compressional travel time is 
exceeded. When this occurs, then there is no cycle within the given limits 
showing that a cycle has been overlooked by the second receiver that would 
correspond with the selected shear wave cycle of the second receiver. 
The method can be employed by recording graphically by recorder 22 the 
transmitter impulse event and the detected returns sensed by both 
receivers 18 and 20 by conventional electronic logging divides operating 
in conjunction with an acoustic logging system. Such logs would develop 
traces similar to those shown in FIGS. 2a and 2b to which the principles 
above described could then be applied. 
However, it is also possible to perform the detection techniques on the 
electronic signals as they occur. One system for doing this is also 
illustrated in FIG. 1. Detector 30 receives the impulse event from the 
transmitter and changes the state (i.e. resets or enable) of flip-flops 32 
and 34 to which are also connected to zero level crossing detectors 36 and 
38 which are connected respectively to sense the outputs from receivers 18 
and 20. The outputs of flip-flops or bistable multivibrators 32 and 34 
then are applied to a compresion wave indicator 36, which measures the 
difference in the time of arrival of the outputs from the respective 
flipflops 32 and 34 corresponding to the respective first zero-crossing 
events from the two receivers and divides by a standard number 
corresponding to the distance difference between the receivers. This 
compression wave indicator 36 produces a compression wave travel time 
measurement signal value on line 38, which can be separately metered 
and/or recorded, if desired. The compression wave indicator also provides 
certain control pulses via lines 40, 46, and 52 which will be described 
further below. 
The compession wave travel time indicator also produces another output in 
the form of a trigger pulse for enabling a flip-flop 42 on line 40. This 
trigger pulse occurs at a time after the output from flip-flop 32 (the 
first zero level crossing received and detected by receiver 18) plus the 
compression wave travel time measurement multiplied by a fixed number, 
namely 0.55 times the number representing the distance between the 
transmitter and receiver 18. Once this trigger enables flip-flop 42, the 
next appropriate (e.g., positive-going) zero level crossing signal 
detected by detector 36 produces an output from flip-flop 42 to shear wave 
indicator 44 via line 43. 
The compression wave indicator 36 also produces still another output in the 
form of a trigger pulse for enabling a flip-flop 48 via line 46. This 
trigger occurs at a time after the output from flip-flop 34 (the first 
zero level crossing received and detected by receiver 20) plus the 
compression wave travel time measurement multiplied by the same fixed 
number, namely 0.55 times the number representing the travel time between 
the transmitter and receiver 20. Compression wave indicator 36 provides a 
verification pulse output to the shear wave indicator 44 via line 52 at a 
time equal to the travel time of the compression wave between transmitter 
18 and receiver 20 plus 0.9 times this travel time indication. Once this 
trigger pulse on line 46 enables flip-flop 48, the next appropriate (e.g., 
positive-going zero level crossing signal detected by detector 38 produces 
an output signal from flip-flop 48 to shear wave indicator 44 via line 49. 
Output 50 from shear wave indicator 44 is a measure of the difference 
between the time of arrival of the signal on line 43 and the time of 
arrival of the signal on line 49, provided that it is within the 
parameters of 1.55 times the output from compression wave indicator 36 and 
1.9 times such output, which is the verification pulse applied via line 
52. If the output is within these parameters, then the ouput is 
representative of the shear wave travel time. If the output is not within 
these parameters, then an indication of error in a detectable cyclical 
event would be shown, thereby self-assuring that the measurement is a 
proper one if one is given at all. 
Of course, other electronic components could be used to practice the method 
described above with respect to the detected signals illustrated in FIGS. 
2a and 2b, if desired. 
The same results can be had by another technique utilizing the same 
principles employed in the method described above. 
This second technique starts in the same manner by determining the 
compression wave travel time. From that value, it is possible to determine 
by the same technique as described above how to select a certain shear 
wave cycle zero-level crossing point for the first receiver. That is, by 
multiplying the compression wave travel time by a number corresponding to 
the distance between the transmitter and the first receiver and further by 
0.55, it is possible to determine the minimum time after the first 
zero-level crossing where cycles corresponding to shear wave cycles can 
occur. That is, only shear wave cycles should occur after that point. 
Therefore, it is convenient to take the next one occurring. 
For the second receiver, consecutive cycles are selected for measuring the 
difference in time of arrival to the corresponding zero-level crossing 
with that of the first receiver. When one is selected that produces a 
shear wave travel time that falls within 1.55 and 1.9 times the 
compression wave travel time, then it is known that the correct cycle has 
been selected for the second receiver for measuring shear wavel travel 
time. 
Again, it is a simple thing to implement the above procedure either in 
terms of determining from recorded traces or to utilize logic and related 
electronic components, much in the same manner as for the first method, 
and to obtain the shear wave travel time value without inadvertently 
picking a cycle of the return signal detected by the second receiver that 
is a wrong cycle. 
It is determined by either of the two alternate methods described above 
that there is a minimum time after the occurrence of the first detectable 
cycle of a return signal after which the next cycle is a cycle suitable 
for shear wave velocity or travel time measurement. Such cycle could be 
the second or even later number of the actual shear wave cycles. But, it 
is better to let such possible earlier, and possible higher amplitude, 
shear wave cycle pass and select a certain shear wave cycle than to select 
a cycle that may not be a shear wave cycle. 
Also, the positive-going zero-level crossing has been used by way of 
example. The negative-going crossings could be selected and used with 
equal validity to the results, if desired. 
Also, it is assumed that the transmitter impulse is at 15 KHz or below. If 
the impulse is higher in frequency, the factors of of 1.55 to 1.9 would be 
adjusted for the difference in travel time of the frequency used. 
While particular embodiments of the invention have been shown and 
described, and several variations therefrom have been discussed, it will 
be understood that the invention is not limited thereto, since many 
modifications may be made and will become apparent to those skilled in the 
art.