Ultrasonic pulse-broadening and attenuation in volcanic rock - A case study

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1 Indian Journal of Pure & Applied Physics Vol. 40, June 2002, pp Ultrasonic pulse-broadening and attenuation in volcanic rock - A case study M VMS Rao, L P Sarma & K J Prasanna Lakshmi National Geophysical Research Institute, Hyderabad Received 10 March 2002; accepted 16 April 2002 Ultrasonic pulse-broadening measurements at 1.0 MHz frequency have been carried out at the ambient conditions in a set of volcanic rocks of south-east Gujarat. The experimental data have been processed along with the results of P-wave velocity (Vp) measurements leading to the determination of internal friction (Qp.l) and attenuation (<Xp). The attenuation in rhyolites is relatively high (average Qp.1 = , <Xp = db/cm) compared to basalts (average Qp. 1 = , <Xp = 0.0 I 00 db/cm) which EIre fine-grained and well crystallized. The mineral composition, in particular, the volume percentage of ground mass in rhyolites is found to have a strong control on both the attenuation and velocity of the rock samples tested. The experimental method is briefly described and the results obtained are presented and discussed. 1 Introduction Rock is a naturally occurring engineering material. Mostly, it is a mineral assemblage and contains grain-size defects (micro-cracks, pores and cavities) ranging from a few Ilm to a few mm even in samples prepared for laboratory experiments. The preferential alignment of its constituting minerals, and either the pre-existing or newly-formed cracks along with their physical dimensions and nature can produce velocity anisotropy of up to - 40% in rock. Further, the fluid-saturated rocks commonly show frequency-dependent attenuation and velocity dispersion when compared to dry rocks ' 3 All such observations have been ascribed to the complex nature of the crack/pore' structure of rocks, and to the behaviour of fluids occupying and flowing within the pore structure 4 S Generally, the ultrasonic testing of rock is carried out in the frequency range of 0.1 to 1.0 MHz, and the rock is mostly characterized by its velocity and density values 4 6 The complex micro-structures of most rocks make.the determination of attenuation a difficult task. In fact, several dissipative mechanisms involving textural as well as structural features contribute to the attenuation and velocity dispersion of ultrasonics in rocks. The most important among them are internal friction, scattering and energy loss at the grain boundaries, besides the external influences such as geometrical spreading and surface reflections s. lii Frequencies showing high attenuation also have the greatest dispersion in rock ll. 12 The conventional 'amplitude-decay' and 'rise-time' measurements among the 'Time-domain methods' have not proved to be adequately successful to produce repeatable results of attenuation measurements in rock R. 9. \3.'5. Hence, the attenuation data (a) is generally presented as Q.I, which is a function of velocity, frequency and attenuation in rocks and other lossy media7.l3 '6. The reciprocal of it is known as Quality Factor 'Q', which is a ratio of the real (Mr) and imaginary parts (M j) of the elastic modulus of the material under test. It is most often defined in terms of the maximum energy stored during a cycle, divided by the energy lost during that cycle '2. 1J The present study of the volcanic rocks of deccan plateau is a part of the on-going laboratory investigations on physico-mechanical behaviour of the rocks of seismically active areas of Gujarat and Maharashtra. The rhyolites and basalts constitute the major varieties among the volcanic rocks, and the former is rich in glass and the later contain pyroxenes (iron-magnesium silicate minerals) to a larger extent and is also dense. Rhyolite is a much more rapidly cooled rock compared to basalt. Its ground mass is mostly glassy and it indicates the eruptive power of magma l7 and also controls the thermo-physical and mechanical properties of the rock 'R.'9.

2 RAO et ai.:volcanic ROCK Experimental Details 2.1 Pulse-broadening and attenuation relationship Pulse broadening occurs due to an increase of wave period of the first arrival following the attenuation suffered by the ultrasonic wavelet in lossy media l4 and a measure of it can also lead to the computation of l/q or a [Fig. I(a)]. The following equations show the relationships among them as reported by Knopoffl4. IIQ = v.t I L I/Q = Val Of a= Oft I L ( I )... (2) (3) where V is the velocity,f is the frequency and t is the change in pulse width or half-wave period of the first arrival in the test sample with respect to the reference signal and L is the path length (i.e., length of the test sample). The reference signal is usually the ultrasonic signal received by direct contact between the driving and receiving transducers [Fig. I (a)]. Also, included in the same figure is a test signal in which the broadening has occurred. The pulse-broadening measurements are not only simple but also have proved to give accurate and repeatable results in rocks I3. 16.H under a variety of experimental conditions. 2.2 Experimental approach The authors used a high-energy ultrasonic Reference signal To Test signal 7 Tl (a) E u '-" 4 Cl c: 3.t:. as Il.. 2 () O r----r----r o Vp t X 100 (em) Material Nylon e TeHon Ebonite o Limestone o Sandstone XRh olite Q Fig. I - (a) Schematic picture of reference and test signals for measuring D.t; (b) Plots showing the linear relationship between path length and VpD.r in a set of materials including rocks. The experimental data were obtained, using P-wave transducers of 1.0 MHz resonant frequency

398 INDIAN J PURE & APPL PHYS, VOL 40, JUNE 2002 0.06,---------------------------------, Rhyolites C'I I/) o 0.05 +----------- llmj_---------------_l C'I.., 0.

