Geoexploration, 22 (1984) 89-105 Elsevier Science Publishers B.V., Amsterdam -. Printed in The Netherlands 89 SHALLOW SEISMIC REFLECTIONS WITH A PROPANE-.OXYGEN DETONATOR SURENDRA SINGH School of Physics, University of Science of Malaysia, Penang (Malaysia) (Received 3 March 1983; accepted 5 September 1983) ABSTRACT Singh, S., 1984. Shallow seismic reflections with a propane-oxygen detonator.Geoexplo- ration, 22: 89-105. In this paper, experimental results of shallow seismic reflections obtained with a down- hole propane--oxygen detonator as a seismic source are presented. The site selected for field experiments was in an area where granite bedrock is overlain by alluvial consolidated sediments, and in turn, by the low velocity layer near the surface. Typically the granite depths and the thickness of the low velocity layer are around 40 m and 3 m, respectively; velocity of the consolidated sediments is about 1700 m s-l and that of the low velocity layer is about 300 m s-l, as determined initially by the surface refraction method and later confirmed by a well-velocity survey. The seismic source is of variable energy, cheap, directional, generates high frequencies and very little surface waves. A single high frequen- cy geophone with near-zero offset distance from the source was used to acquire time rec- ords at different locations. The data illustrate the successful use of shallow reflections to delineate the irregular bedrock. Although evaluation of this seismic source is still continu- ing, there are preliminary indications that reflections from depths as shallow as 5 m may be discernible under favourable conditions, It is demonstrated that application of accu- rate static corrections is critical for shallow and very shallow reflection work. A method- ology, called ABO method here, for computing static corrections quickly and accurately, is discussed. INTRODUCTION A field study was undertaken to evaluate the effectiveness of reflection seismics in mapping irregular bedrocks at shallow depths with a propane- oxygen detonator as a seismic source. This work was part of the overall ef- fort to devise a suitable geophysical method in the search and mining of pla- cer-tin deposits in Malaysia. The investigation also included the feasibility of delineating subsurface intra-alluvial stratigraphy. Mapping of bedrocks for shallow depths has been traditionally done by drilling, however, as the near- surface resources get depleted, the emphasis is shifted to deeper target areas. In addition to being costly and time-consuming, drilling is also found to have a poor horizontal definition for placer deposits which concentrate in the
Transcript
Geoexploration, 22 (1984) 89-105 Elsevier Science Publishers B.V., Amsterdam -. Printed in The Netherlands
89
SHALLOW SEISMIC REFLECTIONS WITH A PROPANE-.OXYGEN DETONATOR
SURENDRA SINGH
School of Physics, University of Science of Malaysia, Penang (Malaysia)
(Received 3 March 1983; accepted 5 September 1983)
ABSTRACT
Singh, S., 1984. Shallow seismic reflections with a propane-oxygen detonator.Geoexplo- ration, 22: 89-105.
In this paper, experimental results of shallow seismic reflections obtained with a down- hole propane--oxygen detonator as a seismic source are presented. The site selected for field experiments was in an area where granite bedrock is overlain by alluvial consolidated sediments, and in turn, by the low velocity layer near the surface. Typically the granite depths and the thickness of the low velocity layer are around 40 m and 3 m, respectively; velocity of the consolidated sediments is about 1700 m s-l and that of the low velocity layer is about 300 m s-l, as determined initially by the surface refraction method and later confirmed by a well-velocity survey. The seismic source is of variable energy, cheap, directional, generates high frequencies and very little surface waves. A single high frequen- cy geophone with near-zero offset distance from the source was used to acquire time rec- ords at different locations. The data illustrate the successful use of shallow reflections to delineate the irregular bedrock. Although evaluation of this seismic source is still continu- ing, there are preliminary indications that reflections from depths as shallow as 5 m may be discernible under favourable conditions, It is demonstrated that application of accu- rate static corrections is critical for shallow and very shallow reflection work. A method- ology, called ABO method here, for computing static corrections quickly and accurately, is discussed.
