17. acoustic logging methods and applications

8/12/2019 17. Acoustic Logging Methods and Applications

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382 Well Logging for Earth Scientistsof lithology and fluid type, detection of overpressured zones and fractures,and estimation of the formation strength. Ultrasonic devices operating in therange of 1 MHz have opened the door to acoustic borehole imaging. Thesecan be used in the detection of naturally occurring fractures and, in anotherapplication, the evaluation of the cementing of cased wells.

TRANSDUCERSAcoustic measurements rely on the production of a pulse of pressure which isapplied, through the borehole mud, to the formation. Two types oftransducers have been in use both as acoustic source generators and receivers.One type is based on a magnetostrictive behavior of certain materials. Forthese, application of a magnetic field causes a volume reduction in thematerial. Consequently the sudden application of a magnetic field initiates apressure pulse which is completed upon the removal of the magnetic field.This is accompanied by a subsequent volume relaxation.

The general form of the magnetostrictive transducer used in logging is atorus. The magnetic field is produced by supplying current to a coil whichcompletely wraps the toroidal core material. Since the magnetostrictivematerial is also magnetized, i t can operate as a receiver. Any impingingcompressional acoustic energy will cause volume distortions in the core andthus vary the magnetic field which threads the coil windings. This changingmagnetic field will produce a voltage at the terminals of the coil which isrepresentative of the acoustic signal.

The second type of device in common use is based on ceramic materials,such as BaTi02, which have piezoelectric properties. This dielectric materialresponds to an applied electric field by changing its volume. Applying avoltage pulse between the inner and outer surfaces of the ceramic torusproduces a subsequent fluctuation in its volume and thus the generation of apressure disturbance. As a receiver, the incoming compressional wavedistorts the ceramic, setting up a polarization charge, which appears as avoltage across the two sides of the torus.

The output power and operating frequency of both types of devices arelimited by surface area and material properties. The dimensions dictated bylogging sondes result in frequencies around 25 kHz. As a transmitter, theapplication of a voltage pulse results in a ringing at the central frequencywhich lasts for several periods.

CONVENTIONAL SONIC LOGGINGConventional sonic logging, for this discussion, is taken to mean thedetermination of the transit time of compressional waves in the materialsurrounding the borehole by a device with two receivers. These are generally


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Acoustic Logging Methods and Applications 383located at 3 and 5' from the transmitter. This type of device is illustrated inFig. 17-1. The technique consists of measuring the difference in the arrivaltimes of the acoustic energy at the two transducers. This difference dividedby the span between the two detectors yields a transit time At (usuallyexpressed in ps ed ft) for the formation. The depth of investigation of thismeasurement is somewhat difficult to define in the case of a uniformformation. Since only the transit time of the first detectable signal is beingmeasured, the measurement will be sensitive only to the acoustic path whichhas the shortest time. This is generally the one parallel to the borehole walland very close to its surface. The notion of depth of investigation willbecome meaningful only when we consider the problems of alteration, anddamage (both imply a reduction in the formation velocity in the vicinity ofthe borehole) to the borehole wall.

Figure 17-1. A standard sonic tool in the centered logging configuration. FromTittman.

The first arrival transit time may characterize the undisturbed formation,depending on the source-to-detector spacings, the velocity contrast betweenthe invaded (or altered) and undisturbed zone, and the thickness of thisaltered zone. Some authors have attempted to define a pseudogeometric


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384 Well Logging for Earth Scientistsfactor for the first arrival time? Their results, shown in Fig. 17-2, can beinterpreted as giving the maximum thickness of an altered zone for which themeasured At is still representative of the virgin formation. This depth ofinvestigation increases with source-to-detector spacing and for increasedvelocity contrasts between the two zones. For the conventional sonic tool atypical value may be on the order of 6.

Various Sonic Sondesv 3 > v 2

(50 Point on Geometric Factor)1.8

1.6-

1 .4 -

1.2-

1.0-

0 5 10 15 20 25 30 35 1Depth of Alterat ion (Inches)

0

Figure 17-2. Estimates of the depth of investigation for three types of conventionalsonic arrays. From Chemali et al?

