BUREAU OF MINERAL RESOURCES, GEOLOGY AND GEOPHYSICS

Size: px

Start display at page:

Download "BUREAU OF MINERAL RESOURCES, GEOLOGY AND GEOPHYSICS"

Clara Ferguson
5 years ago
Views:

1 COMMONWEALTH OF AUSTRALIA DEPARTMENT OF NATIONAL DEVELOPMENT BUREAU OF MINERAL RESOURCES, GEOLOGY AND GEOPHYSICS RECORD No ALBERT RIVER 54.5M SEISMIC REFRACTION SURVEY FOR UNDERGROUND WATER, QUEENSLAND 1959 by P.E..Mann The information contained in this report has been obtained by the Department of National Development, as part of the policy of the Commonwealth Government, to assist in the exploration and development of mineral resources. It may not be published in any form or used in a company prospectus or statement without the permission in writing of the Director, Bureau of Mineral Resources, Geology and Geophysics.

2 RECORD No ALBERT RIVER 54.5K SEIsmia 4FRAc1IOI1 SURVEY FOR untsrazad WATERS QUEIMLAND 1959 by P.E. Mann The information contained in this report has been obtained by the Department of National Development, as part of the policy of he Commonwealth Government, to assist in the exploration and development of mineral resources. It may not be published in any form or used in a company prospectus or statement without the permission in writing of the Director, Bureau of Mineral Resources, Geology and Geophysics.

3 CONTENTS Abstract Page 1. INTRODUCTION 1 2. GEOLOGY 1 3. METHODS 1 4. INTERPRETATION- 2 Seismic Velocities 2 Profiles 3 Comparison between seismic 4 depth determinations and depths from bore logs. 5. DISCUSSION AND CONCLUSIONS^ 4 6. REFERENCES^ 4 ILLUSTRATIONS Plate 1 : Locality Map. (G ) Plate 2 : Layout of seismic traverses. (G ) Plate 3 : Cross-sections on seismic traverses. (G ) Plate 4 : Bore Logs. (G )

4 ABSTRACT The Irrigation and Water Supply Commission is searching for underground water, at the 54.5M Albert River alluvial flats site, near Beaudesert, Queensland. At their request a seismic refraction survey was conducted by the Bureau of Mineral Resources to search for suspected subsurface river channels with aquifers. This Record gives a description of the survey and its results. It was possible to identify the water table by the seismic refraction method. The deepest places in the bedrock profile interpreted as possible locations of subsurface river channels were indicated at the following positions : between X26 and X30, X5, X7, between A7 and A8, and between C17 and 018.

5 INTRODUCTION To provide water for irrigation of farms bordering the Albert River, the Irrigation and Water Supply Commission sank five bores at the 54.5M Albert River alluvial flats site in an attempt to locate an ample supply of water. As the bores did not yield sufficient water it was suspected that an old river channel or channels might exist beneath the alluvial flats, and that a bore drilled to penetrate such a channel might provide sufficient water for irrigation. The Commission requested the Bureau of Mineral Resources to conduct a seismic survey to search for old channels beneath the flats and recommend locations for additional bores. The seismic refraction method was used on.the survey, which is described in this Record. The surveyed site is about 12 miles south of Beaudesert adjacent to the Beaudesert-Ladybl'ook Road (see Plate 1). The approximate coordinates of the site are on the Tweed Heads sheet of the Australian 4-mile map series. The geophysical party from the Bureau consisted of P. E. Mann (Party leader and geophysicist) and J. P. Pigott (Geophysical assistant). The Commission did the topographical surveying and provided four field assistants, additional transport, and survey party supplies. The field work was done between the 24th and 28th July GEOLOGY The site is situated in the mature valley of the Albert River, where the alluvial flat is nearly 3000 ft wide. The present channel of the river on the eastern edge of the alluvium is bordered by unstable banks up to 30 ft high. The river bed is gravel, sand, and silt. The deep channel that the river has cut in the present alluvial flat may indicate that the stream is in the process of degradation. The logs of bores, (see Plate 4) put down at different sites across the alluvium show silt ; clay,sand, and gravel overlying the formation interpreted as bedrock. The bedrock in the area consists of Walloon Coal Measures of Jurassic age, shales with coal measures, weathered sandstone, and probfbly weathered basalt. 3. METHODS i.nd EQUIPMENT, I, general description of the seismic refraction method and the teenique of the "method of differences" used on this survey is given by Polak and Mann (1959). The equipment used was a twelve-channel portable seismograph, maiaufactured by the Midwestern Geophysical Laboratory of Tulsa, U.S.A., arid designed for shallow reflection and refraction work. Midwestern geophones with a natural frequency of about 8 c/s were used to record the vertical motion of the ground. Small explosive charges wore detonated in line with, and at distances of 50 and 200 ft beyond. each end of, a spread of eleven goo- ; phones, spaced 50 ft apart.