INTRODUCTION
A field study was undertaken to evaluate the effectiveness of reflection seismics in mapping irregular bedrocks at shallow depths with a propane- oxygen detonator as a seismic source. This work was part of the overall ef- fort to devise a suitable geophysical method in the search and mining of pla- cer-tin deposits in Malaysia. The investigation also included the feasibility of delineating subsurface intra-alluvial stratigraphy. Mapping of bedrocks for shallow depths has been traditionally done by drilling, however, as the near- surface resources get depleted, the emphasis is shifted to deeper target areas. In addition to being costly and time-consuming, drilling is also found to have a poor horizontal definition for placer deposits which concentrate in the
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“lows” of the pinnacle-and-trough topography of the limestone bedrock in Malaysia (Singh, 1983). This necessitates the use of geophysical methods. With geophysics methods, areas can be delineated where one would use high drilling patterns and other areas in which one might reduce or eliminate ex- ploratory drilling. A judicious use of a combination of geophysical methods and drilling will be the best strategy in these areas.
Seismic refraction methods have been used for the past several decades in areas of groundwater, civil engineering and mineral exploration. The seismic reflection methods, in spite of their definite advantages, have, however, only come to the fore recently. This is particularly true in the case of very shallow reflections; the main difficulties being the identification of reflections and lack of suitable sources. Energy sources tried so far include sledge hammer (see, for example, Meidav, 1969; Singh, 1983), small explosive charges (Pakiser and Warrick, 1956), and rifle sources (Steeples and Knapp, 1982). The surface waves encountered in the beginning of a seismic trace are the pri- mary cause which limits the shallowest reflection that can be identified. Most of the workers have reported using reflections for depths above 30 m or so. Only very recently, Steeples and Knapp (1982) have reported obtain- ing reflections from as shallow as 10 m or less. The advent of high frequency geophones and digital ~~rno~phs has given hope for finally developing a workable seismic method in the shallow target areas of 5 to 60 m.
The site of our field experiments is located on the campus of the Universi- ty of Science on Penang Island, Malaysia. The campus is a coastal part of the Island which is composed entirely of granite hills and coastal regions of allu- vial deposits. Depths of granite rock at the site range from 30 to 60 m, as evi- denced by numerous borings done for the foundation engineering on the campus. The original purpose of this experiment was to evaluate the reflec- tion method using the present source for placer-tin deposits in areas of lime- stone bedrock. However, the present site was chosen because depths and ve- locities of seismic waves at this place are similar to those in the limestone country. Further, because the seismic source was in a developmental stage, the proximity of the site to the facilities of a machine- and electronic-work- shop on the campus proved to be very useful.
The field equipment used, included a propane- oxygen detonator, here- after referred to as POD, a single-channel-signal-enhancement seismograph without AGC or timed gain, a chart recorder, 100-Hz vertical geophones and a portable auger drilling machine. While a multichannel enhancement unit with tape recording would have been preferable for a spatially high density data resulting in possible stratigraphic information after processing, the sin- gle-channel unit seems to be sufficient for delineating bedrock in the present study. The financial constraints have influenced our choice of the equipment and consequent field strategies. The POD has cost us only about U&$3,500. The total cost of all the equipment in this initial experiment amounted to a- bout US$15,000.
We will present here the workings and applicability of the POD for shal-
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low (30-60 m) and very shallow (shallower than 30 m) reflections. Seismic reflection data obtained with the POD over a granite bedrock will be dis- cussed as a case history. A fast and appropriate method of applying static corrections will be outlined. It will also be shown that, in addition to de- lineation of bedrock, very shallow reflections may be obtained which, with proper processing, can help map the intra-alluvial stratigraphy. Finally, the future directions of this work will be outlined.
PROPANE-OXYGENDETONATOR:POD
The cost, workability and applicability are critical factors to consider, when one chooses a particular source for use in shallow reflection work. It would be appropriate and worthwhile therefore to go into some details of the POD in this paper. In the author’s opinion, an investigator with some ma- chine- and electronic-workshop facilities available to him can get such a source made cheaply in almost any country.
The POD, visible in Fig. 1, consists mainly of two parts: down-hole thrus- tor unit on a 1.5-m pipe handle and a control box containing sparker’s elec- tronics and the strapped gas bottles of oxygen and propane. Though we have used it for energies around 1,000 J, it can produce energies up to 15,000 J simply by charging the combustion chamber with additional propane fuel and oxygen. With proper gas mixture for quick combustion and the bottom of the shothole well-tamped before firing, the seismic pulse generated by the POD contains frequencies of several hundred hertz. There is no detectable di- rect sound wave on records from the source. Unlike sledge hammer, weight drop or buried explosives, there is no direct coupling to the overburden soil end consequently only negligible surface waves are produced. The maximum energy is converted into P-wave going vertically downward.
