Jil. 10, No.2 (Vol. 10, No.2) Mac-Apr 1984 KANDUNGAN (CONTENTS) CATATAN GEOLOGI (GEOLOGICAL NOTES) Surendra Singh and G.A. van Klinken: An experimental high-resolution seismic survey over tin-mine ponds 45 PERTEMUAN PERSATUAN (MEETINGS OF THE SOCIETY) 1. V. Watson: A deeply eroded collision orogen: the Caledonides of Britain 57 GEOSEA V-Laporan (Report) 58 Bengkel Penulisan/ Penterje mahan Buku Teks Asas Geosains Pengajian Tinggi 76 BERITA PERSATUAN (NEWS OF THE SOCIETy) GEOSEA V Field Trips-Reports 77 Keahlian (Membership) 82 Keahlian Profesional (Professional Membership) 83 Pertukaran Alamat (Change of Address) 83 Pertambahan Baru Perpustakaan (New Library Additions) 83 BERIT A-BERITA LAIN (OTHER NEWS) Unpublished Final Year Project Reports, University of Malaya, 1983/84 84 Short Course on Quaternary Geology of Malaysia-Update 85 The Royal Society Fellowships for Scientists from Developing Countries 86 Symposium on Cenozoic Basins of Thailand: Geology and Resources 87 AEG Decennial Convention and International Seminar on Exploration Geophysics 88 Applications of Geology and the National Development 88 Kursus-kursus Latihan dan Bengkel-bengkel (Training Courses and Workshops) 90 Kalendar (Calendar) 93 PERSATUAN GEOLOGI MALAYSIA (GEOLOGICAL SOCIETY OF MALAYSIA). Majlis (Council) 1984/85 Pegawai-pegawai (Officers)' Presiden Leong Khee Meng, Petronas-Carigali, (President) P.O. Box 12407, Kuala Lumpur Naib Presiden John Kuna Raj, Jabatan Geologi, (Vice-President) Universiti Malaya, Kuala Lumpur Setiausaha Kehormat Mohamad Ali Hasan, Jabatan Geologi, (Honorary Secretary) Universiti Malaya, Kuala Lumpur Penolong Setiausaha Kehormat Koh Tuck Wai, Petronas-Corigali, (Honorary Assistant Secretary) P.O. Box 12407, Kuala Lumpur Bendahari Gan Ah Sai, Geological Survey (Honorary Treasurer) Malaysia, Ipoh, Perak Pengarang Teh Guan Hoe, Jabatan Geologi, (Editor) Universiti Malaya, Kuala Lumpur Presiden Yang Dahulu Khoo Teng Tiong, Jabatan Geologi~ (Immediate Past President) Universiti Malaya, Kuala Lumpur Ahli-ahli Majlis, 1984-86 Abdul Hamid Mohamad, Jabatan (CounCillors, 1984-86) Geologi, Universiti Kebangsaan Malaysia, Bangi, Selangor Michael Leong, Petronas, P.O. Box 12444, Kuala Lumpur S. Paramanathan, Dept. of Soil Science, Universiti Pertanian Malaysia, Serdang, Selangor Yin Ee Heng, Geological Survey Malaysia, Kuala Lumpur Ahli-ahli Majlis, 1984-85 Andrew Spykerman, Malaysia Mining (Councillors, 1984-85) Corp., P.O. Box 10936, Kuala Lumpur Choo Mun Keong, Malaysia Mining Corp., P.O. Box 10936, Kuala Lumpur Syed Sheikh Almashoor, Jabatan Geologi, Universiti Kebangsaan Malaysia, Bangi, Selangor Yeap Ee' Beng, Jabatan Geologi, Universiti Malaya, Kuala Lumpur Juruodit Kehormat Peter Chew (Honorary Auditor) ****** PubUshed by the Geological Society of Malaysia~ Department of Geology~ university of MaZaya~ Kuala Lumpur 22-11 (Tel. 03-5??036) - 30 June~ 1984. Printed by Art Printing Works 8¢n. Bhd.~ 29 Jalan Riong~ Kuala Lumpur. 45 CAT A TAN G E 0 LOG I ( G E 0 LOG I CAL NOT E S ) AN EXPERIMENTAL HIGH-RESOLUTION SEISMIC SURVEY OVER TIN- MINE PONDS SURENDRA Singh and G.A. van KLINKEN, School of Physics, Universiti Sa ins Malaysia, Penang, Malaysia. Abstract A study was carried out to investigate the usefulness and workability of employing high-resolution reflection seismics in tin­ mine ponds of the Kinta ValleY3 Peninsular Malaysia. Results indicate that strong reflections can be obtained and a field procedure can be worked out for detailed mapping of the irregular topography of the limestone bedrock. It has not been possible to identify the sedimentary lithology and structure3 with the present equipment. Appropriate updating of the acquisition system3 especially the seismic source3 is recommended. The routine use of a combination of seismic survey and drilling will result in cost- and time-savings and also enhance greatly the horizontal definition in the overall program of tin-ore reserve evaluation and mining. Introduction Some parts of the tin fields of the Kinta Valley, Peninsular Malaysia, which is one of the richest areas of placer-tin deposits, are covered by fresh-water ponds. These ponds are old tin mines which had been dredged out for alluvial placer tin and later abandoned. They have a fill of unconsolidated sand with gravel and clay. These mine ponds are scattered allover the tin fields in the country. Such old, dredged-out mines are still rich in tin ore, mostly embedded in crevasses and troughs of the limestone bedrock, and also in alluvial sand which was difficult to dredge out. The ponds can be mined in future for the remaining tin ore, using gravel-pump mining. A review by Rajah (1979) describes the geology of the Kinta Valley in detail. For the purpose of our study, the cross-section in Fig. 1, based on drillhole data, typifies the geological situation in mine ponds of this area. The limestone bedrock has a characteristic trough-and-pinnacle topography, with