The typical presentation format for a standard sonic log is shown inFig. 17-3. The formation transit time is presented in tracks 2 and 3 .Increasing transit times (or slowness, as it is more chic to say in acousticlogging circles) are shown to the left, which is also the trend for increasingporosity. n additional trace consists of a series of little pips every so often.These are seen at the beginning of track 2 in the figure. Each pip represents1 msec of integrated travel time and serves as a reminder of the origin of thesonic log: It was developed to correlate time with depth in seismic sections.The conventional sonic log presentations for portions of the simulatedreservoir are shown in Fig. 17-4. The bottom zone is from the carbonatesection, and the upper from the shaly sand section. To emphasize theporosity sensitivity of the sonic log, comparison with the neutron and densitylogs over the same intervals is provided.The use of two-detector devices, with normal spacing of 3 and 5 fromtransmitter to receiver, introduces some problems with the resolution of thin


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Acoustic Logging Methods and Applications 385Ca I perHole Diarn.Inches

BHC Sonic Log2' Spanpsectft6 16

?f

I

whl0

Figure 17-3. A standard acoustic log presentation format with the integrated travelt ime pips. From Timur.'

beds whose compressional velocity is different from the surrounding medium.This problem is considered in Fig. 17-5. which shows a fast limestone bedsurrounded by slower shales. As indicated in part a, if the span between thetwo receivers exceeds the bed thickness, then the measured At will neverattain the true value but some weighted average over a length which is equalto the difference between the span and the bed thickness. In part b of thefigure, the span is shorter than the thickness of the bed, and for a shortstretch, the value of At attains the true value of the fast formation.

One of the real advantages of sonic logging is its relative insensitivity toborehole size variations. Fig. 17-6 qualitatively compares borehole sizeeffects for the density device, the neutron porosity device, and an acoustictool. In the section of log shown, there is an enormous borehole irregularityover a 20' interval that is seen on the caliper trace. Although the Ap curvefor the density log is not shown, an experienced interpreter might question thevalue of pb indicated in this region. In large irregularities like this, the skidof the measuring device cannot possibly follow the borehole profile andconsequently rather large tool-formation stand-off can develop. Thecompensation can cope only with gaps which are generally less than 1 . Forcases which exceed this stand-off, the correction will not be sufficient, andthe bulk density will indicate a value which may be much too low.In the middle log, the neutron porosity can be seen to be nearly flat justbelow the shale section. However, at the point of largest caliper activity


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wL

-B


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Acoustic Logging Methods and Applications 387

Transit Tlme

Figure 17-5. The effect of the spacing between two receivers on the measuredacoustic travel time of a thin high velocity bed. From Timur?

3100

3150

3100

3150

Figure 17-6. Illustration of the insensitivity of the acoustic measurement toextremely poor borehole conditions. Compared are the responses of adensity and a neutron porosity tool; both show incorrect responses inthe caved section of the borehole. From T i m ~ r . ~


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388 Well Logging for Earth Scientiststhere is also a peak in the neutron porosity, which no doubt is the result of aninadequacy in its compensation scheme. The dramatic data, however, are inthe third log, which shows the measured At in this zone. It is very steadydespite the large caliper excursions. No doubt the porosity of this zone isquite constant, a fact that could not be deduced from either of the other twoporosity devices. The question remains: How is the sonic boreholecompensation achieved?As Fig. 17-7 indicates, in a region of changing borehole size, a singlesource-detector sonic device will measure abnormally long transit times whenthe hole becomes enlarged. This is a result of the increased transit time fromthe transmitter across the mud to the formation and back to the receiver. Apartial solution to this problem is obtained by use of one transmitter and tworeceivers. By determining the travel time to the two detectors and using thedifference to determine the travel time, as indicated in the figure, the effect ofthe borehole diameter is eliminated except at the boundaries, where hornscan appear on the log response.