The thrustor body, shown in Fig. 2 and the inset of Fig. 1, is made up of stainless steel, 6.5 cm in diameter and 63 cm in length. The upper conical part is screwed on to a pipe handle of 1.5 m length and the lower end has a screw-in body plug with three inlet holes for propane, oxygen and exhaust. Spark plug and timebreaker switch, along with sockets for the copper tubes to three inlet holes are all mounted on the upper surface of the body plug. There is a sliding sleeve of stainless steel with a screw-in sleeve plug at the bottom end. It is machined convex for better coupling to the ground. The sleeve has a total travel of 30 cm and is restrained from pulling off the body by two mating shoulders. The weight of the thrustor unit along with that of the pipe handle provides the reaction mass against recoil at the time of com- bustion. The combustion chamber is formed when the gases are fed through fine copper tubes of 2-mm internal diameter which pass through the pipe handle and body, and this pushes the closed end of the sleeve away from the body. The spark plug in the body plug detonates the gas mixture to generate the thrust impulse.
The control box, shown in the inset of Fig. 1, has a gas metering system
Fig. 1. Propane--oxygen detonator recoil-hold-down system. Insets show the control box and thrustor unit that goes inside the shothole (a meter ruler shown for comparison).
SLEEVE COLLAR
*1.7--G-- 46Crn --Ad---- 12.5cm.4*5Cml; cm
cm
Fig. 2. Schematic cross-sectional view of the thrustor unit,
and an electronic sparker circuit. Strapped to one edge is an oxygen refillable bottle with 1 cu. ft. (S.T.P.) of oxygen at 550 psi. A small regulator reduces the pressure to less than 30 psig. The propane comes from a standard prop ane-torch bottle. The propane gas pressure depends upon the ambient tem-
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perature and it can vary from 130 to 180 psig in daytime in Malaysia. Gases from bottles are admitted by two-way toggle valves into two separate mea- suring cylinders when valves are in “measure” position. The measured gases are then admitted through flexible rubber hoses and fine copper tubes to the combustion chamber by flipping the toggle switch to “fill” position. Mea- suring cylinders are presently designed in such a way that we need about 2% to 3 times “fill” of oxygen for one “fill” of propane for proper combustion. One bottle of liquid propane should suffice for several fillings of the oxygen bottle, each of which should itself suffice for several hundred shots.
The sparker circuit works on a 6-V lead-acid storage battery. It contains four main sections. The inverter converts 6-V D.C. to a lO-kHz non-sinus- oidal 500-V A.C. Rectified current from the rectifier section charges two lO- PF capacitors. When two capacitors are fully charged, current starts bypass- ing through a voltage limiter and the “Ready” light comes on. At this mo- ment, a momentary toggle switch triggers a trigger circuit and the stored charge is discharged through the spark coil to give out about 5 kV across the spark plug. The gas mixture in the combustion chamber is thus fired.
The thrustor unit with pipe handle is lowered into the shothole of about 1 m depth. Flexible rubber hoses for gases from the control box are con- nected to the copper tubes sticking out of the upper end of the pipe handle. High-tension cable and “ground” for spark plug are also connected to their respective leads on the handle. For gassing the unit, first keep the “exhaust” valve open. Read the pressure on the propane bottle and then regulate the pressure of oxygen accordingly. Purge the system with one or two propane “measures”. Close the “exhaust” valve. Give about three “measure-fill” cy- cles of both the toggle valves to pressurize the system. The thrustor body now starts to rise. About 1% to 2 times additional “measure-fill” cycles of oxygen alone are now required to make the mixture right for quick and com- plete combustion. The momentary toggle switch is now pushed towards “charging” position. As soon as the red light comes on, the toggle switch is let go and the unit fires. Quantities of gases can be increased for higher ener- gies, but always the propane-to-oxygen ratio must remain at about two-to- three with the present matering system. The unit is exhausted after ignition and the cross-handle, visible in Fig. 1, is pushed down to force out all the water vapour and exhaust gases. The “exhaust” valve is now closed and the unit is ready for re-use.
