A study of shallow reflection seismics for placer-tin-reserve evaluation and mining

Geoexplorafion, 21 (1983) 105-135 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

105

A STUDY OF SHALLOW REFLECTION SEISMICS FOR PLACER-TINRESERVE EVALUATION AND MINING SURENDRA SINGH School of Physics, University of Science of Malaysia, Penang t~alaysia) (Received June 14,1982;

accepted July 9,1982)

ABSTRACT Singh, S., 1983. A study of shailow reflection seismics for placer-tin-reserve evaluation and mining. Geoexploration, 21: 105- 135. This paper presents results of a study of shallow reflection seismics carried out in tin fields of Kinta Valley, Malaysia. The main objectives of the experiment were to develop a field procedure to obtain and identify reflections from the irregular tough-Ed-pi~a~le topography of the limestone bedrock, and to give recommendations for updating the existing data-acquisition system. It is concluded that clear reflections can be obtained from the bedrock using only a hammer and the signalenhancement seismograph. The same is the case over tin-mine water pools where a high-frequency seismic system was used with energies less than 300 Joules. On the basis of theoretical and experimental results, it appears that, with a little updating of the existing system by adding a new source and suitable detectors etc., the refIection seismic method can help in a detailed delineation of the bedrock and determination of thicknesses of atluvium. The procedure discussed here is specially suited to exploration and exploitation of tin ore in small areas of the size of a tin mine. The consequent savings in time and money are tremendous.

INTRODUCTION

A feasibility study was done to investigate the applicability of the seismic reflection method for determining shallow depths, over small areas, to the trough-and-pinnacle topography of limestone bedrock. The field experiments were carried out in the Kinta Valley, Malaysia and the main instruments used were a single-channel signs-enh~cement unit with a hammer as the seismic source. The work was done with two specific purposes in mind; namely, to help in a better assessment of placer-tin-ore reserves and in a better planning of mining activities. The fact of the limited monetary resources available for geophysics in the overall mining activities was the main influencing parameter in deciding for the instrumentation and data collecting and processing techniques adopted. The study covered a wide spectrum of topics such as the energy of the hammer, characteristics of the hammer signal, behavior of source patterns, vertical and horizontal resolution, and finally obtaining and identifying reflections. All these analyses are essential for an appreciation of 0016-7142/83/$03.00

0 1983 Elsevier Science Publishers B.V.

106

the varied aspects of reflection Kinta Valley. Geology

seismics as applied to the environment

in

and tin mining of the Kinta Valley

The Kinta Valley, Malaysia, is the world’s richest tin field and produces more than 10% of the world’s output. Starting from the mountains in the north (see Fig. 1) it widens to about 24 km and is about 48 km long, trending roughly southwest. A review by Rajah (1979) describes the geology of the Kinta Valley in detail. Granitic rocks, generally Triassic in age, encircle the valley on the three sides of the limestone sedimentary rocks of the valley basement. The Main Range granites flank the valley on the east and the Keladang Range granites on the west. The Kinta Valley is underlain by a sequence of sedimentary rocks composed mainly of crystalline limestone with minor argillaceous and arenaceous rocks of Silurian to Permian age. Alluvium covers most of the valley plain, varying in thickness between 6 m in the north to more than 80 m in the southern part. This alluvium is mostly stanniferous and forms the most important part of the placer tin deposits here. Alluvium can be divided into two types: the Old Alluvium and the New Alluvium. Old Alluvium overlying the limestone bedrock includes gravel, sand, silt and clay and is derived from the surrounding granitic rocks. Young Alluvium, on the other hand, is the product of present-day rivers and consists of unconsolidated deposits of sand and gravel with some clay. It is about 2 m thick and contains the top humus layer of organic materials like leaves and roots. The structure of the valley evolved during granitic intrusions in Mesozoic times. The sedimentary rocks were folded, fractured, jointed and faulted. The characteristic ‘trough-and-pinnacle’ topography of the bedrock of the valley floor is believed to be the final result of solution along the well-developed joint system. Pot holes are another feature of the bedrock which is relevant to tin mining. As an example, an 80 m wide pot hole is the site of a well-paying tin mine using gravel pumps, near Ipoh town. The nature of pinnacled-limestone-bedrock topography is illustrated by two typical crosssections in Fig. 2, which are based on drillhole data. The limestone bedrock in the valley, with its trough-and-pinnacle topography and its deep solution channels has formed a series of natural riffles retaining and concentrating the heavy grains of cassiterite that were washed by erosion from the granite hillsides. Rich deposits are also present in granite/ limestone contact zones. Lode tin deposits, which are hydrothermal in origin, have been found in granites as well as in limestone and schist, in the form of fissures, fractures, pipes and veins. Associated with the alluvial deposits are heavy minerals such as ilmenite, zircon, monazite, columbite, gold, wolframite and scheelite which are being recovered as by-products of tin-mining operations. Most of the tin is mined from alluvial deposits, mainly by gravel pump and dredging machines. In many dredged-out areas, gravel-pump mining has been introduced to recover the cassiterite lodged in the crevices. It is

107

Fig. 1. Geology of the Kinta Valley, based on Rajah (1979).

35

I

!

I

I

1

I

I

4

2

3

4

5

6

7

8

9

-14 15

SerKl

“0

Of (80

Fig. 2. Bedrock topography

Boreholes M Apart)

16

on

17

18

I 19

I 21

I 22

, 22

J 23

L L,ne

-

along two borehole lines.

estimated that 80% of the gravel-pump working on previously dredged areas.

mines in Kinta Valley are presently

Estimating tin ore reserves and mine planning Drilling is almost always the method used for evaluating tin ore reserves in the Kinta Valley and for the premin~g phase. ~epend~g upon the field conditions, drilling is done using a hand banka drill or machine drilling. As an example of drilling costs for bedrock depths from 10 m to 70 m, a drillhole costs, on the average, about US$350. For a certain project area of about 2.5 km2, there were 471 d~lholes with a spacing of 80 m, in a grid pattern. The total cost amounted to about US$164,800.during more than two years. In addition to being expensive and very time-consuming, the horizontal definition of drilling is very poor. It is not uncommon, for the bedrock topography in this area, to find two pinnacles spaced only 10 m apart. Noting that placer tin is found in the Old Alluvium as well as in crevices and troughs of the bedrock, the poor horizontal definition with drilling can result in an underestimation as well as overestimation of ore reserves. A detailed depthdetermination is, therefore, highly desirable for reserve evaluation and mining operations. In mining operations, for example, the gravel pump should be ideally situated at the lowest level of the bottom of the minehole and as near to the processing plant as possible. The reason