6 ^ INTERPRETATION Old river channels beneath the alluvium, which were originally formed by erosion of the bedrock when the river was a degrading stream, may appear in a seismic refraction bedrock profile across the valley. An old channel probably would be filled with porous and permeable unconsolidated alluvial material. A porous rock may be considered as a rock frame containing voids. If the voids are filled with water, seismic energy is transmitted by both a "frame" wave and a water wave. For an unconsolidated rock of which the frame is formed by sand and gravel, the water wave velocity is about 4500 to 5500 ft/sec. In unconsolidated plastic rocks such as watersaturated silt, no separate water and frame waves exist, and the seismic energy is transmitted with lower velocities (2000 to 4000 ft/sec) and lower frequencies (Patterson, 1956). In a consolidated rock the frame wave velocity is greater than the water wave velocity and the latter cannot be easily distinuished. Hence, in unconsidated alluvial material mainly consisting of sand and gravel the level of the water table may be identified as the seismic refractor with a velocity within the range of 4000 to 5500 ft/sec. Seismic velocities The interpretation of the results is largely dependent on associating a seismic velocity with a rock type. In Table 1 the seismic velocities recorded have been interpreted in geological terms using the logs of bores (Plate 4) drilled at the site, and general experience accumulated from other surveys. TABLE 1 Seismic Velocity^ Rock TWO ft/sec ^ Unconsolidated dry soil, silt, and sand. 3000^-^Water-maturated, unconsolidated silty clay ^-^Water-saturated unconsolidated deposits of sand, silt, and clay ^ Water-saturated silt, sand, and gravel. 6000^ Partly cemented silt, sand, and gravel, or weathered sandstone Partly weathered sandstone (Walloon Coal Measures). 10,000 t^ Weathered or porous basalt. 16,000-13,400^ Coal shale (Walloon Coal Measures) and possibly volcanic rocks in the higher velocity range. Porosities of water-saturated sediments with seismic velocity 6000 ft/sec are about 40 per cent (Pickett, 1960).

