Geoexploration-v19-1081-PropriedadesGeofísicasLaboratórioRochasÍndia-Ramachandran&Nair

Geoexploration, 19 (1981) 15-32 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

15

ELASTIC PROPERTIES OF LOWER GONDWANA ROCKS OF EASTERN INDIA

C. RAMACHANDRAN and M. RAMACHANDRAN NAIR

Geological Survey of India, 15-Park Street, Calcutta 700016 (India)

(Received May 30, 1980; accepted December 10,198O).

ABSTRACT

Ramachandran, C. and Ramachandran Nair, M., 1981. Elastic properties of Lower Gondwana rocks of eastern India. Geoexploration, 19: 15-32.

Compressional (P) and shear wave (S) velocities have been measured in the laboratory, in about 50 rock samples representative of the different geological formations of the Lower Gondwana rocks of eastern India. The P-wave velocity was measured along the bedding plane and perpendicular to it. The anisotropic factor ,V, /lb, was estimated. The absorp- tion coefficient, (Y, expressed as db/cm was also determined. The effect of fluid (water and oil) saturation on velocity, absorption and anisotropy was evaluated. The elastic con- stants were calculated and the entire data were statistically analysed to obtain some em- pirical results relating the elastic parameters to the fluid saturation, absorption and anisotropy.

The basic data on the elastic properties furnish useful information for seismic prospect- ing in the coal bearing Gondwana rocks. Fluid saturation, in general, increased the P-wave velocity. In the same kind of rock, the absorption coefficient decreased with increasing compressional wave velocity. Absorption increased in medium and fine grained sandstones on saturation with water while it decreased in coarse-grained sandstones and shales. l,V, is noted invariably greater than lb,. In general, fluid saturation decreased the anisotropic factor in sandstones and shales alike.

An attempt has been made to compare the elastic properties of the Gondwana sand- stones with those of the older and younger sandstones in India.

INTRODUCTION

A detailed study of the elastic properties of the different rocks of the Lower Gondwana formations of eastern India is particularly interesting in view of the fact that these formations contain the main coalfields of India and that seismic surveys are now more widely applied for coal seam delinea- tion. Laboratory measurements of compressional wave (P-wave) velocity, shear wave (S-wave) velocity, density, porosity, absorption characteristics and elastic moduli are useful in the interpretation of geophysical and geolog- ical data.

Laboratory determination of P and S-wave velocities had its beginning in the late 1930s, but, was pursued more actively in the 1950s, by among

0016-7142/81/0000~000/$02.50 o 1981 Elsevier Scientific Publishing Company

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others, Hughes (1950,1951,1952,1956,1957) and Wyllie et al. (1956, 1958). Using an ultrasonic pulse method, Birch (1960,196l) studied P- wave velocity in igneous and metamorphic rocks from different parts of the world. Simmons and Brace (1965), Kaarsberg (1966), Levykin (1965), Merculova (1963), King (1962,1966), and more recently Domenico (1976) and Gregory (1976) investigated elastic wave propagation in rocks using ultrasonic pulse method. Investigations involving the effect of overburden pressure and fluid saturation were reported by Wyllie et al. (1956,1958), Cook (1957), Volarovich et al. (1966) and Nur and Simmons (1969). Some studies on the elastic properties of Indian rocks were conducted by Balakrishna (1958,1966,1970), Datta (1967,1971), Datta and Simha (1968, 1969) and Datta and Ramachandran (1978). Absorption of elastic waves in rocks was investigated by Paselnick and Zeitz (1959), Volarovich (1960), Levykin (1962,1965),Vanek (1962), Wyllie et al. (1962) and Datta (1967). An extensive account of the attenuation measurements is given by White (1965).

Some 50 core samples of sandstones, shales, mica peridotites etc., belong- ing to the Lower Gondwanas of Ranigunj and Jharia coalfields have been col- lected for the purpose of the present study, from a dozen boreholes. P and S-wave velocities, absorption coefficient, anisotropic factor and elastic con- stants were determined using an ultrasonic pulse technique. The effect of fluid saturation on velocity, absorption and anisotropy has been investigated. The results provide comprehensive information on the elastic behaviour of the coal environs of eastern India. Besides, the Gondwana rocks being wide- spread in other parts of the world too, these results are significant while ex- ploring in such environs.