0 0c

O n e Receiver Tw o ReceiverT y p e T y p e

True Trans i t T imeMeasured Trans i t T im e- - _ _ _ _ -

Figure 17-7. The effects on the integrated travel time at the boundaries of holediameter changes. From Timur?

The more general logging situation is shown in Fig. 17-8. Not only canthere be changes in borehole diameter, but the tool is not necessarily centeredin the borehole, because of deviation of the borehole and differential stickingof the tool string. As indicated, this more general case can be solved by theuse of two transmitters and two pairs of closely spaced receivers. Two setsof differential travel time measurements are ma&: an up-going one and a


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Acoustic Logging Methods and Applications 389down-going one. In the case sketched in Fig. 17-8, the up-going transit timewill exceed the down-going case. By averaging the two results, the effect ofexisting unequal mud travel paths is eliminated, and the measurement reflectsthe travel time of the formation. Tools of this type re said to be boreholecompensated (BHC).

per Transmit ter

Transmit ter

Figure 17-8. The use of four detectors to compensate. for borehole size and tooltilt

Another variation of this technique, used for a long spacing device to bediscussed later, is shown in Fig. 17-9. In this case, two transmitters and tworeceivers are used to produce the same result as the six-transducer tool ofFig. 17-8. For the long spacing device, there are two receivers at the top ofthe tool and two transmitters at the bottom (a saving of two transducers).The measurement is made in two phases. At one position in the well, thebottom transmitter fires and the transit time between the two top receivers ismeasured. Shortly after, when the tool has moved so that the twotransmitters are nearly in the position previously occupied by the tworeceivers, the two transmitters are fired in succession, and the two transittimes (from the different transmitters) are measured to the lower detector. Asthe figure indicates, this is equivalent to the use of two transmitters and fourreceivers, and the technique is referred to as depth-derived boreholecompensation.


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390 Well Logging for Earth ScientistsR t

i I: 9 8 ' - Act iveTransmitterof Receiver

,.-LT.i 1 1m

Figure 17-9. The principle of the depth-derived borehole compensation. Courtesyof Schlumberger.

Some Typical ProblemsDespite the good performance of the compensated sonic tool, as noted above,there are a few situations which can cause problems. One of them resultsfrom the possibility that, in slow formations, with very large borehole sizes,the direct mud arrival will precede the formation arrival. In the conventionalsonic tool, an amplitude rise in the detected pulse is sensed to determine thefirst arrival. However, it is not necessarily the result of a signal from theformation. Because of the generally large contrast between formationcompressional velocities and mud velocities (generally the formation velocitycan exceed the mud velocity by a factor of 2), the formation arrival and mudarrival separation can be increased by simply increasing the distance betweentransmitter and receiver. However, for a given spacing it is possible for thetwo signals to overlap, if the mud transit time to and from the formation islarge (because of a very large borehole size). This notion is quantified inFig. 17-10. The area of reliable At measurements is indicated for receivers atthree different distances: The slower the formation, the smaller the boreholesize must be in order to see the formation arrival before the direct mudarrival. The situation improves dramatically for increased spacing.One serious environmental effect for the sonic device is that of damageor alteration of the material near the borehole wall. Generally this occurs insome clays, commonly known as swelling clays, which take on water,expand, and suffer changes in density as well as velocity. Another source ofalteration can be induced stress-relief fracturing around the well bore, which


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Acoustic Logging Methods and Applications 391Long SpacingQ - l n t Sonde90-

Conventional140 3 - 5 f t S o n d e\13012011010090

Transmitter iNearReceiver Spacing6 8 10 12 14 16 18 ~

Hole Diameter ( in)Figure 17-10. Areas of confidence for conventional two-receiver tools as a function

of boreho le diameter. From Goetz et alcan largely alter the acoustic properties of the material. A strilcing exampleof the former type of shale alteration is shown in Fig. 17-11, which showsthe transit time measured in the same well two months apart. In general thetransit times have increased by about 20 p e d f t due to the shale alteration.In a case such as this, the fastest travel time is through the slower alteredmedium. This is due to the thickness or depth of alteration; the two-waytravel time through it to reach the faster undamaged formation exceeds thetime difference between them, and thus the first signal to arrive travels onlythrough the altered zone.

