Geol. Soc. Malaysia, Bulletin 27, November 1990; pp. 75 - 102
Application of sonic waveform attributes
in reservoir studies
R. GIR
Schlumberger (Malaysia) Sdn Bhd
Abstract: Bas;d on the field data of the long spaced sonic tool (LSS*), and of the
digital sonic tool (SDT*) from several wells, several potential applications have been
observed for full waveform sonic logging. The amplitude and phase of the waveform are
displayed by combining the signals and their Hilbert transformed signals. From the phase
the various wave arrivals, i.e., compressional, shear,etc. can clearly be identified. The
amplitude logs are presented in a color VDL form. The features shown in the amplitude
VDL are related to the following:
(1)
formation lithology
(2) gas in .the formation
(3) shale types
(4) sand strength
(5) fractures and
(6)
borehole geometry.
Faster formations give weaker compressional wave and stronger shear wave. Shear
is usually not detectable in slow formations (soft rocks). The compressional amplitude is
significantly weaker in gas zones than in water and oil zones. The hard and soft shales,
as well as shale alteration, can all influence the sonic waveforms in predictable ways. It
has also been found that the compressional amplitude is stronger in weaker sands. The
sand strength observation is in agreement with microfrac tests. The sonic amplitude is
also we8kened significantly by induced hydraulic fracturing. The correlation between the
amplitude and the formation has been found to be very useful in assisting the formation
evaluation with other well logging data. In this paper several field examples will be shown
and the data explained with borehole acoustic theory.
INTRODUCTION
For the past 20 years , sonic logging has evolved considerably from simple
compressional velocity logging using the first break of compressional arrival to
using multiple receivers and recording and processing the complete sonic
waveform. This trend is driven by the use of shear waves for determining
mechanical properties ofthe formation for sanding or fracing (Coates and Deneo,
1981; Ingram et al., 1972; Newberry et al., 1985), and stoneley waves for fracture
identification (Brie et al., 1988; Hornby, 1989), and permeability studies (MEA
Review, 1988).
Presented at GSM Petroleum Geology Seminar 1989
* Trademark of Schlumberger
R. GIR
76
However, very little attention has been given to the use of amplitude
information in studying the reservoir rocks.
In this paper, a catalogue of sonic waveforms using examples from different
parts of the world will be presented that will illustrate the usefulness of
amplitudes of compressional (P) and shear (S) waves and Stoneley and their
decay behavior in relation to the following applications, namely:
1. Pore fluid determination
2. Formation strength
3. Formation lithology
4. Fracture identification (induced/natural)
5. Fracture permeability
THE EFFECTS OF BOREHOLE ENVIRONMENT ON SONIC
WAVEFORM AMPLITUDE
In borehole sonic waveforms, there are several acoustic wave types that
travel from the transmitter to the receivers. The earliest part of the waveforms
is the compressional head wave. Trailing the compressional head wave is the
shear wave. The shear wave train is actually a combination of the formation
shear wave and the guided acoustic modes in the borehole. The guided waves are
also called pseudo-Rayleigh waves. In soft rocks, the shear wave is not detectable. In its place there is a borehole fluid wave that follows the compressional
head wave. The last portion of the waveforms is a Stoneley wave. The Stoneley
wave is particularly significant if the sonic tool emits low frequency acoustic
. waves. For the signals near 20 kHz, such as the LSS*, the late arrivals are the
acoustic modes that travel near the sound speed of the borehole fluid. In the
following,the amplitudes of compressional and shear waves will primarily be
discussed.
The amplitude of the compressional and shear arrivals are affected by
several borehole environmental factors, including: formation lithology, intrinsic
formation acoust~c attenuation, formation alteration near the boreh~le, fluid
invasion in a gas zone, and borehole geometry.
Formation Lithology
In hard rocks, the compressional head wave is. weak and the shear/modes
can be very strong. The compressional wave is weak because there is a large
acoustic impedance contrast between the mud and the formation compressional
wave. The shear/modes are strong because the acoustic rays can be totally
reflected inside the borehole, making the borehole a wave guide that traps the
SONIC WAVEFORM ATIRIBUTES IN RESERVOIR STUDIES
77
acoustic energy. As the formation becomes softer, compressional wave becomes
stronger and shear/mode becomes weaker. In the soft rocks, where the formation
shear is slower than the guided mode and the shear wave becomes undetectable.