The recoil of POD at the time of firing can be prevented by clamping the pipe handle to a vehicle in the field. However, at small energies as in our case, a simple recoil-hold-down system was used. It is visible in Fig. 1. A wooden restraining rectangular frame is supported on a recoil-absorbing- spring frame which, in turn, is locked on to the pipe by a pipe-gripping clamp. Two persons, one on either side of the POD, stand on the wooden frame and hold it down with hands on both sides of the cross-handle. The recoil-hold-down system can be fixed anywhere along the pipe depending up- on the depth of the shothole.
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There were several problems with the equipment that we encountered ini- tially. The following miscellaneous comments may be of interest to those in- terested in using this source.
(a) Pressure-regulation of the oxygen bottle and the volume of measuring cylinders inside the control box have to be adjusted in such a way that there is just five times as much oxygen as propane by volume, at equal pressure, for correct chemical propo~ion~g and maximum detonating speed.
(b) Flexible gas hoses for propane and oxygen should be connected to the copper tubes through one-way valves, whereas “exhaust” valve is an on-off type. Carbon and water vapour can clog up the two-way toggle valves inside the control box if we do not use one-way valves to prevent this happening. Externally fitted one-way valves can be easily taken off, cleaned and put back on.
(c) Fine copper tubes, one-way valves and inlet holes in the body plug tend to clog up with carbon deposits after about 150 shots or so. To deear- bonize, we push a paint thinner from a nozzled can through copper tubes un- til it starts spurting out from the three inlet holes in the body plug. Leave it lying for 5 to 6 h in horizontal position, making sure that the thinner is not touching any rubber or plastic parts. The thinner can now be drained out, then blown out with compressed air. Finally, let it dry up for a few hours be- fore screwing the sleeve plug back on.
(d) Appreciable amounts of water can collect in the combustion chamber after about 30 shots. This can result in misfires due to current leakage across moist and carbonized electrodes of the spark plug. The sleeve plug should be opened and water cleaned out after a day’s work. Misfires can presumably occur more often if the sparker voltage is lower than 5 kV. This can happen, for example, if one uses a crank-telephone generator which can charge the capacitors to only 200 V. Sparking is more reliable with the sparker used by us because the rapid pulse-rise overcomes minor current leakage.
Comparison with sledge hammer
Though the signal-to-noise ratio, S/N, from a weak source such as a ham- mer (energies less than 300 J) can be improved by “vertical stacking” (i.e., stacking many identical shots), stacking with a weak source might never achieve a S/N obtained by using a stronger source. The energies from the POD can be varied from about 1,000 to 15,000 J by varying the amount of gases fed to it. This energy source can thus cater to a wide range of depths of investigations. As compared to a sledge hammer, frequencies of the signal from the POD are high. Dominant frequencies of the reflected signal were seen to be around 175 Hz for the POD, whereas they range around 80 Hz for the hammer in our experiments. Factors such as proper clamping, tamping of shothole and the proportioning of gas mixture influence the spectral nature of the signal. Due to these factors, POD does not reproduce the signal so re- liably as a hammer, and a stacking of records in the field may not be justifi- able.
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The reduction or suppression of surface waves is the first and foremost ob jective one has to accomplish in order to detect very shallow reflections with near-zero offsets of about 1 to 3 m. As an i~ust~tion~ surface waves from a hammer at the field site occupy the first 50 milliseconds (ms) and they are saturated in amplitudes. The velocity and thickness of the low velocity layer (LVL) are about 0.3 m ms-’ and 3 m, respectively; the velocity of the con- solidated sediments overlying the bedrock is about 1.7 m ms-‘. Reflected ar- rivals from less than 30 m will, therefore, be totally submerged in the surface waves. Thus bedrocks shallower than 30 m can nut be detected on account of the surface waves from the hammer’s impacts on the surface. Also, a ham- mer can not be used for bedrocks deeper than, say, 60 m due to low S/N from this weak source, in spite of enhancement with vertical stacking. Typi- cal examples of surface waves from hammer and POD are shown in Fig. 3, a and b, respectively. Experiments were also done to suppress surface waves by using a hydrophone in water-filled holes of about l-m depth -. . but with no success. A cylindrical aluminium leak-proof skirt was tried next so as to com- pletely isolate the hydrophone from the walls of the hole. In both cases,
a
/ VW
i L..!_-_ 1. .A-_L_i....i...i. . ..1.