3 398 INDIAN J PURE & APPL PHYS, VOL 40, JUNE , , Rhyolites C'I I/) o llmj_ _l C'I.., mj--+ H I-----IHlI _<i _1.., HiH--:!i! i!ii!!!l _4!!; , C'I C'I C'I C'I 0 0 Basalts I..,.. o 0 o.. LO I C? "i I u.. u... u.. u.... o I u.. Test Sample... I u.. N.. I u.. < Fig. 2 - Bar graph of internal friction (Qp. l) and attenuation (<Xp) data of deccan rhyolites and basal ts pulser-receiver on the driving side and a Hewlett Packard digital storage osci 1I0scope (Model No A) which is equipped with cursor facilities on the receiving side. The measurements of pulse broadening have been carried out using P-wave transducers of 1.0 MHz resonant frequency with an accuracy of ± 0.0 I Ilsec. The test samples have been prepared in the form of right circulars cylinders (25-30 mm diameter and mm long) by core drilling in the rock blocks. A detailed description of the measurement technique was reported earlier The wave period between the first-downward and first-upward pulse (T/ -T/ ) of the received wavetrain constitutes the major measurement parameter [Fig. (I a)]. It is compared to the wave period (T 2 - T 1 ) of the reference signal and the difference between the two gives the t.t value of Eq. ( I) as follows: (4) The time of fight measurement (T.,) is carried out with respect to the onset of the first received pulse for determining the P-wave velocity. The test samples of different lengths prepared from individual rock blocks have helped us in obtaining the linear plots between Land Vrt.t. Fig. I (b) shows an example of a set of curves obtained from the measurements carried out in different materials. Some of them are commonly available synthetic materials and the remaining are naturally occurring rock materials. The slope of each curve is the quality factor from which a p and Q p- I of the respective test material can be deduced using Eqs (2) and (3). 3 Results and Discussion The test samples of the present study were prepared from 12 blocks of rhyol ites and 3 blocks of basalts. All the rock blocks were collected from the Pavgadh hills near Vadodara, south-east Gujarat. Al l the test samples are fine grained and quite homogeneous in composition and texture. A detailed description of their petrography has been reported separately1. Three and more core-drilled samples of different lengths from each block have

4 RAO et al.:volcanic ROCK 399 been selected for the P-wave velocity and pulse broadening measurements and to compute Qp.l and a". The average values obtained are shown plotted as bar graphs in Fig. 2. The average values of (liqp) and a" of each rock block are shown at the end of the corresponding bar graph. The P-wave attenuation in rhyolite samples ranged from to db/cm, and the average a" = db/cm. Whereas, a" of basalts is found to be quite low ( to db/cm) with an average value of db/cm. The results show that the quality of ultrasonic propagation is appreciably high in these two categories of rocks, and both of them have shown less attenuation and ' high Q as compared to the sedimentary, metamorphic and even some of the igneous rocks in generap R. This can be attributed to the presence of large volume percentage of glass in rhyolite and iron-magnesium minerals in basalts. In this context, it may be mentioned here that rhyolites are volcanic rocks which are formed due to the rapid cooling of magma resulting in a fine-grained glassy texture. The results of the present study show that, it is mostly the internal friction due to which the attenuation occurs in these rocks. Since close relation exists between a", Qp and Vp, the results obtained are plotted to show the correlations among them. The results of Qp.l and u p are plotted separately as function of P-wave velocity of both rhyolites and basalts [Figs 3(a) and 3(b»). Although, the data of basalts is not large enough, the observed trends have confirmed and established the relationships among the measurement parameters. The results show an appreciable scatter in rhyolites compared to those of basalts [Figs 3(a) and 3(b)] I.] o Y'"' -9I;O-'Io..=z + 7E-08x R2 = :---r --- '_1r -- y---, eooo.6500' 7000 P-wave velocity (m/sec) (a) O.O'SO !!.! ;.:, Y = -2E-Oox:> + se-06x t ;:."_:: :_ =-0.::..:::.67.: ' 5 0'.030' :a; J c: G> =::..q: 0'.0'20-0 O'1O' y - -7E-06x + 0.0'586 = r---' Qa ' Pve velocity (m/sec) (b) Fig. 3 - (a) Plot showing the relationship between Qp. l and P-wave velocity in rhyolites and basalts. The experimental data were obtained at 1.0 MHz frequency; (b) Plot showing the interrelationships between P-wave attenuation ( Up) and P-wave velocity (V ) in rhyolites and basalts. Up is calculated from Qp.l and Vp p