A new generation 'of sonic tool using dipole transducers will be needed to log
shear directly in soft rocks.
Intrinsic Acoustic Attenuation of the Formation
In gas reservoirs and in fractured formations, acoustic waves can be quite
attenuated in the formation rocks. Results from laboratory experiments of
unconsolidated sandstones have indicated that gas in the pore fluid can cause
the acoustic attenuation to increase significantly (Winkler and Nur, 1982,
Murphy, 1982, 1984). The attenuation (in dB) is equal to 0.027 f S z / Q, where
fis frequency (in kHz), S is the slowness of the formation wave (in microseconds/
ft), z is the travel distance (in ft) of the sound wave, and Q is the quality factor
that is used to describe attenuation. For example, if the slowness is 80 microseconds/ft, frequency is 20 kHz, and Q is 20, then the attenuation is about 20 dB if
the wave travels a 10 ft distance.
Fl~d
Invasion in Gas Zones
Low amplitude compressional waves are observed in gas zone. One possible
reason for the low amplitude is that there is a fluid invasion into the gas
saturated formation. The invaded region has a faster wave speed due to the
larger elastic modulus of the invading fluid than that of the gas. With the zone
near the borehole wall having a faster wave speed than the virgin formation,
the rays will be diverted away from the borehole, causing the compressional
wave amplitude to be significantly weaker than usual.
Near-Borehole Formation Alteration
There can be shale alteration due to the chemical reaction between shale and
water. Shale alteration forms a soft cylindrical layer near the borehole wall.
Stress relief around the borehole in unconsolidated sandstones can also create
a cylindrical layer with a slower wave velocity than the virgin formation. With
a softer layer near the borehole, the acoustic rays can be re-focused into the
borehole, resulting in a stronger formation arrival. This is true even if the
acoustic velocity contrast between the altered zone and the virgin formation is
only in the order of one percent. Since shale types and sand strength are directly
related to the degree and the depth of borehole alteration, the compressional
head wave amplitude can be used to distinguish the type of shales and also to
indicate the strength of sandstones.
78
R.Gm.
Borehole Size, Shape and Conditions
Borehole fluid wave can be quite strong in large holes. In irregular holes and
in holes with cave-ins and other irregularities, the formation arrivals can be less
coherent and consequently weaker than normal. Because of these and other
environmental effects, one should include other available data from logs, from
cores, and from other means in making the waveform interpretation
SONIC ATTRIBUTE ANALYSIS (SONATA*)
A number of properties of the sonic waveform such as phase, amplitude,
frequency and transit times are grouped together in a waveform train. Complex
attribute analysis (Taner and Sheriff,1977) allows a decomposition of the
waveform and separates amplitudefrom angular information. No new information is generated, yet data re-arrangement adds another dimension to the
interpretation. Two attributes (Envelope & Phase) are discussed in this paper.
Envelope display gives a measure of instantaneous amplitude and is independent of Phase, and is useful for lithology formation and strength.
Phase on the other hand, is independent of amplitude and is used to identify
compressional, shear and stoneley arrivals in gas reservoirs/fractured intervals
and washouts. Chevron patterns typically observed in fractured intervals are
enhanced in Phase displays.
A. Application for Fluid Discrimination
Discrimination of gas sands vs. water sands or oil sands is possible whereas
distinguishing oil from water is difficult unless the oil is light. Following criteria
are used:
a. VPNS ratio
DTc is affected significantly by the presence of gas in its pores. The
maximum change occurs between 0-10% gas satUration. Beyond 10% gas
saturation, ·the increase in DTc is insignificant (Figure 1).DTs on the
other hand is much less affected by the pore fluid. Therefore, within a
given lithologic unit, DTs/DTc is indicative of change in pore fluid (gas).