n annoying feature which sometimes appears on acoustic logs is cycle-skipping, which is shown in Fig. 17-12. This condition is immediatelyrecognized by the spiky nature of the At trace; apparent travel time changeson the order of 40 psec/ft are visible. Fig. 17-13 indicates the origin of theseproblems: Either the timing circuitry is triggered by random noise, or theanticipated signal strength falls below that expected and the arrival is notdetected until a full cycle ( 4 0 p e c ) later.

Newer DevicesThe preceding discussion concentrates on devices which extract the simpleinterval transit time, or compressional wave velocity. However, a few


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392 Well Logging for Earth ScientistsBHC SonicTravel Time(psec/ft)Hole Open 4 Days

Hole ODen 79 Davs

Figure 17 11. Log example of the effect of formation alteration observed betweentwo logging runs with 75 days of elapsed time. From Timur?

Induction IntervalResistivity Travel Time(API Units) WM) (psedf t )

Figure 17 12. An example of cycle-skipping on the interval transit time log. FromTimur?


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Acoustic Logging Methods and Applications 393Detection

NearReceiver

FarReceiver

+ I CycleI

Figure 17-13. The origin of cycle-skipping. From Timur?

Var iab le Densi ty Disp lay

Figure 17 14. One method of identification of shear arrivals: the variable densityplot. Courtesy of Schlumberger.


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394 Well Logging for Earth Scientistsdevices extract other information from the complex acoustic waveforms, suchas the shear wave velocity. These newer devices, of varying sophistication,basically provide access to a large portion of the received wavetrain for signalprocessing rather than providing a crude analysis such as the first arrival time.

The measurement of compressional velocity was natural. Since this wavehas a larger velocity, i t always arrives at the detector first. The shear arrivalcan be masked if it arrives in the midst of the ringing portion of thetransmitter signal, or it can be lost in some of the other modes produced byacoustic waves in boreholes. In the best cases, the problem of detecting theshear arrival is one of just looking for it after the compressional arrival. Sucha trivial case is illustrated in Fig. 17-14. In the waveform shown, the sheararrival is clearly distinguishable. The accompanying VDL (variable densitylog) presentation, shows quite clearly the variations in arrival time for thecompressional and shear waves: The positive amplitudes of the waveform arereplaced by dark bands as one waveform after another is reduced in thismanner.

From such a demonstration, it seems possible to extract the shear traveltime by judicious time-windowing of the received waveform from theconventional sonic tools. However, the recorded waveform must be availablefor analysis. Until recently this has been a somewhat tedious specialty, but ithas been done, on occasion, nevertheless. With the development of longerspacing sonic devices, shear velocity determination has become less difficult.

The stacked waveforms from six receivers placed from 3 to 16' from thetransmitter, as shown in Fig. 17-15, indicate that the separation of shear from

Depth591.6591.6

594.6594.7596.3

598.4

S Wave Time Pick--I +W aveform Period Pick T-RSpacing

3'5'

v -10'Ir ,12'vIf 1000 2000

I 61Time, p e c

Figure 17 15. The effect of spacing on the separation of shear and compressionalarrivals. From Timur?


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Acoustic Logging Methods and Applications 395compressional arrivals is greatly aided by spacing. This is one of themotivations for the development of long spacing sonic sondes, devices whichare currently coming into vogue. A second reason for their development is tocombat the problem of the altered zone. As might have been guessed fromthe results of Fig. 17-10 and the discussion on depth of investigation, thelonger the spacing, the more reliably one can measure the transit time of thefaster, undamaged formation. A common detector configuration is at 8 and10 from the transmitter. The results of such a device compared to theconventional 3-5 spacing device are compared in Fig. 17-16. In this zoneof nearly constant velocity, the conventional device is seen to read At valueswhich are much too high.