5 400 INDIAN J PURE & APPL PHYS, VOL 40, JUNE 2002 K iii c.q 0.03 ro :::J 0.02 c <D U Q) <I) '"-E 0> ><: <"> C> a C> :> Deccan rhyolites y = -O.oooax R ' ao Ground mass (Volume percent).... -= y = >< R 2 = a r ,-----, Ground mass (Volume percent) = - -. y = x'" R2 = Ground mass (Volume percent) Fig. 4 - The experimental data of attenuation ( 0;,), acoustic impedance ( p Vp) and P-wave velocity (Vp) are plotted against the volume per cent of ground mass of rhyolites. The best-fit line and correlati on coefficient of each plot are also included and these are refl ected in the nature of best-fit lines and correlation coefficients. The scatter is strikingly more in the data obtained for rhyolite samples that show lower P-wave velocity. But, as the velocity increases, the scatter is markedly reduced. This can be ascribed to the fact that the rhyolite is a rapidly cooled rock unlike basalt which is a deep-seated volcanic rock. In that process, appreciably high differences among the composition and properties of the ground mass (fine-grained glassy material) which is the dominant constituent of rhyolite (volume percentage: %) can arise and such features will mostly account for the wide range of attenuation and Qp.1 values of rh yolites '6 and also the mechanical properties of rhyolite 17. Whereas, the basal ts are more dense, compact and show a homogeneous mineral composition. Consequently, the fine-grained basalt rocks show hi gher wave velocities and low attenuation. The rh yolites can be classified as intermediates between obsidian glass and highly crystalline basalts which have evolved due to rapid and slow rates of cooling of magma respectively. In the case of rhyolites, the ground mass is found to have a strong control on the attenuation, acoustic impedance and velocity (Fig. 4). It shows that, velocity and acoustic impedance wi ll increase smoothly with the increase. of ground mass, whereas the attenuation which decreases with the increase of ground mass shows some scatter and it needs to be studied further. 4 Conclusion The determination of Qp.1 from pulse broadening measurements is not only simple but also gives

RAO et al.:yolcanic ROCK 401 fairly accurate results to characterize the materials. Measurements of pulse broadening in rocks require a high power ultrasonic pulser on the driving side.

6 RAO et al.:yolcanic ROCK 401 fairly accurate results to characterize the materials. Measurements of pulse broadening in rocks require a high power ultrasonic pulser on the driving side. The ground-mass of rhyolite rock is glassy and the quality of ultrasonic propagation in it is fairly good. The rocks show Qp.l values ranging from based on P-wave velocity and pulsebroadening measurements at 1.0 MHz frequency. Pyroxene (an iron-magnesium silicate mineral) which is largely present in basalt gives rise to relatively low values of Qp. l ( ) at 1.0 MHz frequency. The relationship between Qp.l and other mechanical and petrological properties of these rocks is of significant value for some of the engineering applications of these rocks and work is currently in progress in that direction. Acknowledgement This forms a part of the project work which is supported financially by the ESS Division of the Department of Science & Technology, New Delhi. This paper is released for publication with the kind permission of Dr VP Dimri, Director, NGRI, Hyderabad. References Tokoz M N, Johnston D H & Timur A, Ceophys, 44 ( 1979) Titlmann B R, Noadler H, Clark V A, Ahlberg L A & Spencer T W, Ceophys Res Lett, 8 ( 1981 ) Murphy W F, Winkler K W & Kleinberg R L, Ceophys, 51 ( 1986) Nur A, Byerlee J D, 1 Ceophys Res, 76 ( 197 1) O'Connell R J & Budiansky B, 1. C eophys Res, 79 ( 1974) Brown E T, (Ed) Rock characlerizalion, lesling and moniloring, ISRM suggesled mel/lods, ( 198 1), Ramana Y V & Rao M VMS, Pllys Earlh Planel liller, 8 ( 1974) Ramana Y V & Rao MY M S, Ceoexploralion, 12 ( 1974) Rao M VMS & Ramana Y V, Rock Mech Rock Engng, 25 ( 1992) Sarma L P & Ravikumar N, Engng Ceol, 57 (2000) 123. II Murphy W F, Ceophys Res Lett, 12 ( 1985) O'Connell R J & Budiansky B, Cenphys Res Lett, 5 ( 1978) Tonn R, Phys Earlll Planel In ler, 55 ( 1989) Knopoff L, Rev C eophys Space Phys, 2 ( 1964) Liu H P, Anderson D L & Kanamori H, C eoph.'"s 1 Roy ASlron Soc, 47 ( 1976) Rao M VMS, Murthy D S N & Prasanna Lakshmi K J, Tech Reporl No NCRI 2001-L1THOS-323, Report submitted to the DST, (200 I), pp. I Wilson L, 1 Volcanol Ceotll erm Res, 8 ( 1980) Sarma K V L N S, Gogte B S & Ramana Y V, ell,.,. Sci, 48 ( 1979) Rao M VMS, Adesh Kumar, Murthy D S N, Nagaraja Rao G M, Shivakumar S & Mohanty S K, lind Ceophys Union, 5 (200 I ) 83.

Shear-wave propagation in rocks and other lossy media: An experimental study

Shear-wave propagation in rocks and other lossy media: An experimental study M. V. M. S. Rao* and K. J. Prasanna Lakshmi National Geophysical Research Institute, Hyderabad 500 007, India Ultrasonic shear-wave