Fig. 3. Typical examples of seismic traces from a 10thHz geophone at; a near-zero offset distance from: a. a sledge hammer; b. propane-oxygen detonator in a l-m shothole; and c. a hydrophone in a l-m water-filled hole, with a sledge hammer as energy source.
the signal was much worse than that from a geophone on the surface, as seen in Fig. 3c. Also high amplitude surface waves can not be suppressed by sim- ple horizontal stacking unless pre-stack muting or automatic gain control are applied to seismic traces. It was felt that the ideal way to reduce surface waves, under our conditions, would be to use a down-hole source which does not generate much surface waves.
Surface waves from down-hole POD with surface geophones of 100 Hz are very much reduced in duration and amplitude as ihustrated in Fig. 3b. If the POD is carefully clamped so as not to touch the walls of the hole during its recoil, then the early part of the seismic trace which appears to be surface waves dies off as early as 20 ms.
ABO METHOD FOR STATIC CORRECTIONS AND VELOCITY DETERMINATION
Besides special considerations such as closer intervals, use of single geo- phones, low-cut filters, smaller and high frequency sources, there is an im- portant place for static corrections in shallow high resolution reflection pro- filing. This is due to the fact that static corrections can, due to very low ve- locity of the LVL, be a sizable part of the total traveltime for shallow reflec- tions. This fact, in turn, makes it necessary that any routine for static cor- rections take into account the possible lateral variations of velocity in the LVL, irregularity of the bottom of the LVL and the topography of the ground. The conventional statics method (Telford et al., 1976) would not be suitable under these conditions. A better statics method, called the ABCD method, is suggested by Bahorich et al. (1982). The ABCD method, howev- er, assumes that the LVL’s velocity does not have appreciable lateral varia- tions and the bottom of the LVL is horizontal. Further, the method fails to distinguish the effect of the lateral changes in the LVL’s velocity from that caused by the changes in the velocity of the layer below the LVL or the ir- regularity in its boundary.
The method (Singh, 1983) used in this study is briefly described now and will be called, for the sake of convenience, the ABO method. It is totally based on first-arrival data. The field lay-out of the ABO method is depicted in Fig. 4. Let &A and tom be the first-arrival times for geophones at B and
A B 0
LVL 4 v, GRoUND - i
C --em___
DE Fv 2
Fig. 4. Source-receiver configuration for the ABO method of static corrections. The in- tercept time at B, ZB, for determining thickness of the low velocity layer, is equal to tom - (to* - ~BA), with tom being the first-arrival time at 0 from a source at B, etc.
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0, respectively, with the source at A. Similarly, toB is the first-arrival time for geophone at 0 and source at B. Then it can be shown that the intercept time at B, IB, is
IB BEBDm
=- + - - - = toB - (fO*-tg*)
Vl v, vz (1)
Ordinarily, VI is so much lower than V, that the travel paths in the LVL are close enough to vertical. Vertical and normal depths can be assumed to be the same since the critical angle for V,/V, greater than four is smaller than 14O from the vertical, The vertical depth at B, used for static corrections, can then be given as:
d= (
tOB - (tOA - tBA) 1
l VI/~ (2)
The ABO method of static corrections with eq. 2 is quick and convenient. The only inherent assumptions are that distances AB and BO along the line AI30 are larger than the critical distances and that the LVL’s bottom is suf- ficiently regular, not necessarily flat, for critical refractions to occur. It can be seen from eq. 1 that the effect of lateral variations of VI and V2 between the points A and B, and B and 0 are eliminated. S~il~ly, the effect of change in the ground elevations or the topography of the LVL’s bottom be- tween A and B, and B and 0 are eliminated. The intercept time, and then the depth of the LVL, is affected only by the conditions in the region BDE below the point B, a region which is quite small for a velocity ratio, V2/V1, greater than, say, four.
Several refraction profiles were run in the area to estimate the velocity structure. A typical time-distance plot of the first-arrival’s data and the sec- tion of a drillhole at the site are shown in Fig. 5a. The LVL overlies the con- solidated sediments which, in turn, overly the granite bedrock. Comp~son of the time~istance plot with the drillhole’s section indicates that the bottom of the LVL, at about 3m, corresponds to the water table at the site. The velocity of the LVL varies slightly in the area, ranging from about 245 to 294 m s-l, and three different values of this velocity were used in com- puting static corrections at different respective shothole locations. A veloci- ty of 1690 m s-l was estimated for the consolidated sediments. The granite bedrock, at a depth of 39.5 m in the drillhole, does not have a corresponding segment in the time-distance plot due to insufficient traverse length.