LABORATORY EQUIPMENT AND MEASUREMENTS

A block diagram of the ultrasonic pulses equipment (Datta, 1965) used for the studies is given in Fig.1. Briefly, it consists of a quartz crystal oscillator of 1.5 MHz, the output of which is reduced to 125 Hz by a chain of synchro- nised multivibrators through a cathode follower: This output of 125 Hz is now differentiated and after clipping the negative spikes, the positive spikes are used to fire a normally non-conducting thyratron tube. During the 8 msec inter- val of positive spikes, the static capacitance between the silvered faces of a barium titanate transducer are charged to a potential difference of 459 V and then, discharged down to 10 V through the low resistance of the thyr- atron tube. As a result of the voltage surge, a damped seismic wave train of about 2 I.tsec duration which repeats itself at 8 msec intervals is produced. The wave train travels through the rock specimen and is received by a similar transducer which converts it to an electrical pulse that is amplified by a wide band amplifier and displayed on an oscilloscope. A time marker signal, at in-

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I- -I- I I I r

t I

Fig.1. Block diagram of the ultrasonic pulse equipment for laboratory measurements of the elastic properties of rocks.

tervals of 10 psec, that is obtained from the output of the oven-compensated quartz crystal oscillator through a clipper and a buffer amplifier measures the travel time of the pulse in the specimen.

Barium titanate transducers have been used to generate P-waves. The directional dependence of velocity is studied by measuring the P-wave veloc- ity (VP) along the core axis and at right angles to it. (The maximum velocity in a direction perpendicular to the core axis indicated the bedding plane velocity 11 VP, while the other gave the transverse velocity IVp). In a few samples the S-wave velocity was determined by measuring the delay time, t, between P and PSP pulses by a method similar to that of Hughes (1949) mak- ing use of the formula:

where D = diameter of the core sample. On measuring VP, V, and density, p , the elastic constants were calculated using the standard relations:

Youngs modulus, E = V,p(l + u) (1 -- 20)

(I - 0)

Rigidity modulus, p = E/2(1 + u)

Poissons ratio, (VplW - 2

u = 2[(vp/vs)z -11

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Absorption measurements followed the method adopted by Auberger and Rinehart (1961) and Datta (1967). The amplitude A, of the first half cycle of the seismic pulse after its transmission through the sample is measured. The sample is then removed and the amplitude of the pulse, A0 is brought down to the same as A, by an r.f. attenuator placed between the receiving transducer and the wide band amplifier. If 0 is the total absorption and X, the length of the sample, the coefficient of absorption, 01, is given by:

1 AX Q! =p/x = - 20 log,, -

X ( 1 Ao

GEOLOGY AND ROCK TYPES

The rock samples under study belonged to the Lower Gondwana forma- tions of the Ranigunj (Fig.2) and Jharia (Fig.3) coalfields. The Lower Gondwanas extend over the time scale Upper Carboniferous, Permian and Triassic and consist of the general Indian sequence of Talchir, Damuda and Panchet rocks. Of these, the Damuda Series is of paramount importance as the greatest coal-bearing Epoch in the geological history of India (Dutta, 1977). The Damudas have a maximum thickness of about 2.3 km in the Ranigunj coalfields and about 2.5 km in the Jharia coa&l&. The Damuda Series comprises four stages, viz., Karharbari, Barakar, Barren Measures and Ranigunj.

The Barakars are the chief coal bearing strata in the Gondwanas, the coal to strata ratio being about 1:6 to 1 :lO in the coalfields of the Damodar valley. They consist essentially of sandstones and shales alternating with coal seams and occasionally conglomerate bands. These sandstones generally vary in colour from white to yellow and sometimes brownish red and are highly felspathic. The Barren Measures, overlying the Barakars in the Ranigunj coal- fields comprise mainly carbonaceous shales with clay ironstone nodules, known as Ironstone shale formation, and sandstones. No workable coal seams are seen in the Barren Measures. The Ranigunj stage, the youngest of the Damudas is composed of sandstones, shales and coal seams. These sandstones are somewhat finer in texture than those of Barakars. The coarse white felspathic grits and conglomerates, very common in Barakars are not generally found in the Ranigunj stage.

Fig.2. Geology of the Ranigunj coalfields (eastern India) and borehole sample locations.

