PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

The effect of water saturation on the P-wave velocity ofsedimentary rocks
Sair KAHRAMAN1
Hacettepe University, Turkey

ABSTRACT
Since ultrasonic methods are non-destructive and easy to apply, they have been widely used invarious fields such as mining and civil engineering applications. The water content of rock strongly
influences seismic waves. Rock engineers sometimes need to estimate the wet-rock P-wave velocity
from the dry-rock P-wave velocity. In this study, in order to investigate the predictability of wet -rockP-wave velocity from the dry-rock P-wave velocity, P-wave velocity measurements were performedon the thirty-six core specimens pertaining to six different sedimentary rocks. The evaluation of theresults showed that P-wave velocity increment due to saturation was about 30 %. The wet-rock P-wave
velocity values were also correlated to the dry-rock P-wave velocity values. A very strong linear
correlation was found between the two parameters. Concluding remark is that the wet -rock P-wave
velocity values of sedimentary rocks can be predicted from the developed empirical equation.
Keywords: P-wave velocity, Sedimentary rocks, Water saturation

1. INTRODUCTION
Seismic methods are non-destructive and easy to apply. Therefore, they are increasingly being usedin various fields such as mining, geotechnical, civil, and underground engineering applications orlaboratory investigations.
There are a number of factors that influence the sound velocity of rocks. The important factors arerock type, texture, density, grain size and shape, porosity, anisotropy, water content, stress andtemperature. In addition to these factors, rock mass properties also influence the sound velocity.
Weathering and alteration zones, bedding planes and joint properties (roughness, filling material,water, dip and strike etc.) have important influence on the sound velocity.
Early theories were proposed by Gassmann (1) and Wood (2) on the effects of saturation on theseismic velocities. The variation of velocity in sandstone as a function of water content was studied byWyllie et al. (3). They observed a marked decrease in the P-wave velocity as the saturation wasreduced from 100 % to approximately 70 %. Between 70 % and 10 % the P-wave velocity was nearly
constant and below 10 % P-wave velocity usually increased to dry P-wave velocity. Wyllie et al. (4)clearly showed that the velocity of fluid-saturated rocks was dependent on the ratio between thevelocity of rock and the velocity of pore fluid. The influence of saturation on pulse velocity in St.
Cloud granodiorite was examined by Thill and Bur (5). They observed that a remarkable velocitychanges could occur even in compact rock having only a minute amount of porosity. Nur and Simmons(6) measured the compressional wave velocity in a sample of Chelmsford granite, initially saturated
with water but allowed to dry in the atmosphere over a period of four days. They indicated that a rapidchange of velocity occurred in the first a few hours even though the porosity of the sample was only
about 1%.
The study carried out by Ramana and Venkatanarayana (7) on Kolar rocks indicated that in generalboth weight and velocity increased with increasing the time of saturation. Beyond 48 hours thesaturation curves tended to be steady. Gregory (8) studied the influence of saturation by water, oil, gas,and mixtures of these fluids on the densities, velocities, reflection coefficients, and elastic moduli of
consolidated sedimentary rocks in the laboratory by ultrasonic wave propagation methods. He foundthat fluid saturation effects on compressional wave velocity were much larger in low-porosity than in
high-porosity rocks. Lama and Vutukuri (9) and Bourbié et al. (10) reported that the wetting of rocks
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sairkahraman@yahoo.com

usually leads to a rise in the P-wave velocities. However, since the P-wave velocity in water is lessthan the P-wave velocity in mineral skeleton, generally the wave velocity in more porous rocks
completely saturated with water is less than in slightly porous rocks.
Guéguen and Palciauskas (11) state that elastic wave velocities in rocks are closely related tomicrostructural changes such as pore structure and connectivity. A theoretical model was suggested byDavid and Zimmerman (12) for predicting the wet P- and S-wave velocities as function of the dry ones,based on thin, closable cracks having a distribution of aspect ratios, and stiff, nearly spherical poresthat do not close under pressure. Pimienta et al. (13) showed that elastic velocities in Fontainebleau
samples appeared to depend on the moisture content, while such relation were not observed forlimestones. Furthermore, the decrease in the velocities for quartz-pure sandstone increases withporosity. These measurements thus confirm that weakening from moisture does not originate from thepresence of clay minerals, but from the rock microstructure.
Kahraman (14) carried out dry- and wet-rock P-wave velocities on the different rocks types andfound a strong linear correlation between the dry- and wet-rock P-wave velocities. Recently,
Kahraman et al. (15) investigated the correlations between the dry- and wet-rock P-wave velocities for
pyroclastic rocks. They derived estimation equations between the two parameters for the samples
having porosity values of less than 20 % and greater than 20 %, respectively.
In this study, the influence of water saturation was investigated on the P-wave velocity of
sedimentary rocks such as limestone, travertine, and sandstone, and a prediction equation wasdeveloped for the dry-rock P-wave velocity from the wet-rock P-wave velocity.