109

is that in this way the alluvium that is washed down from the mineface will flow to the gravel pump naturally. Situating the gravel pump near the processing plant will be an advantage economically as less piping is needed to pump up the alluvium to the processing plant which is a permanent feature of a mining operation. The size of the processing plant and the machinery used depends on the amount of ore reserve in a mine area. The thickness of the overburden, i.e., the Young Alluvium, and the depth of the minehole are also important factors. If the overburden is thick, then the cost of production is high as the overburden has to be excavated and thrown away. If the depth of the mine to the bedrock is shallow, then the cost of production will be again high as the gravel pump and piping have to be shifted quite often, resulting in loss of production days. Further, for a shallow mine, the throwing away of the top about 2.5 m of overburden may not be wise because, in spite of its low ore content, this part may be a significant part of the total available stanniferous alluvium. It is obvious, then, that detailed depths to the limestone bedrock and, to a less degree, thickness of the overburden are important factors which can severely influence the estimates of tin-ore-reserves and the economics of a mine operation. The de

of geophysics

Considering the economics, difficulty and poor horizontal definition of using only drilling in tin-ore-reserve evaluation and mine planning, the significance of using a suitable geophysical method along with drilling is obvious. With financial and logistic help from the Mines Research Institute, Department of Mines, Malaysia, the work was started on a geophysical research project for the purpose. All the potential geophysical methods were considered. Gravity modelling of the bedrock topography indicated that for closely spaced pinnacles like those in the area, observed readings may, at times, be less than the observable accuracy of 0.05 mgal. The alluvium is stanniferous as a whole, with only very minor association of ilmenite; so the magnetic method also is not of help in delineating the bedrock. The troughand-pinnacle topography also precludes the use of electrical sounding. In a similar fashion, results of electrical profiling do not appear to show any unambiguous, qualitative correspondence with the bedrock topography. Coming to seismic methods, seismic refraction can map the irregular surfaces with dips within f 20 degrees from the horizontal, using delay-times (Barry, 1967), the iterative method (Singh, 1978) or the generalized reciprocal method (Palmer, 1980). However, the refraction method is not feasible for the irregular reflectors encountered in the environment of the Kinta Valley, with dips as great as f 20 degrees from the vertical, even though it can map the overburden thickness adequately, can determine velocities and can give a rough qualitative stratigraphic picture. The only method which held out any hope for detailed delineation of the bedrock and meet the unique require-

110

ments of the problem, was the reflection seismic method. Shallow seismic reflections have been used by various workers (Meidav, 1969; Shepers, 1975; Noponen et al., 1979) for geological mapping in the past. The project area under study comprises two distinct types. The major part of the area is flat land, mostly covered with rubber plantations and other equatorial bushy undergrowth. The other part of the project area is covered under mining pools which were dredged out in the past for placer alluvium and are filled with unconsolidated sediments. The work in pools involved experimental study to examine the feasibility of using a high-resolution marine seismic unit. For the land seismic study, the equipment used was a single-channel and a 12-channel enhancement unit with a hammer as the seismic source and sometimes also a thumper for refractions. The use of explosives was not only unnecessary for such shallow depths, its use was also not feasible for the field procedures adopted by us. In the following, results of our study of seismic reflections on land, and also on water, are discussed and the improvements in the equipment and procedures suggested for the use of the Mines Research Institute. THE HAMMER SIGNAL

A lo-lb tions and as energy, tion etc., procedures the signal following.

sledgehammer was the main source of seismic energy for reflecshort-distance refractions. Knowledge of signal parameters such radiation pattern, frequency structure and reliability of reproducis essential for a theoretical unders~d~g and designing of field for recording and identi~~g reflections. These ch~~r~tics of from the hammer were studied and the results are presented in the

Estimate

of hammer energy

To estimate the energy of impact of a IO-lb (4.54 kg) sledgehammer, the procedure applied is the simple dropping of a 16-lb (7.257 kg) shotput ball on to the stoker-plate from various heights. The energy of the ball dropping through a height h (in m) will be given by Energy = 7.257 X 9.81 X h = 71.2 h (J) Keeping the position of the striker-plate and geophone fixed and the gain of the seismograph constant, the shotput ball was dropped from various heights and the energy calculated. The amplitudes of the troughs of first-arrivals from the shotput ball and hammer, at a fixed distance of 5 m, are plotted against the source energy in Fig, 3. It can be seen that the values scatter around a reasonably straight line in this energy range. The corresponding energy of the sledgehammer can be estimated from the figure to be about 280 J. Several assumptions are inherent in this exercise. The input signal from both the shotput ball and sledgehammer is assumed to have the same structure, i.e.,

111

Hammer-,:

/

/ /

0”“’ 0

20

40

60

60

”

100

120

Energy

Fig. 3. Amplitude of fist varying impact-energy.

in

”

140 Joules

160

a

183

’

200

I”’

220

240

/

260

280

-

arrival, at a fixed distance,

from a weight-drop

and hammer with

both are impulsive. Another assumption is that the part of energy in both cases which is converted into heat and sound is very small or equal. It is also assumed that the striker-plate is firmly embedded in the earth so that no energy is used up at each impact in deforming the surface of the ground. The experimental value of the hammer energy of 280 J seems, therefore, to be an overestimate. Seismic signal from the hammer-geophone

system

The energy from a hammer should be well coupled. The striker-plate should be firmly embedded in the earth for transmitting the maximum of the hammer energy to the ground and reduce the plate’s penetration into the ground, since this will result in the generation of lower frequencies. Further, the hammer should impact the plate in the center and vertically. This will generate more P-wave and less surface and shear wave energy. The way a geophone is planted also makes a difference in the type, amplitude and frequency structure of the received signal. As a general rule, the harder and firmer the surface on which a geophone is planted, the higher the peak frequency of its signal. A poor contact with the surface will result in relatively low frequency of the received signal. The geophone should be planted firmly, vertically for P-wave, by scraping off the top humus and loose soil, if necessary. Conclusively, then, proper planting of geophones and firm embedding of the striker-plate, along with correct hammering, will give a signal which is high in frequency and low in surface waves and shear waves. All these con-

