A SEISMIC REFRACTION STUDY OF THE CRUST AND UPPER MANTLE IN THE VICINITY OF BASS STRAIT* By R. UNDERWOODt [ManWlcript received July 7, 1969] Summary A reconnaissance seismic refraction study of the crust and upper mantle of Bass Strait and adjacent land was undertaken in 1966 under the sponsorship of the Geophysics Group of the Australian Institute of Physics. The shot locations and times, the station locations, distances, and first arrival travel times are presented. Analysis of these data is described; they indicate a P n velocity below 8 km sec-I. Time terms are less than expected and do not agree with previous work. Crustal thicknesses cannot be computed until studies of upper crustal structure are made. These, and several mantle refraction studies, are suggested for future work. INTRODUCTION Although direct knowledge of the solid earth extends only a few kilometres below the surface, seismic and other geophysical methods lead us to suppose that the top 30 or 40 km of continents are approximately horizontally layered with P wave velocities up to 6·5 or 7 km sec-i. Below this is the Mohorovicic discontinuity (M) where the velocity jumps to about 8 km sec-i. Under oceans, M is only about 10 km below the sea bed. To investigate the crust and upper mantle in south-eastern Australia, a seismic refraction experiment, the Bass Strait Upper Mantle Project (BUMP), was sponsored by the Geophysics Group of the Australian Institute of Physics. In designing the experiment, we chose to cover a wide region rather than concentrate on a detailed study. Further more detailed work could then be tied to the framework of our observations. BUMP was linked to previous work (Doyle, Everingham, and Hogan 1959; Doyle, Underwood, and Polak 1966) by recording in the Snowy Mountains and south to the coast. Shots were fired in eastern Bass Strait, with further observations in Tasmania. A second line pivoting on Tasmanian stations with shots off King Island was also observed in western Victoria. This paper present£! the data gathered from the experiment. The interpretation is provisional on further work. * Prepared under the auspices of the Bass Strait Upper Mantle Project (BUMP) Committee of the Geophysics Group of the Australian Institute of Physics: C. Kerr Grant (convenor), R. Green, L. V. Hawkins, B. M. Hopkins, W. D. Parkinson, K. Richards, J. Spinks, R. Underwood, J. P. Webb; and co.opted participants J. R. Cleary, C. Dampney, D. Dyson, B. D. Johnson, D. J. Sutton, and Officers of the Bureau of Mineral Resources. This report was prepared by R. Underwood, in consultation with C. Kerr Grant and other members of the Committee. t Department of Geophysics and Geochemistry, Australian National University, Canberra, A.C.T. 2600. AWlt. J. Phys., 1969,22, 573-87 574 R. UNDERWOOD 34°1 I I 1 ."".!'o.1"-, ODLN --.n \ iNV "\,...... ~ NElw SOU T H W ALE S .,;::!"~ ,.." I:f,(.~ V leT OIR I A I I IOIWT o~~' iii -2 .d 1j v BAS S .- 40° L__ -l ____ 4t~--_t~S~T~RfA---IT--r_--_,~~~----~--~ 6_ "'~ ONAI OONO OMIR svl'O MSlO DAD ONll o OCVD. 42° oacw OQAE Yt! Temporary Itn. o 0aAW oalE MTV TRR o~ crYAIDaCE 0 Fixed leismic Itn. a. • Shot point TAUAb\ Scale I ~~ o 100 km 1 . 144° 146° 148° 150° Longitude E. Fig. I.-Locations of shots and stations. BASS STRAIT UPPER MANTLE PROJECT 575 THE EXPERIMENT AND ITS RESULTS Nine I-ton bundles of obsolescent depth charges were fired on the sea bed at the locations plotted in Figure 1 and listed in Table 1. These sites were chosen for uniform coverage of two lines, but only part of the western line was shot. Locations by Hifix, horizontal sextant, and radar should be accurate to ±200 m (max), except shot lO which may be up to 500 m in error. Errors in the origin times have been estimated; they are based on VNG radio time signals interpolated by marine chronometer. TABLE 1 COMPUTED SHOT POSITIONS AND TIMES Geographic Geographic Height above Location Shot Latitude S. Lougitude E. Mean Sea Level Fixiug Date Time No. Error Remarkst (m) Method' h m (msec) 0 36 26 33·0 148 37 33·4 960 Ground 5.ii.66 17 15 00·3 100 survey 1 38 08 43 148 01 09 -52 Hifix 6.ii.66 18 14 55 '25 16 (1) 2 38 32 58 147 54 14 -65 Hiflx 7.ii.66 11 14 58·690 11 3 38 59 46 147 49 37 -55 Hiflx 7.ii.66 18 13 57'980 6 (2) 4 39 26 04 147 43 54 -50 H.S.A. 8.ii.66 06 14 58 ·275 6 (3) 5 39 52 33 147 37 21 -49 H.S.A. 8.ii.66 12 14 58 '057 11 6 40 23 46 147 31 35 -47 H.S.A. 8.ii.66 18 14 58·532 6 7 40 48 32 147 27 00 -37 Gyro and 9. ii. 66 06 14 58·331 6 radar 8 40 18 53 144 50 58 -37 H.S.A. 17.ii.66 09 14 56'432 11 (4) 9 (5) 10 39 43 50 144 18 20 -46 H.S.A. 17.ii.66 20 15 00·932 9 , H.S.A., horizontal sextant angles. t Remarks are: (1) Large error as VNG time signals hard to pick up above Hifix interference. (2) Shot fired 1 min early. (3) Double break instead of single break iu tone indicating shot instant. (4) Precise time doubtful as ship recorded 110 msec pulses as against nominal 100 msec. (5) Shot 9 omitted as location too exposed to rough seas and also impossible to position accurately. A quarry blast at Jindabyne was also used. The location is accurate, and the shot time is known to within 0·1 sec. Fixed and portable seismographs recorded signals from the shots. Station details are given in Table 2; most locations were taken from available maps and some will not be accurate to better than a few hundred metres. Travel times of seismic signals are listed in Table 3 (upper values), each value being the first discernible arrival on the record. The accuracy of the observations depends on the signal to noise ratio at onset, on the clarity of radio time signals and count-down signals, and on instrument response and paper speed; and the quality of the value estimated by the observer is indicated in Table 3 by the number of decimal digits. Distances are also given in Table 3 (lower values). They have been computed by a geodetic programme and are as accurate as the site coordinates allow. DISCUSSION OF RESULTS Table 3 lists only about 60% of the possible entries. Data are missing where some stations did not attempt recording of some shots or where instruments failed. Some blanks occur because I-ton underwater shots seldom generate large enough " 576 R. UNDERWOOD TABLE 2 BUMP STATION POSITIONS A.N.U., ~ustralian National University; B.M.R., Bureau of Mineral Resources; Vic. C.R.B., Victorian Country Roads Board Geographic Geographic Elevation Operating Name Code Latitude S. Longitude E. 0 (m) Authority Permanent Stations Werombi WER 33 57 01 150 34 49 226 A.N.U. Dalton DLN 34 43 22 149 10 54 550 A.N.U. Inveralochy INV 34 57 54 149 40 01 640 A.N.U. Canberra CAN 35 19 15 148 59 55 700 A.N.U. Cabramurra CAB 35 55 36 148 26 35 1610 A.N.U. Wambrook WAM 36 11 34 148 53 00 1290 A.N.U. Jindabyne JIN 36 26 22 148 35 34 960 A.N.U. Toolangi TOO 37 34 17 145 29 26 604 B.M.R. Melbourne MEL 37 49 53 144 58 24 28 B.M.R. Mt. Tassie MTV 38 24 06 146 34 00 740 A.N.U. Savannah SAV 41 43 17·6 147 11 22·4 180 Univ. Tasmania Tarraleah TRR, 42 18 16·3 146 27 03·5 579 Univ. Tasmania Tas. Univ. TAU 42 54 35·7 147 19 13·5 132 Univ. Tasmania Temporary Stations of Eastern Line Pinch River PIN 36 47 33·2 148 24 37 250 N.S.W. Mines Dept Emu Plains, Black Mt. SIX 36 56 30 148 13 03 1360 B.M.R. Black Mt. Stn BMT 37 01 06 148 15 54 900 B.M.R. Wulgulmerang WUL 37 05 40 148 15 20 940 Univ. Adelaide I Gelantipy GEL 37 10 40 148 15 55 800 Univ. Adelaide Butcher's Ridge BCR 37 14 42 148 15 14 740 Univ. Adelaide W. Tree CDW 37 20 11 148 13 49·5 530 Comm. Dept Works I South Buchan NSW 37 33 53·5 148 07 24·5 184 Univ. N.S.W. NowaNowa NOW 37 43 59 148 05 31 8 Univ. Sydney Lakes Entrance CRE 37 51 46 148 00 40 60 Vic. C.R.B. I, j Flinders Is. FLI 39 53 45 147 47 50 30 Univ. Melbourne Nabowla NAB 41 10 33·5 147 22 22·7 105 B.M.R. Mt. Barrow MBR 41 21 09·2 147 22 38·4 460 B.M.R. I Musselboro MSL 41 27 29 147 26 52·5 460 Univ. Tasmania lI j Nile NIL 41 39 13 147 21 28 190 B.M.R. I Cleveland CVD 41 47 51·5 147 25 40·5 210 ! B.M.R. i Goldsmith GLD 42 00 13 147 19 33 400 Univ.