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20

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RESULTS

Table I presents the P-wave velocity in some of the typical Lower Gondwana rocks in dry and saturated (with water) states, as also, the respec- tive anisotropic factors (coefficients), A, defined as:

The superscripts d and s indicate dry and saturated conditions of rocks.

TABLE I

Compressional wave velocities VP (m/set) in some typical Lower Gondwana rocks in dry and water saturated states

Sample Formation Description of number sample

_____-

Coarse grained sandstones JM-16;15B Barakar Coarse gr. to

granular sst. JM-15;25 Barakar Coarse gr. to

granular sst. JM-15;24 Barakar Coarse to medium

gr. sst. JM-15; 23A Karharbari Coarse gr. sst.

with pebbles and carb. matrix

JM-15;23B Karharbari Coarse gr. sst. with pebbles and Carb. matrix

JPK-1;44 Barakar Coarse gr. sst. JBH8;172 Barakar Coarse gr. sst. JBH-6;169 Barakar Coarse gr. sst. JBK-4 ;202 Barakar Coarse gr. sst. RBR-3 ;382 Ranigunj Coarse to very

coarse gr. and gritty sst.

RBR-3;386 Ranigunj Very coarse gr. gritty sst.

Medium and Fine grained sandstones JM-16;lA Barakar Medium gr. sst. JM-16;9A Barakar Medium gr. sst. JM-16;9B Barakar Medium gr. sst. JM-16;7 Barren Medium gr. sst.

measure JM-16;ll Barakar Fine gr. sst. JM-16;13 Barakar Medium gr. sst. JPK-1;43 Barakar Medium gr. sst. JPK-1;115 Barakar Medium gr. sst. JBJ-2;124 Barakar Fine gr. sst. JBH-6;163 Barakar Medium gr. sst. JBK-4 ;204 Barakar Medium gr. sst.

3050 4260 3570 4570 1.17 1.08

3340 3870 3800 4130 1.14 1.07

3140 3990 3700 4380 1.18 1.10

3080 4130 3510 4240 1.14 1.03

3220 4140 3510 4240 1.09 1.02

3180 4300 3700 4320 1.17 1.0 3050 3980 3520 4250 1.15 1.07 3140 3650 4100 3770 1.20 1.03 3820 4410 4260 4510 1.12 1.02 2510 2970 3020 3130 1.20 1.05

2680 3750 3180 4100 1.19 1.09

3970 4580 4450 4570 1.12 0.99 3700 3950 4420 4910 1.19 1.24 3550 4280 3870 4240 1.16 0.99 4010 4790 4380 4670 1.28 0.98

3930 4670 4240 4780 1.08 1.02 5100 5250 5180 5330 1.02 1.01 2620 3480 3390 3700 1.29 1.06 3630 4270 4020 4420 1.11 1.04 3540 4050 5300 4760 1.50 1.18 3190 3280 3880 3520 1.22 1.07 4430 4790 4850 5000 1.09 1.05

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(Table Z continued)

Sample number

Formation Description of sample

3340 3940 3200 4080 0.9 1.04 RJK-1;351 Ranigunj

RBR-4;372 Ranigunj

JBK-3;146 Barakar JBK-3;144 Barakar *JM-16;12 Barakar *JM-19;84 Barakar *JBK-4;206 Barakar

*RBR-4;370 Ranigunj

Shales and other rock types JM-16;5 Barren

measure Barakar

Medium to fine gr. sst. Medium to coarse gr. sst. Medium gr. sst. Medium gr. sst. Fine gr. sst. Fine gr. sst. Medium to fine gr. sst. Medium to fine gr. sst.