We ran some experiments on ‘shooting’ the d~llhole to accurately deter- mine velocities, and severai useful things were learnt from this effort, The top 12.2 m of the hole is encased with a PVC pipe of 114 mm outer dia- meter and a thickness of 5 mm; the uncased hole extends from 12.2 to 25 m. The part of the hole below 25 m down to the bedrock had already been caved-in and could not be used. A hydrophone was lowered down the hole and the first-arrival times recorded with a hammer at an offset of 2 m from the hole. To record the times above the water table at a depth of 3 m, water
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was poured into the hole and data taken quickly within a few minutes be- fore the water went down to its original level at 3 m depth. Values of the average velocity (V) and the interval velocity (Vi) from the well-velocity sur- vey are given in Fig. 5b. A velocity of about 1700 m s-l, below about 4 m depth, can be seen from the interval velocity curve. Average velocity below 3 m depth can also be computed from the vertical times. This value is equal to (25-3) m/(20.3-7.2)ms, which is 1680 m s-l, as compared to 1690 m s-’ estimated from the refraction survey. The abrupt increase in the interval ve- locity at a depth of 3 m corresponds to the water table. The PVC pipe does not seem to have any effect on velocities. It is considered, therefore, advis- able to always encase the hole to prevent cave-ins. There are many areas where it is extremely difficult to determine velocities from the refraction method due to a top layer of highly attenuating dry and loose sand. Well-ve- locity survey is probable the only way to reliably estimate velocities in such areas. Our experiments suggest that the well-velocity survey can be routinely
VELOCITY h’) AND INTERVAL VELOCITY h/i)(M/MSEC.) -_)
Fig. 5. a. A typical time-distance plot of first-arrivals and the section of a borehole at the field site. b. Results of the well-velocity survey.
used in tin-mining areas, with about the same equipment as utilized for col- lecting surface reflection data. Additionally, useful indications about the bedrock reflection times are available from the recordings of full traces at various depths down the hole. This information can prove quite useful in the interpretation of reflection data.
Having estimated the velocities from the refraction time-distance plots, static corrections can be easily computed with the ABO method. The LVL’s thicknesses ranged from 2.6 to 3.5 m, amounting to a two-way traveltime of 20.3 to 27.9 ms. To account for topograhic elevations and different thick- nesses of the LVL, static corrections were computed relative to a horizontal datum plane lying below the bottom of the LVL.
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DISCUSSION OF REFLECTION DATA
The major part of these field experiments was to investigate the workabil- ity of the POD in delineating irregular, shallow bedrocks and the possibility of detecting very shallow reflections. The seismic method is intended to be initially used for small-size areas such as those encountered while exploring and mining placer-tin ore, and thus should be reasonably inexpensive with a minimum of laboratory processing of field data. The reflector’s shallowness and the large velocity gradient of the earth near the surface make data pro- cessing a difficult job, particularly for non-zero offset geometry. For these and other similar considerations (Singh, 1983), near-zero offsets were used in generating the seismic sections. Shotpo~ts were spaced IO m apart along the seismic profile. A portable motorized auger drill was used to drill holes of 1 m depth for the down-hole source. One seismic trace at a time was obtained from a lOO-Hz geophone placed 0.5 to 1 m from the shothole. At times, more than one shot was needed to get a reliable signal by relocating the geo- phone around the shothole. The energy of the source is sufficient for a single shot to generate reflections from the bedrock and thus no enhancing was necessary as, for example, that needed with a hammer. Our primary interest being in mapping of the bedrock, the amplifier gain had to be adjusted to re- ceive visible reflections from the bedrock on the seismic trace. But, as a con- sequence, very shallow reflections in the beginning of the trace were saturat- ed in amplitudes.
The processing scheme used on the field data involved only static correc- tions. Surveys with shallow targets necessitate a careful application of static corrections to the field data. After surveying the area for elevations, we took first-arrival data at shotpoint locations using the ABO method and computed the LVL’s thicknesses below each location. Finally, the elevation corrections and the LVL corrections were computed with respect to a horizontal datum plane which lies 7 m below the shotpoint at the 90-m location in Fig. 6. The seismic section displayed in Fig. 6 is an example from the field data, with static corrections applied relative to the shotpoint at the 240-m location.