SP

250 psecl f t

Figure 17-16. Comparison of the response of a conventional and long spacing sonictool in an altered formation. From Timur?The quantification of this improvement can be found in Fig. 17-17. It

indicates the reliable zone of measurement (to the left of the indicated c w e s )for the conventional and long spacing sonde. The change in velocity of thedamaged zone which can be tolerated as a function of its thickness isindicated as a function of the formation travel time. In a formationcharacterized by a At of 100 pec/ft, a 20 pec/ft alteration can be toleratedup to 5 thick for the conventional sonde, but it can be up to 14 thick with


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396the long spacing device. This tolerance to alteration also eliminates problemswith direct mud arrival in very large borehole sizes.

Well Logging for Earth Scientists

Conventional Long SpacingI (3-5 f t (8-10 ft)

Alteration Depth(Inches from Borehole Wall)Figure 17-17. The effect of alteration as measured between the observed intervaltransit time and the formation transit time as a function of alterationdepth into the formation. As expected, the long spaced tool is able totolerate deeper alteration before noticeable effects appear. From

Goetz et al.'

Acoustic array tools containing a battery of receivers, variable detectorspans, and waveform digitization are making progress in extracting reliableshear measurements. An example of one of these devices with eight receiversis shown in Fig. 17-18. In an array tool such as this, waveform processingand signal extraction is aided by the possibility of stacking signals recorded atthe same depth from different receivers for noise elimination as well asdiscrimination against other acoustic signals produced within the borehole.Typical waveforms from the eight receivers are shown in the upper portion ofFig. 17-19 with a clear indication of the compressional, shear, and Stoneleyarrivals. To deal with cases less clear than this example, an elaborate signalprocessing scheme known as slowness-time coherence has been successful inextracting the various arrivals Basically, it measures the similarity of theeight wave forms by comparing a portion of wavetrain 1 to shifted portions ofthe other seven waveforms. Using this processing, a plot such as that shownin the bottom portion of Fig. 17-19 can be developed. The ordinate is thetime (in msec) along the wavetrain of receiver 1. The abscissa is At orslowness, determined from the conversion of the delays applied to the other


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Acoustic Lo ggin g Methods and Applications 397eceiver Electronics

Fluid-Delta T Measurements

\Eight Wideb and Receivers,Spaced 6 Apar t

R9'Two Stan dard Ceram ic Recei ver s

ransm it ter ElectronicsU

Figure 17-18. A n eight-receiver sonic array tool. From Moms et al?receiver waveforms. The contours which clearly delineate the three arrivalsindicate regions of largest similarity between the shifted waveforms and theoriginal signal. A sample log section from this type of processing is shownin Fig. 17-20. In addition to the interval transit time of the compressional,shear, and Stoneley waves, a log of their amplitude can be produced whichhas application in the detection of fractures.

ACOUSTIC LOGGING APPLICATIONSOne of the first extensive uses of borehole sonic logs was for correlation.Wyllie and others observed that there was a strong correlation between sonictravel time and the porosity of consolidated formations. This resulted in theso-called Wyllie time-average equation discussed earlier. The laboratory dataseemed to indicate that a volumetric mixing law held for the case of transittime. Knowing the matrix transit time and the fluid transit time, one couldobtain the appropriate porosity from a measurement of any intermediate traveltime.

Figure 17-21 is an example from a chartbook which shows the solutionto the time-average expression for the three common mat rice^ ^ However,


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398 Well Logging for Earth ScientistsComp. Shear Stoneley

1601

@ Compressional40500 1500 2500 3500 4Time (ps)

Figure 17-19. Waveforms from the eight receivers recorded in a formation showingdistinct compressional, shear, and Stoneley arrivals. In the lowerportion, the slowness-time coherence processing of the waveformsidentifies the three arrivals and quantifies the interval transit time andrelative amplitude of each. From Moms et al.