3860 4340 4340 4740 1.13 1.09

2720 3740 4360 4170 1.60 1.33 3240 3800 3550 5210 1.10 1.31 3730 4690 4320 5100 1.13 1.11 2770 3780 3680 4500 1.33 1.19 3160 4020 3850 4390 1.22 1.09

3410 3810 3730 4020 1.10 1.06

Sandy shale 2310 2960 4210 4550 1.84 1.54

JM-16;16

JM-16;21 Barakar JM-16;22 Barakar

JM-15;29 Barakar

JPK-1;48 JBK4 ;208

Barakar Barakar

RCL-1;264

RJK-1;367

Barren measure Ranigunj

RBR4;371 Ranigunj

RBR4;376 Ranigunj

RBR4;373 Ranigunj

RBR-4;398 Ranigunj

*JM-19;91 JBH-6;168 JBK4;209 JBK4;212 RBR4 ;393 RBR-4;394

Barakar Barakar Barakar Barakar Ranigunj Ranigunj

Intercalations of shale and sand Sandy shale Shale with sst. laminations Intercalations of shale and sand Shale Carbonaceous siltstone Ironstone shale

3730 4520 4340 4570 1.16 1.01

2610 3020 4740 4720 1.82 1.57 4090 4090 4930 4930 1.21 1.21

5100 4100 5670 4800 1.11 1.17

3440 3400 4380 4380 1.27 1.29 3310 3330 5520 3650 1.69 1.10

5220 5480 5630 5630 1.08 1.03

Fine gr. interbanded 4740 sst. and shale Fine gr. sst. with 2750 shale laminations Interbanded fine 2240 gr. sst. and shale Sandy shale with 1630

3970 5410 4590 1.14 1.16

2240 4390 4120 1.60 1.84

1920 3750 3870 1.67 2.02

2500 3700 4050 2.27 1.80 carbonaceous streaks Shale and Jhama intermixed Sandy shale Mica peridotite Mica peridotite Mica peridotite Mica peridotite Mica peridotite

2560 3100 3140 3330 1.22 1.08

2500 3550 4300 4670 1.72 1.32 6000 4780 4440 4020 0.89 0.84 4170 4720 4200 4390 1.01 0.93 4600 4150 4550 4240 1.05 1.02 4190 4620 3610 3940 0.86 0.85 4270 3940 4230 3890 0.99 0.99

*Weathered sample. The first part of the sample number shows the borehole number; sst. = sandstone.

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P-wave velocity in rocks

As to the rock texture and composition related to the seismic wave propa- gation in Lower Gondwanas, they may be grouped into: (1) coarse-grained sandstones; (2) medium and fine-grained sandstones; (3) shales; and (4) mica peridotites. Table I gives a general picture of the seismic wave velocities in these rocks. Coarsegrained sandstones exhibit a P-wave velocity in the range 2500-3400 m/set with the (younger) Ranigunj rocks having a lower velocity than the (relatively older) Barakars. Medium and fine grained sandstones are characterised by a higher-order velocity, 3000-5000 m/set. Sandy shales have typically low seismic velocity, 1600-2600 m/set. In more compact shales and carbonaceous shales, the velocity increases and lies in the range 34404290 m/set. Among the shale group, ironstone shales show a high order of velocity, 5200 m/set. The intrusive sills and dykes of mica-peridotites too have a high order of velocity, 4000-5000 m/set.

Effect of fluid saturation

The change in P-wave velocity in rocks on saturation with water has been studied by conducting velocity measurements on samples soaked in water for about 48 h. Table I gives the results. It is observed that water saturation increases the P-wave velocity considerably, in some cases as high as 40-50% if compared to the dry state.

Figs.4 and 5 illustrate the relations between the P-wave velocities in saturated and dry conditions in the case of coarse grained sandstones as well

xx x

x x3 J x I; I ,

0 1000 2000 3000 4000 5000 6000 P-wave velocity (dry sample), mjsec

kf

6000 -

E 2

Ag 5000 -

E 8 Qs

$ 6

4000.

: @ o

:: 8

: 3000- 0

B

B

; 2000

_/

0 0

? : 1000

s 0 Medium and F;ne grafned Sandstones

ol

0 ,000 2000 3000 4000 5000 6000

P-Wove velocity (dry sample), mjsec

Fig.4. Illustrating the effect of water saturation on P-wave velocity in coarse grained sand- stones.

Fig.5. Illustrating the effect of water saturation on P-wave velocity in medium and fine grained sandstones.

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as medium and fine grained sandstones respectively. Empirical relations have been derived from least square curve fitting. The relations are: vP = 790 + 0.996 r$ for coarse grained sandstones; VP = 1660 + 0.69 V$ for medium and fine grained sandstones; and, vP = 1140 + 0.7 V$ in the case of shales and sandy shales. Notably, the coarse grained sandstones exhibit the highest in- crease in velocity on water saturation.