Reflection data in Fig. 6 consists of one trace from each location along the profile. These individual traces are aligned by placing them sequentially. The portion of the profile in Fig, 6 is 240 m long and the sweep time for ah records is 100 ms. The identification of bedrock reflections was facilitated by one-way vertical times and full-trace recordings at various depth levels from the well-velocity survey. The dashed line across the seismic section shows the two-way traveltimes to the bedrock relative to the datum plane. Depths to bedrock below the datum can be computed from one-way travel-
Fig. 6. A sample of a seismic section with traces for different shothole locations along the profile, Each trace is obtained with a single loo-Hz geophone and propane-oxygen de- tonator as a seismic source. The seismic section is corrected for statics relative to the shotpoint location at 240 m.
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.., ,,!., .‘ /
102
times multiplied by the velocity of 1690 m s-’ of the consolidated sediments overlying the bedrock. The depths of bedrock below the surface of the ground were determined by adding the respective heights of each location above the datum to the depth of bedrock below the datum. The correspond- ing depth section is displayed in Fig. ‘7. Depths range from 30 to 48 m. The drillhole, shown in Fig, 5, about 3 m from location at 90 m in Fig. 7, gave us a depth of about 39.5 m to the bedrock, compared to a depth of about 41 m at this place. Any diffractions caused by the ~e~la~ty of the bedrock are not clearly evident on the seismic section. This could be due to the fact that the POD is very directional and the data were collected with single geo- phones at near-zero offsets.
GROUND
I
LVL ’
DATUM
CONSOLIDATED
SEDIMENTS
Fig. 7.
0 20 40 60 80 100 I20 140 160 I80 200 220 240
DISTANCE IN METERS 3
Depth section computed from the seismic section with statics applied. Borehole depth to the granite bedrock at the about 90-m location is about 39.5 m.
One of the purposes of our field experiments was to investigate the nature of surface waves from POD and subsequently the fe~ibi~ty of obta~ing very shallow reflections which arise from the bottom of the LVL in our case. While the energy and gain need to be relatively high for obtaining clear re- flections from the bedrock, very shallow reflections can not be seen due to saturation of signals at the beginning of records as seen in Fig. 6. Muting of the early part of the seismic trace needs to be done to preserve all possible data. Low source-energy and low amplifier gain were used to study the re- cords in the range of 0 to 30 ms. Three examples in Fig. 8 illustrate the character of the seismic traces in this time-range, The first wavelet on the trace of Fig. 8a is the first-arrival which travels directly from the bottom of the shothole to the geophone on the surface. The second wavelet seems to be of a polarity reversed to the first wavelet. We might expect this reversal for reflections from the bottom of the LVL because a nearby geophone on surface should detect a rarefaction as the first part of the upgoing direct wavelet whereas the downgoing wavelet which gives rise to reflections from
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b
,I I’ -~-~-~-~-~~T~~Tr-~_~~~ ,L.“.“~,A4Lt\.-
0 4MS
TIME IN MILLISECOND (MS) -
Fig. 6. Examples of early part of the seismic trace from a lOO-Hz geophone and propane- oxygen detonator, obtained at low amplifier gain and low source energy. Second wave- lets with troughs at a) 20 ms, b) 16 ms, and c) 24 ms, appear to be reflections from the bottom of the low velocity layer.
the bottom of the LVL should have compression as its first part. Assuming that the second wavelets are LVL-reflections, their times of arrival in Fig. 8, a, b and c, are roughly 20, 16 and 24 ms which correspond to depths of 3, 2.4 and 3.6 m, respectively. These depths compare favorably with thick- nesses of the LVL here. On account of these factors, it seems reasonable to assume that these second wavelets are very shallow reflections from the bottom of the LVL and that the POD does not generate, if properly operated, any detectable surface waves. The latter arrivals after the primary reflections from the LVL’s bottom are most probably the multiples in the LVL, which can probably be suppressed with processing such as predictive deconvolu- tion (Steeples and Knapp, 1982). The very shallow reflections can then be detected with the POD provided that the suitable instruments are available. Unsaturated very shallow reflections can be obtained with a provision of an automatic gain control or timed gain in the seismograph. Alternatively, if the field data can be recorded on a magnetic tape, then two sets of modified data can be printed from the same one magnetic tape copy - one with very low amplifier gain for very shallow reflections and one with high amplifier gain for reflections from relatively deep bedrocks. Almost all of the modern
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12-channel-enhancement seismographs have one or more of these features. Static corrections can be determined directly from field records if very shallow reflections from the bottom of the LVL are discernible on the seismic sec- tions.