This implies that the presence of gas will be detected even if the water
saturation is greater than 90%.
b. Attenuation of P and S Waves
Amplitude reduction ofP and S Waves due to attenuation is dependent
upon the percentage of gas saturation. Winkler and Nur, 1982, suggest
that S wave attenuation decreases. Winkler and Nur, 1982 suggests that
S wave attenuation increases with degree of saturation, reaching a
maximum at total saturation. P wave attenuation also increases with
saturation at low degrees. P wave attenuation reaches a maximum
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between 60 to 90 percent water saturation. As saturation continues to
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Example 1 - Sandstone Reservoir (gas vs. water)
In this example, the array sonic (SDT*) was run in cased hole. Figures 2a,
b, c show the colour VDL (Amplitude, Envelope and Phase) along with a
computer processed interpretation (VOLAN*) based upon several logs. The
volumetric breakdown of various components-sand, silt, dry clay, bound water,
hydrocarbon, water filled porosity, is shown in different colours-yellow, brown,
brown dots, black, red and white respectively. The three main sands A-I, A-2 and
A-3 have a different hydrocarbon saturation. Sand A-I is considered a water wet.
The following observations can be made:
1. Presence ofhydrocarbon causes strong attenuation ofcompressional and
shear wave arrivals seen in sands A-2 and A-3 (Figure 2b, c). A stronger
attenuation ofP wave in sand A-3 which has a higher gas saturation is
observed. The attenuation seen in the Envelope of sand BI in Figure 2c
implies a much higher gas saturation and is one of the producing sands.
This is in contradiction with the formation evaluation result showing low
gas saturation for this sand. This is most likely the result ofhigh irreducible water saturation in these sands.
2. A strong casing arrival is observed (seen as red colour) coming earlier
than the first compressional arrival. This occurs in zones where the
cement bond is poor. The hydrocarbon sand A-3 has a poor cement bond
on the top and bottom part olthe sand. The instantaneous phase display
helps in identifying zones of poor cement bond by enhancing the casing
arrivals that show constant transit time. Also, P and S wave arrivals in.
the reservoir sands that are masked by the casing arrival.in conventional displays are enhanced in the phase display.
Example 2 - Shaly Sand Reservoir (gas/oil/water sands)
In this field, the sonic waveforms were acquired with long spacing tool in
open hole. The formation consists essentially of sands and shales with varying
amount of silt and clay and coal streaks. DTc in water bearing sands is typically
80-85 microseclft. Shales have a larger DTc of about 95-100 microseclft . Figure
3a show a typical signature of a water sand. The DTc is about 80 microseclft and
VpNs is in the range of 1.7 to 1.75 In Figure 3b, shallower inte:r:vals from the
same well are displayed. Envelope response of these sands have a longer P wave
Envelope (or a slow decay of;P wave) indicative of an increase in shaliness.
Figure 3c shows the differences in the Envelope of the P wave within the
given sand interval. The caliper appears to be a good indicator for the sand limits.
Three zones can be identified based upon amplitude differences. We note DTc in
the range of 90-100 microseclft. In the lower part, and increasing to 105-110
ENVELOPE
AMPLITUDE
LITHOLOGY
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LITHOLOGY
AMPLITUDE
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83
SON IC WAVEFORM ATIRIBUTES IN RESERVOIR STUDIES
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In F igure 2b, a strong red uction ofP and S wave am plitude is observed, inspite of casing
arrivals .
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SONI C WAVEFORM ATTRIBUTES IN RESERVO IR STUDI ES
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SONIC WAVEFORM ATIRIBUTES IN RESERVOIR STUDIES
87
microseclft in the upper part ofthe sand. The Envelope shows strong attenuation
of both P and S waves in the top part. Combined with the fact that DTc increases
to 110 microseclft in this zone suggests a gas cap which is not obvious from
neutron and density logs, which do not show any crossover (probably due to shale
laminations). These observations were confirmed by Production Tests. The oiV
water contact from resistivity log is marked on the Envelope. The differences in
Envelope between the water/oil zone is seen because the oilis light (with possibly
ahighGOR) ..
Example 3 - Clean Sandstone Reservoirs ( gas vs. oil)
In this example, the digital sonic tool acquired the data in open hole.