Some samples of sandstones were saturated with oil as well, and the P-wave velocities determined. Saturation with oil too increased the compressional wave velocity. Figs.6 and 7 show the P-wave velocities (I$) in oil saturated samples vis-a-vis the velocities in dry condition. The empirical relations found here are: I$ = 740 + 1.24 I$ for coarse grained sandstones; I$ = 560 + 0.96 I$ for medium and fine grained sandstones; and I$ = 2040 + 0.5 I$ in the case of shales.

Again, these relations show that the increase in velocity in coarse grained sandstones is more than that in medium and fine grained sandstones or shales.

Y C6000 E r z ;: 5000 -

:

P 4000 - d

:

+i 3000 -

a

x -5 2000 -

0

5

0 $ ,ooo-

y 6ooor T

2 5000 -

E LA

D 4000-

J 0

: 2 3000 - 7

. . w /J . .

.

n I I a I 0 ,000 2000 3000 4000 5000 6000 0 ,000 2000 3000 4000 5000 6000

P-wave velocity~dry samples), m,sec ~-wow velocfty (dry sample), mjsec

Fig.6. Illustrating the effect of oil saturation on P-wave velocity in coarse and medium and fine grained sandstones.

Fig.7. Illustrating the effect of oil saturation on P-wave velocity, mainly in shales.

Absorption

A study of the absorption of compressional waves in rocks is of consider- able importance in seismic exploration problems. The method by which elastic energy is lost while travelling through a medium is quite complex and is not clearly understood. Of the several loss mechanisms proposed in the past, the absorption involved in the relative motion of a solid frame of a porous rock and the viscous liquid in the pore space is believed to be the chief contributing factor for the elastic energy absorbed in a fluid saturated rock.

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Table II presents the results of absorption studies in rock samples in dry and fluid saturated states as well as at a high temperature of 100C. P-wave absorption is more in coarse grained sandstones than in medium and fine grained sandstones in dry state. In coarse grained sandstones the absorption coefficient is about 2.574.0 db/cm while in the medium and fine grained sandstones, it is about 0.93-3.5 db/cm. In shales, sandy shales etc., the coefficient varies over a wide range, 2.12-8.05 db/cm. In burnt coal, it is observed to be about 11.1 db/cm.

TABLE II

Absorption coefficient 01 (db/cm) in some Lower Gondwana rocks in dry state, at 100C and fluid saturated states

Sample Depth Formation Description number (m) of sample ~~__

Coarse grained sandstones JM-15;15B 983 Barakar JM-15;25 486 Barakar

JM-15;23B 548 Karharbari

JM-15;23A 548 Karharbari

JM-15;24 486 Barakar

JPK-1;44 453 Barakar JBH-6 ;172 728 Barakar JBH8;169 612 Barakar JBK-4 ;202 937 Barakar RBR-3;382 253 Ranigunj

RBR-3 ;386 350 Ranigunj

Medium and fine grained sandstones JM-16;lA 407 Barren

measure JM-16;9A 611 Barakar JM-16;7 554 Barren

measure JM-16 ;ll 611 Barakar JM-16;13 850 Barakar JM-1143 345 Barakar JPK-1;115 - Barakar JBJ-2;124 105 Barakar JBH-6;163 240 Barakar JBK-4 ;204 921 Barakar

Coarse gr. sst. Coarse gr. to granular sst. Coarse gr. sst. with pebbles and carb. matrix Coarse gr. sst. with pebbles and carb. matrix Coarse to medium grained sst. Coarse gr. sst. Coarse gr. sst. Coarse gr. sst. Coarse gr. sst. Coarse to very coarse gritty felspathic sst. Very coarse gr. gritty sst.