112

siderations may not be too important for first arrivals, but, they are important for obtaining good reflections. First arrivals at short distances can be assumed to be the input signals into the ground for reflection and refraction surveys. However, the input energy becomes attenuated as it travels along various paths, and the received signal may look quite different along the length of a traverse. This is particularly true of the signals from weak sources such as a hammer. Phase changes can occur due to energy coupling and/or geophone coupling to the ground. But the common and major cause of the change in the ‘look’ of the signal is its journey along different paths inside the earth. Depending upon the offset distance, a first arrival can be a direct arrival through the topmost layer, a head wave through various interfaces or even a sound wave at short distances, A sound wave, however, can be easily distinguished by its existence as a first arrival only at very short distances and by its very high frequencies. As the signal grows weaker and signal-to-noise ratio is reduced, picking the first arrivals becomes more difficult. F~ili~ty with the ch~acte~sti~ shape of the signal, when it is strong, will help in picking the correct signals. Timing the received signals, whether first arrival or reflection, is another important factor. As the signal travels a longer distance through the earth, the signal wavelet broadens and the amplitude of the upward kick decreases and finally it is submerged into the background noise. The trough of the wavelet, however, stays alive consistently throughout the length of the survey and thus should be used for timing purposes. In addition to its consistency, the lowest point of the trough is also theoretic~ly the point which travels with the propagation velocity of the wavelet (Ricker, 1977) and, therefore, we should measure the time up to this point. Radiation pattern The radiation of energy from a vertical force such as a hammer on a homogeneous ground surface is one of the commonly known versions of the Lamb’s problem. This has been extensively discussed in the literature (Miller and Pursey, 1955; Mooney, 1976). Angular dependence of the ~plitude of a radiating seismic wavelet is shown in Fig. 4, which is based on eqs. 1,3, 5 of Miller and Pursey (1955). For a vertical detector virtually at the same place as the source, the reflected energy will be reduced by cos 0. The combined effect of the source radiation and detector’s reception then will make such a source-detector system very directional, This directivity can prove to be very advantageous in suppressing diffractions and reflections outside a cone; theoretically a 45” cone for a 50% reduction. Frequency

strutters of the hammer signal

The first arrival at short offset distances, such as 5 m in our case, can be regarded as representing the input signal to the ground for reflected events.

113

10

IO

Fig. 4. Radiation pattern of P-waves with a vertical force, and receptivity pattern with a vertical detector (after Miller and Pusey, 1955).

A small upward rise followed by a big trough and then a positive peak are typical of P-wave wavelets from hammers. The wavelet changes this character, resulting in broadening of the wavelet, with reduced amplitudes as the intervening distance increases. The trough remains, however, the most consistent phase of the wavelet. Surface waves, on the other hand, consist of several cycles with high amplitudes. These waves also change their character with distance but they are always the strongest events on the seismic record. Designing of a proper field strategy for reflections needs a sufficient knowledge of the frequency structure of the seismic signal. Discrete Fourier spectra of P-waves and surface waves from a hammer and the known-energy source of a shotput were analysed as shown in Fig. 5. These are typical of spectra computed for many P-wave and surface waves from hammers at short distances. The main spectral features can be described with the help of results in Fig. 5. Except for some energy at low frequencies around 40 Hz, surface waves as well as P-waves peak around 120 Hz. This peak or dominant frequency f for P-waves can also be computed in the time domain by Ricker’s formula (Ricker, 1977, p. 93), f= 0.778/b, where b is the time interval in seconds between two small positive peaks of a reflection signal wavelet (see Fig. 10). As an example, the value of b for a wavelet was 6.4 msec which gives a dominant frequency of 121.5 Hz. This compares well with peak frequency 117 Hz from the spectra. There is thus a good agreement between the dominant frequency given by Ricker’s formula and that computed spectrally. The seismic wavelet broadens as the high frequencies are attenuated in the earth. However, the form of the wavelet remains the same (Ricker, 1977, p. 95). This feature is of importance to the stacking and enhancing of the reflection signals in the field.

x--x

‘0

2

4

6

8

10

Frequency

12

14

Surface

WOWS

l -.-....*

First-Arr,vo,

O---O

Hammer’s

16

18 20

I” Mult!ples

22

24

of 9.77

First

26 Hz

28

1

Shotwt

--Arrival

30

-

Fig. 5. Spectral amplitudes of first arrivals and surface waves from hammer and weightdrop (shotput).

In order to investigate the effect of source energy on the spectral form, seismic wavelets from several locations in the area were studied over the range of hammer energy. It was found that spectral forms remained very much unchanged, even though the peak spectral, amplitude changed 8 times over the range of hammer energy. Next, we wanted to investigate the change in the spectrum of the signal as the number of impacts increased for the same energy of the impact of about 142 J. The peak spectral amplitude again increased with each impact but the spectral form remained almost unchanged and the peak frequency also was pretty much the same. These results then essentially indicate that, for a particular location, the signal spectrum does not change much over the range of hammer energy and also remains unchanged from one impact to the next. This result is very useful again for stacking and enhancing of the reflection signal. The dominant frequency of surface waves in Fig. 5 falls close to the dominant frequency of the P-wave. The dominant frequencies of surface waves, for example, in a particular case, varied from 110 to 125 Hz, over the source energies up to 180 J; the spectral form again remained unchanged. This situation, however, changes considerably with different offset distances and different locations. For a fixed location, surface waves are very much reproducible. Their reproducibility can be used to advantage by d@gning a suitable field procedure for suppressing surface waves and enhancing reflections.

115 ESTIMATION OF VELOCITIES AND STATIC CORRECTIONS

Estimation of seismic velocities of various layers overlying the bedrock is an essential part of the interpretation of seismic reflections. Seismic refraction is used to estimate the average velocities. The geology of the area helped us to know in advance that the bedrock of limestone is overlain by old and young alluvial sediments. We can consider it a 3-layer case - the top lowvelocity-layer (LVL), compact layer of sand with clayey intercalations and gravels interspersed, and then the bedrock at the bottom. A plot of time-distance for first arrivals from a refraction profile in the area is shown in Fig. 6. Data were collected with a 12channel signal-enhancement unit. There are three main segments. The first layer is about 2.5 m thick with an average velocity of 345 m/s. The second layer of Old Alluvium appears to have a continuous range of velocities, increasing with depth. However, for the interpretation of reflection data, only an average velocity is chosen which will be assumed to be constant for areas of small extent. The average velocity of the second layer is about 1500 m/s, and that of the underlying limestone bedrock is about 5128 m/s. In addition to these three layers, there is always a humus layer of rotten leaves and other organic material at the top. The area is a rubber plantation with flat land covered with bushes and other undergrowth. The humus layer of about 15 cm, blackish to greyish in colour, can have a velocity as low as 55 m/s and should always be scraped off for planting geophones or hammering. The information obtained from seismic velocities was checked against a drillhole section in Fig. 7, drilled close to the refraction profile, At drillhole K3, a layer of compact, interlacing clay and sandy layers is overlain by a layer of soft, sandy clay of 2.7 m thickness and is underlain by bedrock at 32.3 m

I ‘0

I

I

5

10

I

I

15 20

11

25

I,

30

35

I,

40

45

Offset

Fig. 6. Time-distance

50

I

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I,

55

60

65

70

Distance

I

75

in Meters

curve for first arrivals.