Envelope of different sand intervals along with gamma ray log are shown in
Figure 4. Gas clearly shows strong attenuation ofcompressional arrival, whereas
oil and water sands look about the same. This is probably a more typical example
as against the one shown in Figure 3 where we differences in oil and water zones
were observed because the oil was light
B. Application ofAmplitude Information for Formation Strength
MECHPRO*IROCKPRO* are traditionally used to quantify the formation
strength in terms of sanding i.e. estimation of optimum drawdown to prevent
sanding during production. This requires input from various data types (DTC,DTS
volume of clay, regional tectonic stress, calibration factor etc.) It is however
possible to estimate on a qualitative basis the relative strength of the reservoir
sands and any changes in strength within the sand from a study of the P wave
Envelope.
Two criteria are used:
1. Amplitude ratio of shear to compressional wave (As/Ap)
2. Amplitude and length of the P wave Envelope.
Example 1- High Porosity Sandstone Res~rvoir (oiVwater)
In this field the sands have a high porosity of about 30% and are friable in
nature. Figure 5 shows the Envelope display of the pay sand along with GR,
resistivity, DTc, DTs, VpNs ratio. The following observations can be made.
Shear waves are generally lower in amplitude as compared to compressional
with the exception of some intervals in the pay sand. In this example the
following observations are made:
1. Within the hydrocarbon sand, a significant change in As/Ap at different
depths, even though the other logs do not indicate any major difference.
DTc remains practically the same within the entire hydrocarbon sand
interval. Obviously, zones where As/Ap are high indicate a harder sand
and therefore should be selected for testing.
R. GIR
88
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sands. Gas sands are identified by a longer P wave Envelope.
SONIC WA VEFORM ATTRIBUTES IN RESERVOIR STUD IES
89
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is identified by higher shear wave amplitude. In the lower part of sand below owe, weak
sands are identified by a longer P wave Envelope.
R. GIR
90
2. Below the oil water contact in the water sand, zones oflow strength were
-identified by their longer P wave Envelope. These zones were selected
for Mini-Frac. The zone that had a longer Envelope failed earlier by 750
psi (Figure 6) as compared to the one with a higher As/Ap and a shorter
Envelope.
c.
Application of Formation Lithology
In general faster formations give weaker compressional and a stronger
shear wave. In soft rocks, shear is usually not detectable ifits slowness is greater
than mud slowness. The hard and soft shales and the shale alteration can all
influence the sonic waveform in predictable waves.
Example 1 - Mixed Lithology
In this example (Figure 7), the sonic waveform was recorded with the LSS
tool. We can make several observations:
1. Zones that have much higher shear wave amplitude as compared to compressional are limestone streaks.
2. Zones marked as shale A and shale B are distinctly different. In the
upper shale,there is a long P wave Envelope and no shear wave, whereas
in the lower shale the P wave Envelope is shorter and the amplitudes of
shear and compressional are comparable. Shale A with a ringing of P
waves is observed (i.e., a longer P wave Envelope) is softer than Shale B.
Shale A does not support any shear waves, implying that DTs is slower
than borehole fluid slowness. Log analysis and core measurements have
confirmed that the clay types are different. The upper shale with a higher
bound water is Montmorillonite and the lower shale is Kaolinite.
3. The interval shown as sandstone is quite different in waveform character as compared to the lower zone which is a shale
4. Zones where a strong attenuation of energy (deep blue colour) are
abserved, correspond to high angle (>77 deg.) open fractures confirmed
by BHTV and'core measurements.
Example 2 - Shale/Silt
In this example (Figure 8), the Envelope display is presented with a set of
logs. Very clearly, the-differences in the Envelope within a long shale interval
highlight differences within the shale which could be due to different amounts
of alteration due to changes in minerology/texture.
D. Applications to Fractures (natural)
Almost all components of sonic waveform are sensitive to fractures. The
main effects of fractures on P and S waves are:
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FRACTURE PRESSURE = 4350 PSI
PRESSURE VERSUS TIME:
Figure 6b:
Mini Frac result
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SONIC WAVEFORM ATTRIBUTES IN RESERVO IR STUD IES
LITHOLOGY
93
ENVELOPE
sandstone
Shale A
Laestone
Shale B
100
Laestone
Laestone
o
5
10
Figure 7: Limestone streaks are seen as high As/Ap. Note a marked difference between shale A and
shale B in terms of the length ofP wave Envelope and shear wave arrival.