3.48 3.2

2.57

3.62

2.71

3.55 4.0 3.90 2.72 4.21

4.02

Medium gr. sst. 1.50

Medium gr. sst. 1.52 Medium gr. sst. 1.30

Fine gr. sst. 1.27 Medium gr. sst. 0.93 Medium gr. sst. 2.52 Medium gr. sst. 2.79 Fine gr. sst. 2.88 Medium gr. sst. 3.36 Medium gr. sst. 2.13

3.33 2.75 3.08 -

2.17 1.68

3.35 2.62

2.50 -

3.12 - 3.64 - 3.78 - 2.38 1.82 5.80 -

4.18 2.86

1.50 -

2.66 2.17 2.00 1.77

1.61 1.40 1.56 0.93 3.86 - 3.68 3.68 4.35 - 4.35 - 2.71 -

3.61 2.77

2.63

4.83

2.26

2.98 3.67

1.90 5.44

3.80

1.13

1.42 1.37

0.90 1.66 2.30 4.00 2.95 3.36 2.39

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(Table II continued)

Sample Depth Formation number (ml

RJK-1;391 497 Ranigunj

RCL-1;365 596 Ranigunj RBR-4;372 253 Ranigunj

JBK-3;146 176 Barakar

Shales and other rock types JM-16;5

JM-16;20 JM-16;21 JM-16;16

JM-16;22

JM-15;29

JFK-1;48 RBR-4;371

JBK-4;213

JBJ-2;123

JBK-4 ;208

JBK-4;209 RBR-4;392 RBR-4 ;393 RBR-4 ;394 RCL-1;364

- Barren measure

963 Barakar 963.1 Barakar 761 Barakar

961 Barakar

- Barakar

360 Barakar 253 Ranigunj

983.1 Barakar

100 Barakar

838 Barakar

983 Barakar 294 Ranigunj 394.9 Ranigunj 395 Ranigunj 322 -

-

Description of sample

Fine to medium gr. sst. Fine gr. sst. Medium to coarse gr. sst. with carb. streaks Medium gr. sst.

Sandy shale

Carbonaceous shale Sandy shale Intercalations of shale and sst. Shale with sst. laminations Intercalations of shale and sst. Shale Fine gr. sst with shale laminations Burnt coal with minor baked carb. shale Siltstone with minor shale bands Carbonaceous siltstone Mica peridotite Mica peridotite Mica peridotite Mica peridotite Ironstone shale

ad

1.62 2.78 2.32 1.28

2.26 2.84

3.5

4.8 3.92 - 4.72

2.12 - 1.30 - 2.58 - 2.08 1.29 2.30 1.66 - 2.26

3.24 - 3.00 -

2.50 3.08 - -

7.1 5.48 5.32 - 8.05 - 6.25 8.5

11.1 10.8 - 11.75

- 6.9 4.76 9.52

1.15 1.64 0.95 2.34 1.69 2.28 1.06 3.09 2.85 2.81 2.15 2.51 8.06 6.93 4.92 6.45 3.24 0.83 - 1.16

- - 3.20 2.91 - 2.57

4.00 - -

7.06 2.69 6.72

*Weathered samples. The first part of the sample number shows the borehole number.

Fluid saturation decreased the absorption in coarse grained sandstones while it increased the absorption in medium and fine grained sandstones. No systematic trend was observed in the absorption coefficient in rock samples at 100C. Fig.8 relates the absorption coefficient (as) of saturated rocks with that (ad) of dry rocks in the case of coarse grained sandstones and medium and fine grained sandstones. Least squares analysis gave the following rela-

x

27

k5

DRY SAMPLES

x coarse grained sandstones

0 Medium and Fine gralned SondStOneS

I J 0 1DDD ZDOD 3000 4000 5000 6000

P-wave velocity, mlsec

Fig.8. Relating the absorption coefficient of sandstones in dry and saturated states.

Fig.9. Relating the absorption coefficient with the P-wave velocity of sandstones in dry state.

tions: (YS = -1.79 + 1.51 crd for coarse grained sandstones; (YS = 0.47 + 1.07 ad for medium and fine grained sandstones.

Another interesting relation is between the compressional wave velocity and the absorption coefficient as given in Fig.9. The absorption coefficient is observed to decrease with increasing P-wave velocity. Least square curve fitting provided the relations: ~1 d stones and (ud =

= 6.5 - 0.97 V$ for coarse grained sand- 6.36 - 1.11 6 for medium and fine grained sandstones.

The effect of water saturation on a-VP relation is illustrated in Fig.10. The relations are given by the equations: 4 = 9.6 - 1.5 VP in the case of

6- x

0 lOcl0 kooo 3000 4000 5000 6000 0.9 IO 1.05 1.1 1.15 1.2 1.25 1.3

P-wove velocity , m/set AiSrztrOplC Factor

Fig.10. P-wave velocity and absorption coefficient of sandstones, saturated with water.