It has been demonstrated that the POD as a down-hole source does not appear to generate any detectable surface waves and we can thus obtain shal- low as well as very shallow reflections. A multichannel instrument with mag- netic tape recording along with suitable processing routines can considerably widen the capability to include the possibility of even mapping the strati- graphy in the alluvium above the bedrock. Prices of such 12-channel units, without any processing software, range currently from US $20,000 to US $50,000. While the use of the POD with a single-channel unit in our experi- ment has been mainly to demonstrate the system’s capabilities and workabil- ity in the field, this set-up is not meant to be used frequently for usual com- mercial surveys. It is slow and cumbersome, though cheap, for even a small- size survey such as, for example, a tin-mine area which is usually in the range of 400 m X 400 m. Processing and interpretation of field data acquired with a single-channel unit is time-consuming and laborious. In spite of its useful characteristics in our case, the POD is inherently slow due to its being a down-hole source. The only other appropriate source used for shallow and very shallow reflections has been a 0.22 caliber rifle. A rifle is a surface source and generates some surface waves, of energies depending upon the energy of the bullet. If the surface waves can be suppressed by multichannel processing such as horizontal stacking and by the use of small-energy bullets, then a rifle source is very convenient and the production rate is fast. This source has not as yet been tried by us due to very strict regulations on fire- arms in Malaysia. However, we are planning our experiments with a rifle in the future. The important part of the experiment will be to study surface waves generated by a rifle under soil conditions existing in Malaysia. A hori- zontal stacking procedure may not be advisable for very irregular bedrock’s topography. The best solution, therefore, would be to use an energy source which generates negligible surface-wave noise.
CONCLUDING REMARKS
Results of our field experiment with a propane--oxygen detonator as the seismic source demonstrate the capability of the source to delineate shallow and irregular bedrock between 30 m to 50 m; analyses of initial data indicate that very shallow reflections from reflective interfaces in the intra-alluvial stratigraphy from as shallow a reflector as 5 m can also be detected if proper muting of the early part of seismic traces is achieved with proper instrumen- tation and appropriate data processing. No discernible surface waves appear to be generated by this down-hole, directional, variable-energy, high-frequency source. Signals reflected from the bedrock can be seen to have dominant frequencies as high as 175 Hz on the field records. Two-way traveltimes in
10s
the low velocity layer of 3 m to 4 m thickness was found to be almost half of the total traveltimes of reflections from the bedrock. Accurate static cor- rections are, therefore, essential in any such shallow work where high velocity gradients exist near the surface. A convenient and accurate method, called the ABO method, for determining thicknesses of the low velocity layer under shotpoint locations is presented. It is suggested that well-velocity sur- vey be always run in the area. The ~fo~ation gathered can be highly use- ful in identifying bedrock reflections and determining depths accurately.
ACKNOWLEDGEMENTS
This work was supported by the University of Science of Malaysia. I am specially grateful to Dr. John C. Cook of the Teledyne Company, Dallas, for many advices and suggestions about the seismic source and other related equipment. Mr. J. van Winden, electronic engineer, UNDP, Bangkok, was of great help in the initial stages when the seismic source had several break- downs. I am also thankful to Dr. Chong Hon Yew of the School of Physics for assistance and advice on many occasions during the course of this work.
REFERENCES
Bahorich, M.S., Coruh, C., Robinson, ES. and Costain, J.K., X982. Static corrections on the southeastern Piedmont of the United States. Geophysics, 47 : lS40- 1649.
Pakiser, L. and Warrick, R., 1956. A preliminary evaluation of the shallow reflection seis- mograph. Geophysics, 21: 388-4OS.
Singh, S., 1983. A study of shallow reflection seismics for placer-tin-reserve evaluation and mining. Geoexploration, 21: 105-135.
Steeples, D.W. and Knapp, R.W., 1982. Reflections from 25 feet or less. Presented at the 1982 Meeting of the Society of Exploration Geophysicists, Dallas, Texas.
Telford, W.M., Geldart, L.P., Sheriff, R.E. and Keys, D.A., 1976. Applied Geophysim. Cambridge University Press, Cambridge, U.K.