2. SAMPLING
The block samples of rocks were collected from the stone processing plants in Kayseri and Konya
areas of Turkey for the laboratory testing. In order to provide the test specimens free from fractures,
partings or alteration zones, block samples were inspected for macroscopic defects. A total of 6
different rock types were sampled, 4 of which were limestone, 1 of which were travertine, and 1 ofwhich was sandstone. The locations and the types of the rocks sampled are given in Table 1. Thirty-six
core specimens in NX size were prepared from the block samples.
Table 1 – The rock types and the locations of the tested rocks.
Rock code

LocationRock type1

Yahyali/Kayseri

Dolomitic Limestone2

Bunyan/Kayseri

Limestone (Bunyan rosa)

3

Bunyan/Kayseri

Limestone (Bunyan beige)

4

Bogazliyan/Yozgat

Limestone (Bogazliyan beige)

5

Bogazliyan/Yozgat

Travertine

6NiğdeSandstone

3. LABORATORY STUDIESThe samples having a diameter of 54.7 mm and a length of 110 mm were used in the P -wave
velocity measurements. The end surfaces of the core samples were polished sufficiently smooth toprovide good coupling. In the tests, the PUNDIT 6 instrument and two transducers (a transmitter and areceiver) with the frequency of 1 MHz were used. A good acoustic coupling between the transducer
face and the rock surface is necessary for the accuracy of transit time measurement. Stiffer grease wasused as a coupling agent in this study. The transducers were pressed to either end of the sample and thepulse transit time was recorded (Figure 1).
The sound velocity tests were first carried out on the samples oven-dried at a temperature of 105ºC
for a week. Then, the samples were saturated with tap water for a week and the tests were repeated. The
P-wave velocity values were calculated by dividing the length of core to the pulse trans it time. The
average P-wave velocity values are given in Table 2 for each rock type.

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Figure 1 – P-wave velocity measurement.

RockcodeTable 2 – The average P-wave velocity values for each rock type
Number of theDry rock P-wave
Wet rock P-wave
Increment in thespecimen
velocity (km/s)
velocity (km/s)
P-wave velocity (%)

17

5.01

6.50

29.8

2

7

4.67

6.30

34.9

38

4.51

5.86

30.0

47

4.98

6.52

31.0

54

4.19

5.61

33.9

63

3.90

4.99

28.0

4. EVALUATION OF THE RESULTS
The dry rock P-wave velocity values range from 3.90 km/s to 5.01 km/s as shown in Table 2.
Correspondingly, the wet rock P-wave velocity values vary from 4.99 km/s to 6.52 km/s. Table 2 alsopresents the increments in the P-wave velocity values due to the water saturation. It can be said that theaverage increment in the P-wave velocity due to the saturation is about 30 %.
The dry- and wet-rock P-wave velocity values were evaluated using the method of least squares
regression. As shown in Figure 2, there is a very strong linear correlation (r = 0.96) between the twoparameters. The equation of the line is:ܸ௉ሺ௪௘௧ሻ ൌ ͳǤ͵ͳܸ௉ሺௗ௥௬ሻ(1)where, where V P(wet) is the wet-rock P-wave velocity (km/s), and V P(dry) is the dry-rock P-wave
velocity (km/s).
Kahraman (14) derived the following equation between the dry- and wet-rock P-wave velocity for
sedimentary rocks:ܸ௉ሺ௪௘௧ሻ ൌ ͳǤͳͻܸ௉ሺௗ௥௬ሻ ൅ ͲǤ͸͹

(2)

In order to see if there is a difference or similarity between the derived equation and Kahraman’sequation Figure 3 was plotted. The lines of the two equations are quite similar at high P-wave velocity
values. However, there is a significant difference between the two lines at low velocity values. The reason
for the differences between the two equation is probably due to the fact that the differences of the tested
rock types and the number or the tested specimens. Thirty-six specimens were tested in this study while
Kahraman (14) tested fifteen specimens. It can be said that the derived relation is more reliable than
Kahraman’s equation since it has a very high correlation coefficient and far more data points.

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Figure 2 – The correlation between the dry- and wet-rock P-wave velocity values.

Figure 3 – The comparison between the derived relation and the previous relation.

5. CONCLUSIONS
The influence of water saturation on the P-wave velocity of sedimentary rocks was investigated.
The relation between the dry-rock and wet-rock P-wave velocities was also studied. The results
indicated that P-wave velocity increment due to saturation was about 30 %. A very strong linear
correlation was also found between the dry-rock and wet-rock P-wave velocities.
It can be concluded that the wet-rock P-wave velocity values of sedimentary rocks can be estimatedusing the developed empirical equation. Further research is required to check the validity of thederived equation for the other sedimentary rocks such as marl, shale, siltstone etc.

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