Fig. 11. Absorption and anisotropy of Barakar sandstones.

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coarse grained sandstones; and (Y s = 8.15 - 1.3 vP in the case of medium and fine grained sandstones.

Anisotropy

It is observed that ,,V is always greater than IV: the increase in velocity being as much as 100% in some of the samples of shale, sandy shale and other laminated rocks. The anisotropic coefficient decreased on water satura- tion of the rock sample, in most cases. It varied from 1.0 to 1.3 for sand- stones while it was much higher for shales, sandy shales, laminations of shale and sand, and Jhama (burnt coal)

The observed anisotropy in seismic wave propagation in rocks can be attributed to the layering when it is clearly visible. However, in shales, no layering is visible and the samples appear homogeneous. Nevertheless, shales possess pronounced anisotropy. White (1965) considers this anisotropy of shales as of an intrinsic nature. In rock formations of shales and sandstone laminations, the observed gross anisotropy can be attributed partly to the above intrinsic anisotropy of shale and sand and partly to the layering.

Fig.11 shows the variation of the anisotropic factor, A with the absorption coefficient, a, in some Barakar sandstones. Samples of low anisotropic factor have low absorption too. An increase in anisotropic factor enhances absorp- tion. This trend is however regular only in unsaturated rocks.

Elastic constants

Making use of the V, and V,, the elastic constants Youngs modulus (E), rigidity modulus (p) and Poissons ratio (u) have been computed and given in Table III. (The V, measurements have been made using the PSP signal wherever sharply defined). The results provide a fair estimate of the elastic constants to be expected of Lower Gondwana sandstones.

Generally, the density of sandstones varied from 2.49 gm/cm3 to 2.75 gm/ cm3. V,/V, ratio lay between 1.5 and 1.9 and the Youngs modulus between 2.75.105 kg/cm* and 5.52~10 kg/cm2. The range of corresponding rigidity modulus was from 1.14.10 kg/cm* to 2.0.10 kg/cm*. And the Poisson ratio varied from 0.11 to 0.38. It is observed that E and p in general increase with

VP.

P-wave velocities in sandstones and their ages

A comparison of the P-wave velocities in sandstones of different ages (in the Indian context) would be of interest in seismic exploration. Leaving the present studies of the Lower Gondwana rocks, no systematic studies have as yet been reported on other formations. However, a fair comparison is made from available data from laboratory measurements on sandstones in dry state.

The Tertiary (1.5-64.5 million years) sandstones of the oil-bearing Cambay

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TABLE III

Elastic properties of some typical Lower Gondwana rocks

Sample Rock type P v, VS number (g/cm) (mlsec) (mlsec)

_

*JM-16;lA *JM-16;9B *JM-15;23B *JM-15;24

RJK-1:361

JM-16;13 JBJ-2;124 JPK-1;115 JM-16;7 JM-16;ll JM-15;25

RBR-4 ;392 RBR-4;393

Medium gr. sst. Medium gr. sst. Coarse gr. sst. Coarse to medium gr. sst. Fine to medium gr. sst. Medium gr. sst. Fine gr. sst. Medium gr. sst. Medium gr. sst. Fine gr. sst. Coarse to granular sst. Mica peridotite Mica peridotite

2.67 4580 2540 2.62 4280 2420 2.56 4140 2360 2.55 3990 2480

3.35 3440

2.75 5100 2230 2.66 3540 1990 2.63 3630 2170 2.69 4010 2050 2.07 3930 2100 2.49 3340 2210

2.25 3210 2020 2.62 4190 2350

1970

VP 0 E F v, ( lo5 kg/cm*) 1.80 0.28 4.54 1.78 1.77 0.26 4.00 1.59 1.75 0.26 3.67 1.46 1.61 0.186 3.78 1.59

1.75 0.25 3.37 1.35

2.29 0.38 5.52 2.00 1.78 0.27 2.72 1.07 1.67 0.22 3.10 1.27 1.96 0.32 3.62 1.37 1.88 0.3 3.70 1.42 1.51 0.11 2.75 1.24

1.59 0.17 2.68 1.14 1.78 0.27 3.76 1.48

*Measurements are made in water-saturated samples.

basin (northwest India), Cauvery basin (southeast India) and of Makum coal- fields (northeast India) are characterised by a P-wave velocity range 1850- 3400 m/set with the exception of Lakadong sandstones of Meghalaya. The medium grained Lakadong sandstones bear a considerably lower velocity of 1000-1600 m/set.