note the cluttered appearance of the chart. The three principal linescorrespond to a solution of:A t - A t ,A$- a h =

where the fluid velocity has been fixed to 5300hec. In addition to the threelinear relations expected for three matrices of differing matrix travel times,there are three slightly curved lines for the same three matrices. Raymer etal., established these additional transforms, which correspond to theirjudgments based on the observation of much field data.8 They basically takeinto account that the sonic travel time seems to consistently underestimate theporosity in midrange. The additional lines to the right, carrying the notationBcp, or compaction factor, correspond to an empirical method for correcting


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Acoustic Logging Methods and Applications 399

0Slowness (Nft)Figure 17-20. A log of compressional, shear, and Stoneley interval transit time froman array tool and signal processing. From Moms tthe transit time measurements for formations which are not sufficientlycompacted or which do not have sufficient effective stress (Ap = 4000 psi).

Despite the very empirical approach to the common interpretation ofsonic logs, they do yield useful porosity estimates under many circumstances.For the technique to work well, the type of rock and its appropriate matrixtravel time must be known, or a local transform between travel time andporosity must be established. Often, in cases of extreme hole rugosity orwashout, when porosity readings from the density or neutron devices areuseless, the sonic measurement will still be reliable.

Determination of other interesting formation properties can be made byusing the sonic measurement in conjunction with other logging tools. Oneexample of the sonic measurement in conjunction with two other porositymeasurements is shown in Fig. 17-22. In the middle of the log, both theneutron and density readings indicate an increase in porosity, to a maximumof about 3 PU. The rest of the zone appears to be a zero porosity dolomite.However, in this same zone, where the dolomite appears to have 3% porosity,notice that the At curve has not shifted. It still indicates zero porosity.

The explanation for this type of occurrence, especially in carbonate rocks,is the presence of so-called secondary porosity. This is thought to be porositywhich is unconnected with the majority of the pore systems in the rock. For


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400 Well Logging for Earth Scientists

. . . 1. . . . . . .

. . , . .. . . . . . . .

30 40 50 60 70 80 90 100 110 120 1At, Interval Transi t Time

Figure 17-21. Chart for estimating porosity from compressional interval transit time.In addition to the Wyllie time-average solution and compactioncorrections, another empirical solution by Raymer et al is shown.8From Schlumberger?

inclusions of porosity, unconnected to the rest of the rock, the acoustic waveenergy follows the faster path around the inclusion in the rock matrix. In thiscase, there is no alteration of the travel time from that of the zero porositymatrix. In this sense, the sonic measurement does not see the secondaryporosity.

Lithology and Pore Fluid IdentificationThe determination of lithology from borehole acoustic measurements is basedon the variations of elastic parameters between rock types. These variationsare reflected in the shear and compressional velocities. One convenientmethod of classifying lithologies is to compare the compressional velocity to


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Acoustic Logging Methods and Applications 401Neutron Porosity-%Transit Tirne-psedft......30..........15............._ ......3 :

Figure 17-22. A log example of a carbonate section, showing indications of thepresence of secondary porosity. This is inferred from the increaseof density porosity and neutron porosity with no correspondingchange in the At. From Timur?

the shear velocity, as suggested by Pi~ket t .~ is laboratory and field data formany different formations showed that measurements corresponding tolimestone and dolomites were found along lines of constant but differentratios: VJV, = 1.9 for limestone and = 1.8 for dolomite. Sandstones showeda variation of velocity ratio from about 1.6 to 1.75, with the upper limitcorresponding to high porosity sands under low effective stress.

Picketts compilation of field points was made from painstaking manualanalysis of recorded wavetrains. With the availability of routine shear andcompressional velocities this technique becomes useful for lithologydetermination and the identification of gas. Figure 17-23 is a cross plot ofthe interval transit time for shear and compressional waves in the format ofPicketts original work. It is a composite of logging data from four differentwells which contained, dolomite, limestone, halite, and sand formations.Some of the latter were gas-bearing. As found earlier, the limestone anddolomites fall on lines of constant ratios. The water-filled sandstone ratios


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402 Well Logging for Earth Scientists

60

AtC

80

40r

-

--

t4 nn

mireL,., L imestone

Sandstone/-

17. acoustic logging methods and applications

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