80 -

I

65

I

90

I

I

95

100

116

Coarse Cloy

----

with

Coarse

1641~1

--

sand

wrth

some

sand

httle

50”~

fine

sand

llttle

cloy

cloy

, grey~sh

16Wl j

,

---I--.-----------225m

32.3 m

Sand

wth

llttk

clay

Sand

with

gravel

Sand

with

fragments

of

limestone

i

Ltmestone

Fig. 7. Cross-section

of a drill-hole,

K3.

depth. The top layer of soft, sandy clay here corresponds to the LVL and the layer underlying LVL to the consolidated sediments of velocity 1500 m/s. The average velocities thus determined are used routinely in the interpretation of the reflection data. As long as these velocities can be assumed to be the same in a particular area, the only variables are then the depth to the bedrock and the thickness of the LVL. The only kind of static correction that we used under these conditions was to correct for the variable thickness of the LVL. The velocity of the LVL is about one-fifth of that of the second layer, and thus a 1 m increase in LVL thickness will correspond to a false increase in bedrock depth by 5 m. The relative effect of static errors is thus more pronounced in shallow seismic surveys than in deep ones. To determine the LVL thickness for.static corrections, consider the situation depicted in Fig. 8. The difference between first-arrival time at the point 0, tOA , and that at B, t&, , with hammer at A, is: ttOA

-

tBA )

DE

EF

GO

v*

v,

VI

=F+-+----=

JL

--

where GF is taken to be equal to BD. OF is not necessarily parallel to BD . Next, the time of first arrival at 0, tOB , with hammer at B is -tOB

BE

EF

GF

GO

VI

vz

VI

VI

=z--+~++++=Ko.

(2)

Similar equations will follow if BD is greater than OF, i.e., thickness at 0 is smaller than that at B. Due to parallelism, time KM = time JL. The intercept

117

n

B

”

Distance

0

B

-

Ground

-------+

Consolidated Sediments

Bedrock

Fig. 8. Schematic diagram of a time-distance curve corresponding to first arrivals from the bottom of the top layer (LVL). The intercept time, P , at point l3 is equal to time (KOJL) which is the same as t 0~ - (t,~ - tB~), with toB being the first-arrival time at 0 from source at B, etc. time

at B, P , is then given by:

IB = time KO -time

KM = time

KO -time

JL = eq. 2 - eq. 1.

Therefore: --

BE

BD i%? IB = 7 I + 7 1 - 7 = toB - (toA -t,,

)

2

-Assuming that m NBE, BE = GF = h, . cos $J/COSilz, where hB is the vertical depth at B, C$is the dip angle and iI2 is the critical angle,

= 2h, co@ = 2h, co@ dm V, cosi,, v, - VI

r”? and:

DE

z

v2

= 2h,

cob v2

2hB cos f$ sin2i,,

tar& =

V,

cosi12

(3)

118

Finally : P=_

2h,

cos$

VI cosz~2

-

2h, co@ sin?

12 =

7, COG,,

and then (normal depth)

2h, co@

qF$-=Ff - - V2Vl

at B = :P

The determination of the ‘modified’ intercept time by this procedure has the following advantages. (1) It is fast and LVL depths can be determined at every reflection site. All that is needed is a rough knowledge of the critical distance corresponding to the LVL. We can determine the value of modified intercept time from only three first-arrival times. (2) The effect of lateral variations of velocities of VI and V2 between the points B and 0 is automatically eliminated. Similarly, the effect of variation in ground elevations or bedrock topography between points B and 0 is also eliminated. In the conventional, graphical method, the estimation of intercept time is influenced by conditions between points B and 0. (3) The intercept time is affected only by the conditions below the point B, as can be seen from eq. 3.

( b)

Fig. 9. Schematic diagram of (a) a geophone pattern, and (b) its transfer function for n geophones, with respect to apparent wavelength A, and dominant period Tf.

119 DESIGN OF GEOPHONE PATTERNS

Geophone patterns, or equivalently source patterns, act like filters to the seismic wavelet, selectively modifying different wavelengths and frequencies. As we have used a single-channel instrument, only source patterns concern us, which are, however, theoretically equivalent to geophone patterns. Based on the theoretical derivations, design parameters pertaining to this study will be discussed. Wavelength filtering Surface waves at such short distances as in our study are characterized, in comparison to the reflection signal, by low velocity, high amplitudes and their near-horizontal travel path along the surface. They have, consequently, wavelengths different from those of the reflection signal. Consider a geophone pattern such as in Fig. 9a. It is symmetrical around the central geophoneg, and uniformly weighted equal to l/n, where n is the total number of geophones in the pattern with equal spacing AL. If AT is the increase in time for a wave, with apparent velocity V,, to travel from one geophone to the next, then V, = A L/AT and also V, = Vt,,/sinO , where 0 is the angle the wavefront makes with the surface. It can be easily shown that the transfer function G,(Tf, V,) of the pattern to the wave with period Tf is: n-l 2 GnU’f,

Va) = +

+

c

(5)

j=l

which causes no phase change. But: F

=f $

= (AL/V,)/(l/f)

= F a

a

where X, is the apparent wavelength of the noise (or signal). Hence, we can also write the transfer function in terms of wavelengths as: n-l 2

G,(AL,h,)

= +

+

(6)

A schematic behavior of G, is shown in Fig. 9b. Geophone patterns thus behave like a wavelength filter. They can be used to suppress the wavelengths of surface waves. The design of a pattern can be planned looking at Fig, 9b. It can be shown that for surface waves to have reduced wavelengths in the

120

central region of suppression, n>

h a,max ~ + 1, and length of the spread n +AL > Xa,max x a ,min

(7)

For surface waves with dominant frequencies in the range of 75 to 125 Hz and a velocity of 200 m/s, dominant wavelengths are about 1.5 to 2.5 m. Thus a pattern with more than 3 geophones and more than 2.5 m as the spread length will be desired for suppressing surface waves. Stacking filter It can be seen from Fig. 9b that the geophone pattern also acts as a Iowpass filter. If geophones are planted on the surface, then the time increment A 2’ between geophones for the reflection signal arriving almost vertically will be given by AT = AL/V,. Then AT/Tf = AT * f = f(A/LIV,). Looking at Fig. 9b, we can see that there is fast suppression of amplitudes after frequencies of about: ffAL/V,)>

%(1/n)

or

f> $

O’,/ALt .