The Upper Gondwana (137-225 million years) (?) sandstones of Rajmahal (east India) also exhibit a lower P-wave velocity in the range 1500-2400 m/ sec.

The present experiments on Lower Gondwana (225-270 million years) establish a velocity range 2500-5000 m/set with distinctly lower velocities of 2500-3400 m/set in coarse grained ones and 3000-5000 m/set in medium and fine grained types.

A considerably higher velocity range of 5100-5500 m/set is observed for Vindhyan (500-900 million years) (?) sandstones from Rajasthan (west India) and Andhra Pradesh (south central India).

The Cuddapah (1000-1400 million years) (?) sandstones have typically a compressional wave velocity of the order of 4200 m/set.

Obviously, characteristic velocity spectrum can be ascribed to sandstones of a particular formation. However, in spite of the vast differences in ages of these sandstones from the Tertiary to the Precambrian it may not be said that older sandstones have typically higher velocities.

30

SUMMARY AND CONCLUSIONS

As regards the Lower Gondwana rocks of eastern India, the results of laboratory measurements and conclusions thereon may be summarised as follows.

The Lower Gondwana sandstones have a density range 2.37-2.76 g/cm3. Ironstone shales have a density around 3.29 g/cm3 while mica peridotites and siltstones have a density around 2.85 g/cm3.

In general, the compressional wave velocity increased with the density of the sandstones (the porosity of these sandstones having varied from 5% to 15%).

Broadly speaking, the compressional wave velocity of formations like sandy shale, Jhama, intermixed shale and Jhama and very coarse grained sandstones in dry state falls between 1500 m/set and 3000 m/set. The velocity of coarse grained sandstones varies from 2500 m/set to 3400 m/set while that of medium and fine grained ones varies between 3000 m/set and 5000 m/set. Mica-peridotites, ironstone shales and some samples of sandstones with shale laminations or interbanded sandstone and shale formations have a velocity between 4000 m/set and 5220 m/set. Though there is occasional overlapping, the compressional wave velocities generally confine to the above ranges. Fluid saturation increased the P-wave velocity. In coarse, medium and fine grained sandstones, saturated with water, the velocity variation is between 3280 m/set and 5200 m/set.

Most commonly, the compressional wave velocity parallel to the stratifica- tion is greater than in the transverse direction. The anisotropic factor llVp/~Vp is observed to be (1.3-2.0) higher in shales than in sandstones (where it is about 1.0-l .3). Fluid saturation invariably reduced the anisotropic factor. Also, the absorption decreased with a decrease in the anisotropic factor.

The absorption coefficient in Lower Gondwana sandstones lies between 0.93 and 4 db/cm, being less in fine grained sandstones and more in coarse grained ones. Fluid saturation reduced the absorption in coarse grained sand- stones while it increased the absorption in medium and fine grained sand- stones. Also, the absorption was less in rocks of higher P-wave velocity. Em- pirical relations between the absorption and the P-wave velocity have been established.

Limited V, measurements indicate a range of 1970-2540 m/set for the shear wave velocity in sandstones, the VP/V, ratio being 1.5-l .96. VP/V, ratio is the characteristic of the consolidation of a formation (Gregory, 1976) and it helps in identifying the lithological variations (Erickson et al., 1968).

The elastic wave velocities and the respective elastic constants presented, in Table III, provide useful data for the purpose of seismic exploration in the Gondwana basins, and also, provide basic physical property data for geotech- nical projects.

The available data on the velocity of sandstones varying in age from the Tertiary to the Precambrian suggest that these formations exhibit distinct velocity spectra notwithstanding that the older rocks necessarily do not have a higher compressional wave velocity.

31

ACKNOWLEDGEMENT

The authors wish to thank Dr. S. Datta (now with the Indian School of Mines, Dhanbad) for instrumentation and helpful suggestions in the problem. They also thank Shri P.M. Mathew, Chief Geophysicist, GSI, for his encour- agement in the work. Thanks are due to the Director-General, GSI, for his permission to publish this paper.

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