7 3. Seismic profiles Plate 2 shews the layout of the seismic traverses. The seismic refraction work has been interpreted to give the depths to formations with different seismic velocities, and the results are shown in the cross-sections on Plate 3. Formations with seismic velocities between 3000 and 5500 ft/sec are interpreted as unconsolidated water-saturated sediments. The level of the water table marked on the cross-section by a broken line has been interpolated between stations where the depth to formations with this velocity was recorded on normal spreads. The lower values of seismic velocity (less than 4500 ft/sec) are characteristic of water-saturated silt and sand with clay, and the higher values (4500 to 6000 ft/sec) are characteristic of water-saturated silt, sand, and gravel. This interpretation of seismic velocity in terms of rock type is consistent with the geological description from the bore logs. In this report the term "bedrock" is defined as the refractor with the highest seismic velocity recorded (i.e. within the range 10,000 to 13,400 ft/sec), and is interpreted by using the rock type descriptions obtained from the bore logs. On Traverse A the depth to "bedrock" ranges between 25 and 57 ft. At A2 the time-distance curve of the normal spread between A2 and Al2 indicates that there may exist a formation of seismic velscity from 3000 to 4000 ft/sec with a maximum thickness of about 10 ft. If this layer is present, the water table would be about 16 ft below the surface level, and this would be consistent with the depths calculated at ether points in the spread. On Traverse B the water table averages about 18 ft below ground level. The higher seismic velocity (5000 to 6000 ft/sec) of the formation overlying bedrock indicates that it consists of porous sand, silt, and gravel. The depth to bedrock is between 35 and 55 ft, being en the average about 10 ft shallower between B2 and B12 than between B12 and B22. On Traverse C the water table is about 19 ft below ground level. Between 02 and 012 the seismic velocity of the formation overlying bedrock is between 5000 and 6000 ft/sec, but at C22 seismic velocities of 3000 and 6000 ft/sec have been recorded. In seismic refraction shooting a layer must have thickness in excess mf a certain minimum to be detected as a first event on a selmogram (Leet, 1950). The formation -with velocity 3000 ft/sec near 022 may be too thin to 1., e detected at 012, and it has been plotted en the cross-section as pinching out. Refractions from a layer with velocity 13,400 ft/sec were recorded only at 017 to 019. The depth to the highestvelocity refractor based on the adopted velocity distribution at 012 is 128 ft. If the 3000-ft/sec layer is present under 017 to 019 the depth to the 13,400-ft/sec layer coull be as low as 103 ft; i.e. 25 per cent shallower. The depth to bedrock on Traverse X ranges from about 6 ft under the present river channel to 65 ft at X26. On the right and left banks the level of the bedrock relative to the river channel area is lower in two areas, namely between X2 and X9, and between X17 and X38.

8 4. Comparison between seismic depth deterpinatione and depth q from bore loan The locations of the five bores in the area are shown in plan and crocs-section on Plates 2 and 3. The bore logs are shown on Plate 4. Table 2 shows a comparison of the depth to discontinuities as determined by boring and by seismic methods. TLBLE 2 Bore Number Location referred to Traverse X Depth to discontinuity(ft) By Boring By Seismic methods Geological interpretation of discontinuity 1 28 Water table Grey basalt 2 X Lpproximately Water table Brown coal 3 X33 - X Water table Basalt 4 Offset. 27 ft down Water table stream from Decomposed X sandstone 5 X Water table Coal shale. L comparison of the seismic and drilling data shows that the depth to "bedrock" determined by drilling may be in error by about * 20 per cent of the depth to the highest-velocity refractor determined by the seismic method. In comparing the depth to the water table determined by the seismic method with the depth shown by drilling, some allowance must be made for the seasonal fluctuation of the water table. The water table shown on Plate 4 was determined several months before the seismic survey, when the water table level may have been different. DISCUSSION LUD CONCLUSIONS In this survey the water table could be determined by the seismic refraction method because the ground water is present in porous unconsolidated alluvial material characterised by seismic velocities greater than

9 ft/sec. Within the 3000 to 6000-ft/sec velocity range, refractors with seismic velocities of 4000 to 5000 ft/sec represent water-saturated unconsolidated silt, sand, and gravel and hence aquifers which are primary drilling targets. Refractors with seismic velocities of 3000 to 4500 ft/sec usually represent sandy, silty clay and hence aquifers which are not considered favourable targets for drilling. Old subsurface water courses or river channels, probably marked by the deepest places on a bedrock profile, are considered the most likely places for aquifers to be found. Hence the deepest places in the bedrock profile are considered good drilling targets in the search for underground water supplies. On the bedrock profile of Traverse X the deepest areas in bedrock and the best locations for drilling are between X26 and X30, and between X5 and X7. It is noteworthy that the bores put down before the geophysical survey lie just outside the area between X26 and X30. On the other traverses the locations of other favourable drilling targets are between L7 and L8 on Traverse L I and at 017 or 018 on Traverse C. 6. REFERENCES LEET, L. D., PliTTERSON, N PICIMTT,.G. R., POLLK, E. J. and P. E., 1950^ELRTH WLVES. John Wiley & Sons Inc., New YOrk, 1956^Seismic waves propagation in porous granular media. 222Rhysics, ^The usc of acoustic logs in the evaluation of sandstone reservoirs. Geophysics 25, ^L seismic refraction survey at the Moogerah dam site, near Kalbar, Queensland, Bur. Min. Resour. Lust. ROC, 1959,-62