steep slopes. The bedrock is overlain by old alluvial sand and unconsolidated fill of sand. Estimating the tin-ore reserves and planning a gravel-pump mine to exploit the remaining ore, requires a reliable map of the bedrock's topography and the thicknesses of the overlying alluvium. Presently, this information is obtained by drilling to the bedrock. Depths to bedrock vary from 0 to almost 80 m. Though it depends upon the depth, a drillhole can cost, on the average, about M$900, all direct and indirect expenses included. In addition to being expensive and time­ consuming, the horizontal definition of drilling is poor. Drillholes ISSN 0126-5539 Warta Geologi3 vol. 103 no. 23 Mar-Apr 1984 46 POSITION OF DRILL HOLES ~ LO LI L2 L3 L4 L5 La L7 L8 L8 LIO LII LI2. LI3 t 0 tI) - -- - - - -- - a: 10 - - - - - -- - -- I&J - - -- - t- - - - ALLUVIUM - - I&J 20 - - - - - - - ~ Z 30 ~ .t- 0.. I&J 0 10 0 100 200 300 400 500 100 700 100 100 1000 DISTANCE IN METERS ~ Fig. 1. Cross-section of a mine pond; based on drillhole data. ( 8 - ELEMENTS) 300 Hz _ 5 I< Hz HYDROPHONE ~ CHART ....., FILTER)+ RECORDER ) 400 Hz - 51<.Hz POWER ) ... 1 SOURCE TRIGGERED 230V POWER 11115 V ~ ~ CAPACITOR ~I I SUPPLY BANI< GENERATORS I --- ---~ J 7 HP T 500 Hz- 81< Hz SYNCHRONISED TRIGGER PULSE < 300 JOULES Fig. 2. Block diagram showing the components of the seismic data acquisition system. 47 in this area are usually put 80 m apart, in a grid pattern. It is not uncommon for the limestone bedrock here to find two pinnacles spaced only 10 m apart. Noting that placer tin is found in the alluvium as well as troughs of the bedrock, the poor horizontal definition with drilling can result in underestimation as well as overestimation of ore reserves; an overestimation can prove economically tragic to a miner. A detailed knowledge about the bedrock's topography is similarly useful in planning a mine operation. In mining operations, for example, the gravel pump should be ideally located at the lowest level of the bottom of minehole and as near to the processing plant as possible. The size of the processing plant and the machinery used depends on the amount of the ore reserve in a mine area. The thickness of the overburden and the depth of the mine hole are also important factors. A relatively thick overburden, which is to be thrown away, and a shallow mine hole will increase the cost of production. A detailed depth determination, therefore, is highly desirable for tin-ore reserve evaluation and mining. Considering the economics, time and poor horizontal definition of using only drilling, the significance of using a suitable geophysical method along with drilling is obvious. An experimental study was carried out to investigate the feasibility of using a high-resolution reflection seismic survey for a detailed delineation of the bedrock's topography, and also to check the possibility of identifying the lithology and structure of the unconsolidated sediments overlying the bedrock. With a detailed knowledge of the topography, reserves can be assessed by drilling at potential locations such as at troughs located at Ll, L4, L7, Lll, etc. in Fig. 1. The seismic instruments used for the survey are shown by a block diagram in Fig. 2. The data-acquisition system employed is a typical analogue seismic EG & G unit used in conventional high­ resolution offshore marine seismic surveys. The seismic source consists of an electromechanical transducer which cODverts a high voltage (about 3500 volts) electrical pulse from the power-supply-capacitor-bank unit into an acoustic~pressure pulse, with a booming sound. The energy of the source is below 300 Joules and is quite sufficient for shallow depths encountered in the Kinta Valley. All the component parts of the system operate in the 300 Hz - 5 kHz range, making it a high­ resolution survey. Time differences of up to 1 millisecond can be easily read off the seismic record. This implies that vertical depths can be determined within 0.85 m, using an upper velocity value of 1700 m/s for the alluvium velocity in this area. Wavelengths of the reflected signal would be less than 3.5 m for a signal frequency of 500 Hz. The radius of the central quarter-wavelength Fresnel zone (Sheriff, 1978, p. 125) will then be 9.4 m for a depth of 50 m and 6.7 m for a depth of 25 m. Theoretically, then, the energy at the hydrophone comes from a portion of the reflecting surface of 5 m - 10 m radius, for depths encountered in the Kinta Valley. General suryey considerations The purpose of the survey, size of a pond, boat size and its speed, source type, and other similar variables make a survey over a mine pond somewhat different from the usual marine seismic surveys, as far as their problems and solutions are concerned. We discuss here the features of the seismic survey over a fresh-water mine pond. Pond boundaries are mostly irregular, making it difficult to organize the survey lines.