R. GIR
94
RH08
140
DT
2.85
40
Figure 8: Envelope of a thick shale section. Zone H shows a higher degree of alteration than zone
F. Zone G corresponds to an overpressured shale that stands out as a geological marker
on the sonic waveform.
SONIC WAVEFORM A1TRIBUTES IN RESERVOIR STUDIES
•
some delay in the arrivals;
•
amplitude reduction of the compressional and lor shear arrivals;
•
reflections causing Chevron patterns;
•
enhancement of borehole coupling causing spikes.
95
Fracture indications from P and 8 waves detect both open and healed
fractures.
Example 1 - Fractured Basement (Granite) Reservoir
In this example (Figure 9) the amplitude of the waveform is displaced along
with GLOBAL* results showing formation evaluations. The purple colour
represents granite, brown represents weathered diabase, red represents diabase
and green represents hydrocarbon (oil). The yellow curve shows a reasonable
hole, yet there are several zones where the P and 8 arrivals are strongly
attenuated.
Two main observations can be made on this display:
1. The zones of amplitude reduction of P,8 waves are deep formation
fractures.
2. The zones where there is no apparent reduction ofP and 8 amplitudes,
but a reduction in pseudo Raleigh waves is due to a local phenomenon
fractures induced by drilling. It can be seen that the caliper reading
shows a kick in these intervals.
The main production in this field is from these natural fractures in the
basement.
E. Application to Fractures Induced by Hydrofracing
Fracture height is one of the important parameters in the design of a frac
job (Newberry et al., 1985). 80nic waveforms recorded before and after a frac job
can be used to determine induced fracture height. Frac-hite parameters can then
be adjusted in a given field using the waveform results as a control to predict with
reasonable accuracy the fracture height prior to a frac job.
The example shown here is of sonic waveform recorded with L88 in open hole
(Figure lOa) before fracing and in cased hole (Figure lOb) after a frac job.
Attribute processing enhance effects seen on the raw waveform data. For
instance, instantaneous amplitude (Envelope) shows a strong attenuation of
compressional(P) and shear (8) energies after fracing. Instantaneous frequency
shows a sharp drop in frequencies caused by induced fractures.
In the example shown, the bottom of the fractured zone reaches below the
R. GIR
96
Dri lling I nduced
Fract ures
500
,(J; SEC
2100
Figure 9: Example offractured Granite Reservoir, Amplitude of sonic waveform showing attenuation of P and S waves amplitude are unaffected but the pseudo-Rayleigh waves show
attenuation-could be indicative of drilling induced fractures ,
(BEFORE FRAC)
AMPLITUDE
ENVELOPE
FREQUENCY
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Figure lOa:
Example showing amplitude, Envelope and Instantaneous frequency display over a reservoir sand interval
before Fracing. Vertical height of the fra cture can be verified from the waveforms.
\D
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(AFTER FRAC)
ENVELOPE
AMPLITUDE
FREfiUENCY
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2900400
2900400
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2900
P-SEC
Example showing a mplitude, Envelope and In stantaneous frequency dis pJay over a reservoir sand in terval
after Fracing_ Vertical height of the fracture can be verified from the waveforms.
SONIC WAVEFORM AITRIBUTES IN RESERVOIR STUDIES
99
oil-water contact. The well produced with 50% water. After recognizing the
origin ofthe water production from sonic waveform analysis, a special operation
was conducted with cement injected in the lower part of the reservoir and gel in
the upper part. The well now produced clean oil
F. Applications to Fracture Permeability
Qualitative information about permeability can be obtained from Stoneley
waves Occurring in the 1-5 kHz frequency band, the Stoneley wave propagates
along the borehole wall with characteristics of both compressional and shear
waves. On encountering an anomaly in the formation, sayan open fracture,
some of the efergy of the pressure wave will be dissipated in and out of the moving
fluid in the opening. The wave's transmission time along this path may also
increase slightly, and the wave's amplitude could be further reduced by reflections from the fluid/rock interfaces within the crack.These reflected events
further complicate the waveform display.