For reflection signals in the LVL, the velocity Vh, = 350 m/s; taking AL = 0.3 m, and n = 7, V,/(2n AL) = V,/4.2. Further, for a pattern center at 3 m and a depth of the reflection point about 30 m, the angle of the reflection trajectory will be @= tan-’ (1.5/30). Using this value of 0, V’ = 3501sinB = 7000 m{s. Substituting the value of V, in the expression off above, f > 1670 Hz. It seems, then, that there will be no appreciable suppression of frequencies in the reflection signal. In conclusion, the stacking produces no degradation of the reflection signal. However, degradation in the process of stacking can occur due to other reasons such as topography of the reflecting surface. SEISMIC RESOLUTION

Seismic resolution concerns the ability of the seismic signals to show that separate features exist, vertically as well as horizontally. An understanding and estimation of the resolution is all the more important in our case as the depths and horizontal spacing of peaks of the limestone bedrock are small. Vertical resolution Reflection wavelets shown in this paper indicate a signal that is reasonably symmetrical about a central point. It can be assumed, therefore, that the reflection signal is a good approximation to a zerodelay wavelet. Because of the convolution properties, this means that the source wavelet and earth’s filter also are zero-delay functions. For such zero-delay source wavelets, a

121

hypothetical signal reflected from the top and bottom of an embedded layer is shown by solid line in Fig. 10. We can estimate the vertical resolution for wavelets in the figure. If the two-way travel time is more than about 6 ms, the appearance of the resultant waveform makes it clear that more than one reflection is involved. For a velocity of 1500 m/s of the alluvium overlying bedrock, this time will correspond to 4.5 m. Using Ricker’s formula, the dominant frequency with b = 13 ms, is 60 Hz and then the dominant wavelength will be 25 m. A resolution of 4.5 m is about 18% of this dominant wavelength. It appears that with the knowledge of the wave shape and no background noise, a resolution of lo-20% of the dominant wavelength is quite possible.

-2

0

4

8

12 Time

16 in milliseconds

20

24

28

32

36

40

--c

Fig. 10. Diagram illustrating the resolution vertically displaced layers.

of two reflection

wavelets

coming from two

Vertical resolutions as discussed above do not impose serious constraints, however, in mapping the bedrock topography as there are no wedgings or thin layers of hard rock. More important to the resolution in vertical direction are the timing errors which decide, in fact, the accuracy of vertical depths. Timing error, again with a known waveshape and no background noise, is about 2 ms, which corresponds to about 1.5 m error in vertical depths.

Horizontal resolution Horizontal resolution can be defined as the minimum horizontal distance between two features which can be resolved on a seismic record. To understand its significance and compute values in the present context, the concept of quarter-wavelength Fresnel zones will be utilized. But as depths are small in comparison to wavelengths, the treatment in the following is somewhat different from the formulations in optics. Simple, ideal conditions are assumed for deriving an estimate of the horizontal resolution. Energy from source S radiates to a plane, horizontal bedrock surface at depth d. The source energy emanating from S is of the same phase in all directions, but has different amplitudes, according to the radiation

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pattern in Fig. 4, at different angles. The reflected energy is received at geophone G after being reradiated upward, according to Huyghen’s concept, from Fresnel zones on the bedrock. The geophone G is taken to be virtually at the same place as the source S. With center of zones C vertically below the S-G point on the bedrock surface, the first quarter-wavelength Fresnel zone will contain all the incident wavelets which will reach the geophone G with a phase change less than 180” with respect to the reflected wavelet from the central point C, and so on. An area element of the Fresnel zones contributes to the disturbance at geophone G according to: (1) the area of the element, (2) amplitude of disturbance of the element, (3) the inverse proportionality to the distance, (4) obliquity factor due to directivity of a vertical geophone, and (5) Huyghen’s obliquity factor. To determine the amplitude of energy of an element, let us take the amplitude corresponding to the central point C as unity. The factors to consider for determining the relative amplitudes of the incident energy at any area element on the bedrock are the distance from source and the obliquity factor due to radiation pattern. The obliquity factor can be put equal to cos 0 = d/[d +j(X/4)] , where j is the number of the Fresnel zone and 13is the angle of incidence from the vertical. Considering the factor due to distance also, the amplitude of the incident energy at the jth Fresnel zone will be given by d*/[d +j(X/4)] *. The contribution of reradiated energy to the geophone G from thejth zone will be proportional to: (1) area, n[A(d/B) + A*/16 (2j - 1)] ; (2) amplitude of the area element, d*/[d + j(A/4)] *; (3) distance to the geophone G, d/Ed + j(h/4)] ; (4) obliquity due to verticality of geophone, d/[d + j(h/4)] ; and (5) Huyghen’s factor (1 + co& )/2. The resultant contribution at G due to the jth areal element of the Fresnel zones on the bedrock surface can be given as F(h,dj), where:

It is noted that the last factor in the derivations above is assumed to be the same all over a particular Fresnel zone. Contributions can now be computed, using eq. 8, for Fresnel zones (0, h/8), (X/8, h/4), (X/4, h/2), (h/2, 3/4 X), (3X/4, h), (h, 5h/4), (5h/4,3A/2), (3h/2, 7/4 X), (7/4 h, 2h), where (X/4, X/2) means the annular zone corresponding to inner and outer travel paths of lengths equal to (d + A/4) and (d + X/2), respectively, etc. For a wavelength of 25 m and depth d of 25 m, these respective contributions are 3.9154, 2.7359, -2.8292,1.7016, -1.0986, 0.7485,-0.5320, 0.3912, -0.2958. The corresponding contributions for a depth of 50 m are 4.9086, 4.0305, -5.4720,3.9639, -2.9678, 2.2623, -1.7672,1.4053, -1.1352. Contribution from outside the (0, X/8) zone, as compared to the total, is about 17% for a

123

depth of 25 m and about 6% for a depth of 50 m, for eight quarter-wavelength zones considered. The (0, h/8) zone has a radius of 13 m for a depth of 25 m and 18 m for a depth of 50 m. Amplitudes decrease rapidly as we move away from C; for example, the amplitude of the 8th zone is only 7.5% of the (0, X/S) zone, for 25 m depth. Apparently, then, only small energy is contributed from area outside the (0, X/8) zone. In conclusion, most of the resultant energy received by the geophone, for the field procedure and depths encountered in our study, can be seen to be reflected from a circle of radius of about 15 m. The use of higher frequencies will reduce the size of the main central zone, (0, h/8). As an example, the radius of this zone will be 13 m for 120 Hz as compared to 18 m for a 60-Hz signal and a depth of 50 m. It should be further noted that a wide-spectrum signal will have various wavelengths and consequently different corresponding portions of the reflecting surface are responsible. A change in the reflecting surface could influence one frequency more than another, with a resulting degradation of the reflected signal. It is also easy to see how the geophone starts “seeing” an abruptly changing feature like a fault even before the source-geophone system comes over it and continues to see it even after passing it. This gives rise to diffractions. Further, a body smaller than the (O,h/8) zone will act like a point reflector and will diffract. Another important point to note here is that diffractions will die fast with increasing horizontal distance because of very rapid decrease in amplitude outside the (O&/8) zone. This fact apparently will be of great advantage to the interpretation of our results where a minimum amount of processing is possible. DISCUSSION OF REFLECTION DATA