10 4 4 ^" 0 i 9- ' PLATE I Ne.rmanten BEAUDESERT CZ) (:) CZ n.. ca OCir-s^ 0 0 E; Townsvill QUEENSLAND^Mackey. Winton Loaroach.^Rockhampho 5 undabery Q dly its Chmleyill BRISBANE Brisbane IPSWICH WARWICK REFERENCE TO AUSTRALIAN 4 MILE MILITARY MAP SERIES TWEED pal, SEISMIC REFRACTION SURVEY AT THE 54.5 M ALBERT R. ALLUVIAL FLATS,NEAR BEAUDESERT, LD 1^T.. LOCALITY MAP SCALE IN MILES 0 2 AFTER I.W.S.C. PLAN N0 L KERRY^ca_ Ui (.0 z --. Z.ze 54.5 M SIT CHRISTMAS^'Ps CREEK r 41 :1 1' LADYBROOK HILLVIEW. HILLVIEW1 LAMINGTON TOF CLIFFS i197',,sis MT. BISHOP 1688'.nex, _ '5r 4..^1;4 ' 9to ^. //, 1 3/, " LliTLE^.S ;. ^,w// WIDGEE MTN. 2495' 1,^,, Geophysics/ Branch, Bureau of Minero/ Resources,6eology and Geophysics. G I TO ACCOMPANY RECORD No

13 N 0 4^ NO 3^ NO2 No 5 No I TOP TOP SOIL TOP SOIL TOP SOIL TOP SOIL AND WW STONES 19, (/000) (woof S.W.L 23' GRAVEL AND (6000) S.W.L. 23' 0, T0F/ BROWN AND GREY SILTY CLAY S.W.L. 23' WOW (5000 Y CLAY S.W.L. 26' S.W.L. 28' Y CLAYEY SAND SEISMIC REFRACTION SURVEY AT THE 54-5 M. ALBERT R. ALLUVIAL FLATS,NEAR BEAUDESERT,CiLD. CORRELATION OF SEISMIC DATA AND GEOLOGY FROM BORE LOGS AND WW STONES VERTICAL SCALE IN FEET BOULDERS AND SILT (5000) 43^ (8500) GRITTY WHITE BROWN SHALE DECOMPOSED SANDSTONE ( N5o.9) BOULDERS GREY BASALT SAND* SHALE ( i4 e b) AND WW STONES COAL SHALE AND ASH BROWN COAL (Aqoa9/ AND WW STONES SAND GRAVEL 40.(5000/ (/0,000) 55' Y CLAY WITH STONES GREY BASALT LEGEND. BOUNDARY OF FORMATION (5000) DETERMINED BY SEISMIC (5500) METHODS. DEPTH AND SEISMIC VELOCITIES SHOWN S.W.L STANDING WATER LEVEL COAL SHALE 60' W.W WATER WORN

DONALDSON DAM SITE, PIEMAN RIVER SEISMIC REFRACTION SURVEY,

COMMONWEALTH OF AUSTRALIA DEPARTMENT OF NATIONAL DEVELOPMENT BUREAU OF MINERAL RESOURCES, GEOLOGY AND GEOPHYSICS RECORD No.- 1963/98 DONALDSON DAM SITE, PIEMAN RIVER SEISMIC REFRACTION SURVEY, TASMANIA