Example 1 - Fractured Carbonate Reservoir Computer Based Formation
Evaluation
In this example (Figure lIa) the phase display is plotted along with lithology interpretation and production logging tool results (PLT) and the Stoneley
wave transit time and energy. An increase in DTST combined with a decrease in
Stoneley energy 'A' st could well indicate zones of increasing permeability. In the
zones of high fluid flow into the borehole there is a sharp drop in Stoneley energy
and an increase in DT Stoneley- the former is derived from a single receiver, and
the latter from the eight receiver array, so there is a difference in vertical
'resolution'. In addition, there is a prominent reflection event in the waveform
itself. This waveform, used to derive Stoneley energy, is from the same single
receiver and has been filtered to Stoneley frequencies and translated into and instantaneous phase form to highlight the subtle changes in the waveform events.
The lithology results show that the fracture, as indicated by Stoneley
analysis, is occurring where a dolomitic zone meets a more porous limestone
region. Similarly Stoneley events can be seen next to Production Logging Tool indications offluid flow into the borehole. Once again, some reflected events may
be seen in the waveform. Another way to evaluate the Stoneley results is to
compare them with core permeabilities (Figure lIb). This is a different approach
to the previous example, as such core measurements are no longer taken in-situ
and larger contributors to a formation's permeability-such as fissures and
fractures-may not be included.
CONCLUSION
We have described simple criteria for analyzing the Amplitude and Phase
information present in a complete sonic waveform train for studying reservoir
R. GIR
100
INSTANT. PHASE
LITHO LOG Y
10 FEET
240
SLOWNESS (J..lsAt)
150
ENERGY (d8)
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00
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TIME
Figure lla:
o
."
o
o
'"oo
o
( u s)
Stoneley transit times, Energy and Instantaneous frequency display over a reservoir
sand interval before and after Fracing. Vertical height of the fracture can be verified
from the waveforms.
SONIC WAVEFORM ATIR IB UTES IN RESERVOIR STUDIES
CORE PLUG AIR
PE RMEAB ILITIES
STONELEY IN DICATOR
0.1
(mD)
1000 0
(dB)
-5
Figure lIb: Comparison of Stoneley energy with permeability measurements from cores.
101
102
R. GIR
properties.
Sonic waveforms, when used in synergy with other logging measurement,
provide a powerful tool for the geologists, petrophysicists and reservoir engineers.
ACKNO~DGEMENTS
The support ot the various oil companies is gratefully acknowledged. I wish
to thank B.Felder and C. Poster for providing some of the examples used in this
paper.
REFERENCES
BRIE, A, Hsu, K, ECKERSLEY, C., 1988. Using the Stoneley Normalized Differential Energies for
Fractured Reservoir Evaluation, SPWLA, 1988
COATES, G.R., DENEO, S.A, 1981. Mechanical Properties Program Using Borehole Analysis and
Mohr's Circle, 1981
HORNBY, B. E., 1989. Imaging of Near- Borehole Structure using Full-Waveform Sonic Data, Geophysics, 54,6,747-757,1989
INGRAM, D.S., ANDERSON, R.A, ZANIER, A. M., 1972. Fracture Pressure Gradient Determination from
Well Logs, SPE 4135,1972
MEA Review, 1988. Middle East Well Evaluation Review, No. 5,1988
MURPHY, W.F., 1982. Effects of Partial Water Saturation on Attenuation in Massilon Sandstone and
Vycor Porous Glass, J. Acoust. Soc. Am., 71, 1458-1468, 1982a
MURPHY III, WILLIAM F., 1984. Acoustic Measures of Partial Gas Saturation in Tight Sandstones, J.
Geophys. Res., 89, B13, 11549-11559,1984
NEWBERRY, B.A,NELSON, R.F., AHMED, U., 1985. Prediction of Vertical Hydraulic Fracture Migration
using Compressional and Shear Wave SloWness, SPE 13895,1985
TANER, M. T., AND SHERIFF, R.E., 1977. Application of Amplitude Frequency and Other Attributes to
Stratigrapic and Hydrocarbon Determination, AAPG Memoir 26, Seismic Stratigraphy Application to Hydrocarbon Exploration,pp 301-327,1977
WINKLER, K W., AND A NUR, 1979. Pore Fluids and Attenuation in Rocks, Geophys. Res. Lett., 6,
1-4,1979
Manuscript received 10th October 1990