The primary purpose of this study has been to develop a seismic reflection method to determine detailed depths to bedrock over areas of the size of a tin mine. Tin mines are on the scale of about 300 X 300 m. Depths to bedrock range up to about 100 m. The velocity of the LVL at the place of survey was about 345 m/s and that of the Old Alluvium underlying the LVL was 1500 m/s. The surface waves travel along the surface with velocities less than about 200 m/s, with frequencies, at short distances, ranging above 80 Hz. Their wavelengths thus vary around 2.5 m. A very low velocity in the LVL, where the source and detector are situated, means a low wavelength of the surface waves. This is helpful to us in suppressing these waves with wavelength filtering, using only a small spread. A small spread is important because a large one degrades the reflection wavelet, and interference from refractions can also occur. The field procedure was designed on the basis of the various considerations discussed above. After clearing the traverse lines of the bushy undergrowth, the first thing was to run a refraction velocity survey. As the area is geologically uniform and flat, one or two velocity profiles are generally suf-

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ficient. The next step is to prepare reflection sites by scraping off the humus layer, etc. Based on the geological information and the objective of the survey, it was decided that 5 to 10 m spacing between reflection sites would be sufficient. Determination of LVL thicknesses at the sites was done next, using the modified-intercept-time procedure discussed previously. Velocity survey and LVL-thickness survey were both carried out with a 12-channel signal-enhancement seismograph, and a hammer. To obtain reflections along the traverse, the distance of the hammer to geophone was kept in the range of 5 m or so. This geometry gives almost vertical reflections from points on the bedrock right below the geophone. The main reasons for this geometry were the trough-and-pinnacle topography of the bedrock and the surface waves. A large offset distance to have the surface waves come after reflections is not feasible because, besides other problems, the reflections may not reach the geophones due to their being blocked by intervening bedrock topography. With a 5 m offset, surface waves arrive and finish before reflections arrive, except for reflections from some very shallow bedrock peaks. Also, at wide angles, the transmitted energy for reflection from the bedrock is poor and it is difficult to get reflections with a hammer. There is also the problem of multiples in the LVL for wide angles; they are strong at wide angles. There are several other advantages in our case in using vertical reflections. At vertical angles, the conversion from P to S phase does not take place, the transmission from the LVL to deeper layers is more effective, and transformation of data from observed time sections to depth sections is simple and quick. Keeping the geophone fixed, hammering was done over a distance of about 1 to 5 m. This is equivalent to a pseudo-commondepth-point method, hereafter called just CDP. Move-out times are small enough to be ignored. Instruments used do not have any facility for mixing different channels. So we made use of features such as signal enhancement and solid-state digital memory to stack the records in the field. For example, a record from an impact at 4 m was first recorded on the strip-chart recorder as a permanent copy and only then was this record stacked in the solid-state memory with a new record corresponding to the impact at 3.6 m. One by one, records corresponding to 4 m, (4 m + 3.6 m), (4 m + 3.6 m + 2.8 m), etc. are obtained. The permanent copies of all these stacked records at a particular site can be pasted on a graph paper, all synchronized, as shown in Fig. 11. A CDP stack should only enhance a reflection and tend to suppress surface waves. A true reflection should enhance through all, or a majority of, the stacked records. Such reflection enhancement is illustrated in Fig. 11 at 61 ms. After applying corrections, this amounted to a computed depth of 31.3 m. A drillhole at this place gave a depth to the bedrock of 32.3 m. In order to see the signal build up, the signal on all records should be below saturation level and thus a low gain has to be used. A strong source such as explosives, therefore, will not only saturate the geophones but is also unnecessary. Further, the first record of the stacked set should be at the farthest

125

t

c d 4.0E” 3.7.&

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ki i ?

1.6-

z j

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.z Q l.O-

.~*-~~.~_.*.~...Cf__.“.i~

8

t

0

200 Time

in

Milliseconds

(ms)

-

Fig. 11. Stacking of seismic traces for varying hammer-togeophone distances, with geophone fiied, at a location. The top trace corresponds to an impact at 4.0 m, the next trace corresponds to impacts at 4.0 m and 3.7 m after stacking, and so on; the bottom trace is the resultant of ail traces stacked together. The event at 61 ms is interpreted to be a reflection from the bedrock.

distance as impacts at very short distances like 1 m have a tendency to saturate the record, making it useless for any further stacking. Even with all due care being exercised, we may still not get reflections at all at a particular place. However, before marking a record ‘no reflection’, it is worthwhile to try hammering in a direction different from the profile direction and see if the signal then shows up. This point is illustrated in Fig. 12. The top set of stacked records represents hammering in the direction of the profile and it has no definite, consistent reflection-type signal. The bottom set is for hammering in a perpendicular direction; it shows a very definite reflection signal. This situation probably arises due to the bedrock topography. The reflection data from all sites along the profile are finally aligned by putting one record for each site sequentially. An example of such a record

126

t

.c

4.0-

: 3.6 ,o g3.2 8

-

E ?2.8LL E E 2.4 I” 62.0-

7

. . . . . . . . ..\...i

i .,

L

.

A_._,

0

\

.

200 Time

in Millijeconds

(ms) --o-

Fig. 12. Two record sections corresponding to hammering done along (top) and perpendicular (bottom) to the profile direction. Each trace in a record section is a stack of ail the previous traces, starting from the top and going toward the bottom. A clear reflection event at 64 ms can be seen in the bottom section.

127

section is shown in Fig, 13. The bedrock as interpreted from the record section is indicated by the dashed line. The portion of the profile in Fig, 13 is 190 m long with a record length of 100 ms. A reflection at the 90 m site is shown at 62.5 ms which corresponds to a depth of 40.3 m to the bedrock. A

2030406060I

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160170 180190-

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Fig. 13. A record section with traces for different diiances along the profile. Each trace is one of the stacked traces, usuatly the bottom one, see Fig. 11, corresponding to a particular location. Stacked sections corresponding to each trace on this record are used to identify and mark the reflection events.

128

subsequent drillhole at this place hit the bedrock at 38.4 m. Due to the absence of automatic gain control (AGC), this record section does not look the same as the conventional multich~nel record. However, the increase in the reliability of reflection identification from the CDP stack compensates, to some extent, for factors such as the apparent lack of quality due to absence of AGC and high-amplitude surface waves. The continuous profiling done to identify reflections may not be useful in situations where structures change very fast laterally. In such cases, there is no resemblance to a continuous reflection from, for example, the limestone topography. Another difference between this survey and the one with multichannel recording with AGC is time and money. About 2 to 3 days are taken for velocity- and LVL-thickness surveys. The actual reflection work at a site may take less than 30 min. The comparison should, however, be made with the presently used method, i.e. drilling, rather than the seismic surveys done w&h different facilities and aims. Signal-enhancement units are not expensive and the Mines Research Institute in Malaysia has access to three such units. There is a permanent staff of three geophysicists available at the Institute, Thus the time factor can be cut down if desired. It takes about 7 to 12 days to drill a 50 m hole in the area and it is expensive. Based on these reasons, the Institute is planning to use a combination of seismic reflection and drillholes for their future operations. HIGH-RESOLUTION-REFLECTION

SURVEY OVER MINING POOLS

As mentioned before, some parts of the area under investigation consist of mining pools. These pools are the old tin mines which were dredged out for alluvia1 placer tin and now have a fill of unconsolidated sand with gravel and clay. Such old mines are still rich in tin ore mostly embedded in crevasses and troughs of the limestone bedrock and they can be mined in the future with gravel pumps. It is useful, therefore, to know the bedrock topography and accurate thickness of the alluvium so that reserves can be assessed by drilling at potential locations such as Ll, L4, L7, Lll, etc. in Fig. 14. This Posltlon LO

Ll

L2

L3

14

OF L5

Dr,ll

Holes

-

L6

L7

LB

600

L9

LlO

Lll

900

Fig. 14. A cross-section of a mine pool, based on the drillbole data.

LIZ

113

1003

129

figure illustrates a typical cross-section of the mining pool surveyed seismically. Drill-holes L4 and Lll are unbottomed, i.e., do not reach the bedrock. The seismic instruments used for the survey are shown by a block diagram in Fig. 15. The survey results presented here indicate again, as in the case of land seismic, that a seismic source with less than 300 J energy is good enough for our purpose, provided there is an efficient energy-coupling to the ground. Operation of the source, detector and recording apparatus at 400 Hz-5 kHz makes the survey a high-resolution one. (0 3OOHz-5K

Hz

Elements)

Hydrophore

Filter 400H

--_c

-5KHz

Capacitor

Generators 7HP

500 <300

Hz

-

8 K Hz

Joules

Fig. 15. Block diagram of a seismic acquisition system used in mine pools.

The purpose of the survey, size of the pool, size of the boat and its speed, source and detector types, and other such variables make a seismic survey over a mining pool somewhat different from the usual marine seismic surveys, as so far as their problems and solutions are concerned. Some of the main features of the survey done can be enumerated as follows. (1) Water depths can vary from very shallow to depths of more than 20 m. Only small boats with small draft can be used. The size of an average mining pool is less than 500 m in any one direction. At a speed of about 25 m/min, the boat frequently has to make sharp turnarounds. This makes it necessary for one person to frequently maneuver the hydrophone cable and the heavy source at turnarounds to keep them from getting entangled and cut by the propeller, or becoming stuck in shallow mud and sometimes weeds. A wire cage around the propeller is highly recommended. (2) There are strong multiples from the water layer. With these water multiples, weak signals on the record are difficult to identify (see Figs. 16 and 17). (3) Good reflections are obtained from the limestone bedrock, indicating that an energy of the source of less than 300 J is satisfactory for the depths and sediments encountered in this area. The source consists of an electromechanical transducer which converts a high-voltage electrical pulse from the energy source into an acoustic, pressure pulse. (4) Even though good reflections can be obtained from the bedrock, the

Fig, 16. A seismic time-section speed was about 25 m/min.

in a mine pool. The record

.~.l-.---lll_

length

is 125 ms and the boat

.--

Fig. 17. A seismic time-section in a mine pool. The mxwd length is 125 m and the boat speed was about 25 mlmin. The deeper pinnacle below the shallow bedrock is believed to be the result of side-reflections from a peak.

131

132

overall quality of the records is rather noisy. A part of the reason for this is the use of an 8-element hydrophone, but most of it is inherent in the size and topography of a mining pool. Individual elements in an 8-element hydrophone act as separate detectors for reflections from the shallow-water bottom, sides of the pool and for reflection from the water surface. The source-tohydrophone distance should not exceed the depth of water; however, it is difficult to satisfy this condition where water-depths decrease to almost zero as the boat approaches the end of the pool. This makes the record noisy. Weak signals and any fine lithological information from seismic records is lost. A single-element hydrophone should remedy the situation considerably. Also the hydrophone should be towed as close to the source as possible. (5) Bedrock reflections have strong diffractions. A sharp peak acts like a point scatterer and is accompanied by two sloping, almost straight-line diffractions. A narrow trough is indicated by an inverted, convex-upward, curve with its highest point at the lowest point of the trough. Diffractions from surfaces such as a flat surface of a limited lateral dimension are also theoretically known and can be identified on the seismograms. The bedrock topography can be considered as a combination of these geometrical elements as far as diffraction effects are concerned. To remove diffractions by migration procedures will be extremely difficult and uneconomical for surveys under study. Besides, migrating the seismic records is not a necessity for our purpose. The locations of peaks, troughs, sloping surfaces etc. can be marked on the seismic records, without precisely delineating the bedrock between a peak and a trough, for example. The main objective of the geophysical survey for tin exploration and exploitation is to know the locations of these peaks, troughs, etc., so that drill-holes can be placed at the appropriate sites, or the opening of a mine can be better planned. Figs. 16 and 17 show some examples of seismic data collected in a mining pool. It might be worth mentioning one special feature in Fig. 17. The deeper pinnacle here is actually a side-reflection event and not one right below the seismic line as it appears on the seismic record. As a matter of fact, side pinnacles and other such features are more difficult to interpret than the diffraction events. The records obtained in the field are a three-dimensional geological picture projected seismically on a twodimensional time section. This poses problems in interpretation. The interpretation can be helped partly by running several parallel seismic lines and looking at them simultaneously. However, the best solution to the problem of side-reflections and diffractions is to use a source with a very narrow cone of energy. We have made a prototype of such a source, specifically for our use in mine pools. Essentially, the source is a sparker of about 0.3 m length with about 30 electrodes, ranging from 2 to 10 J per electrode. The sparker is enclosed in a rectangular box of about 0.6 X 0.6 m base and 1 m length, entirely submerged in the water of the pool. The energy will emerge only through the thin plastic bottom, with a cone of about 20”) and is sufficiently attenuated by absorbers on four long sides. The sparker remains in a salt solution sealed from the fresh water

133

of the pool. The prototype has cost us less then US$ 100. It is very light and will be very suitable for surveys in pools of small areas. SUMMARY

AND CONCLUSIONS

The work presented here concerns a feasibility study supported by the Mining Research Institute, Malaysia, to develop a reflection seismic system for delineating the limestone bedrock and to determine the thickness of the overlying alluvium, for the purpose of tin-ore-reserve evaluation and planning tin-mine operations. The bedrock has a characteristic trough-and-pinnacle topography with depths of less than 100 m. The desired system was to be as portable and inexpensive as possible and also geology-specific. Emphasis was on the improvisation using existing data-acquisition systems. Further, the field system was to be self-sufficient, as far as possible, in the sense that results should need a minimum of laboratory data-processing and interpretation. The present field system includes single- and 12channel signal-enhancement seismographs with hammers as the main source of energy. The experimental results have shown that strong reflections can easily be obtained from the bedrock down to about 60 m with a small seismic source of less than 300 J energy. A small part of the area studied is covered with tin-mining water pools and seismic results of a high-resolution-reflection seismic survey over such pools in the area are also presented. The P-wave velocities of the top lowvelocity layer and the underlying consolidated alluvium for land seismic survey are about 350 m/s and 1500 m/s, respectively. Ground roll has velocities of less than 200 m/s. A quick method to determine the thickness of the low-velocity layer at reflection sites for static corrections has been suggested. The frequencies of the reflection signal and ground roll were estimated to be around 60 and 100 Hz, respectively. Vertical resolution, dependent on timing errors only, is about 1.5 m, whereas the horizontal resolution, under favorable conditions, can be less than about 15 m. The feature of using the field equipment to enhance signals by stacking of several identical impacts permits the use of a relatively weak source and helps in identifying the reflection signals. These reflections appear after dying-away of ground roll. A source array, equivalent to a pseudo-CDP stack, is used with a single-channel unit to stack records from individual impacts. It appears at this time that the factors and procedures which allowed the use of the reflection seismic method for this problem are: (1) a low-velocity, loose, sandy soil which is favorable to reducing ground roll; (2) source-array-CDP stacking to suppress ground roll, and enhance and identify reflections, and also reduce other extraneous signals such as multiples, etc.; (3) directional nature of the source-geophonearray system to generate less diffractions, side-reflections and ground roll; (4) short geophone-to-source distances and short array dimensions; and above all, (5) a good knowledge of the geology of the area. After appropriate studies, it has been demonstrated that the reflection

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seismic method can satisfactorily help in tin-ore exploration and exploitation. A practical and inexpensive field technique can be worked out. Besides the tremendous cost savings, a better horizontal definition with reflection seismics enhances the overall tin-ore evaluation program. There are some unfavorable situations when difficulties arise in obtaining reflections or in interpreting them. Problems can exist in four key areas, They are in the realm of surface waves, energy source, diffractions and the resolution. An ideal energy source would be a variable-energy source which is a portable, repeatable, directional, nonexplosive, high-frequency generator of seismic waves, and applicable on the surface as well as downhole. We are in the process of having such a source fabricated; it is a propane-oxygen detonator at the end of a cylindrical tube about 2% inch in diameter. This is expected to impart energies from 1,000 to 15,000 J, depending upon the need. The surface waves, arriving before reflections as they do in the procedure used, set a limit to the shallowest depth that can be detected. The duration of these waves is typically about 50 ms. Assuming a thickness of 2.5 m for the low-velocity layer, it turns out that bedrock peaks shallower than about 26 m can not be detected. It may be necessary to plant geophones in shallow holes about l-2 m deep; or still better, to use hydrophones which should be buried or used in water-filled holes. Hydrophones have better coupling to the earth and a high-frequency response. For such shallow depths, a hammer is good enough as far as energy is concerned. If a downhole source is used, then an in-series resistor may be needed to reduce the sensitivity of the detector. The detectors inside the holes should be kept at a higher level than the downhole source in the nearby hole. Due to directionality of the source, this should reduce the direct as well as surface waves reaching the detector. Migration techniques will not be practical in our context without substantially changing the present equipment and procedures. Besides, migration is not a necessity in the sense that a knowledge of locations of peaks and troughs is sufficient for deciding the sites for drill-holes. An ideal solution will be to use a directional and high-frequency, acquisition-system, with a narrow cone of energy. It appears that a combination of vertical downhole source, vertical detectors, preferably hydrophones in water-filled holes with some cylindrical absorbent skirt all around, source- or detector-array, small detector-to-source distances, should make the acquisition system sufficiently directional. The present equipment is being updated with the new source, highfrequency detectors and other needed improvisation. Addition of a digitalmagnetic-tape-recording unit with intermixing facility for the channels should increase the flexibility for laboratory data processing if the need arises. ACKNOWLEDGEMENTS

I would like to express my special gratitude to Dr. H. Hussin and Mr. Abdullah Ismail of the Mine Research Institute, Malaysia, for their support

135

and help in carrying out this work. I am also thankful to Dr. John Ringis of UNDP, Bangkok, for some helpful discussions on the manuscript. Finally, the constant encouragement and support from the Dean, Dr. R. Ratnalingarn, of the School of Physics is acknowledged. I appreciated the clerical help from Mrs. Sabariah Ahmad. REFERENCES Barry, K.M., 1967. Delay time and its application to refraction profile. In: A.W. Musgrave (Editor), Seismic Refraction Prospecting. SEG, Tulsa, Okla., pp. 348-361. Meidav, T., 1969. Hammer reflection seismics in engineering geophysics. Geophysics, 34: 383-395. Miller, G.F. and Pursey, H., 1955. On the partition of energy between elastic waves in a semi-infinite solid. Proc. R. Sot. (London), Ser. A, 233: 55-69. Mooney, H., 1976. The seismic wave system from a surface impact. Geophysics, 41: 243-266. Noponen, I., Heikkinen, P. and Mehrotra, S., 1979. The applicability of seismic reflections sounding in regions of Precambrian geology. Geoexploration, 17: l-9. Palmer, D., 1980. The generalized reciprocal method of seismic refraction interpretation, K.B.S. Burke (Editor), SEG, TuIsa. R&h, S.S., 1979. The Kinta tin field, Malaysia. Bull. Geol. Sot. Malaysia, 11: 111-136. Ricker, N.H., 1977. Transient Waves in Visco-Elastic Media. Elsevier Scientific Publishing Co., Amsterdam, 278 pp. Shepers, R., 1975. A seismic reflection method for solving engineering problems. J. Geophys., 41: 367-384. Singh, S., 1978. An iterative method for detailed depth determination from refraction for an uneven interface. Geophys. Prospect., 26: 303-311.