BUREAU OF MINERAL RESOURCES, GEOLOGY AND GEOPHYSICS

Transcription

1 COMMONWEALTH OF AUSTRALA O(l{<- Lb.'.j fct"jj- F.JA. '+ G- {.. DEPARTMEN'T OF NATONAL DEVELOPMENT t q " BUREAU OF MNERAL RESOURCES GEOLOGY AND GEOPHYSCS \ RECORDS 958 No. 86 '. '\ SESMOLOGCAL RESULTS OBTANED FROM THE: SNOWY MOUNTANS EXPLOSONS OF DECEMBER 956 and FEBRUARY 957 by.b. Everngham ' f': The nformaton contaned n ths report has been obtaned by the Department of Natonal Development as part of the polcy of the Commonwealth Government to assst n the exploraton and development of mneral resources. t may not be publshed n any form or used n a company prospectus or statement wthout the permsson n wrtng of the Drector Bureau 0 Mneral Resources Ge'ology and GeC?physcs.

2 RECORDS 958 No. 86 SESMOLOGCAL RESULTS OBTANED FROM THE SNOWY MOUNTANS EXPLOSONS OF DECEMBER 956 and FEBRUARY 957 by.b. Everngham The nformaton contaned n ths report has been obtaned by the Department of Natonal Development as part of the polcy of the Commonwealth Government to assst n the exploraton and development' of mneral resources. t may not be publshed n any form or used n a company prospectus or statement wthout the permsson n wrtng of the Drector Bureau of Mneral Resources Geology and Geophyscs.

3 CONTENTS Page ABSTRACT (). NTRODUCTON 2. RESULTS 3. NTERPRETATON OF RESULTS f. FUTURE PROGRAMME: 2 Table. Exploson tmes and postons. Table 2. Stat6n postons. Table 3. Sesmc phases and travel tmes. LL U S T RAT ON Fg.. Provsonal Travel Tme curves for South-Eastern Vctora.

4 ABSTRACT. Provsonal nterpretaton of results from three sesmologcal statons set up bp the Bureau of Mneral Resources to record the large explosons n the Snowy Mountans area n December 956 and February 957 ndcate~ - (a) (b) (c) the exstence of a grantc layer wth a P wave velocty Of 5.9 km/sec. and an S wave velocty of km/sec. that the velocty of P waves below the Mohorovcc dscontnuty s 8.0 km/sec. and the depth to the Mohorovcc dscontnuty s at least 35 km. ()

5 . NTRODUCTON On 23rd December 956 and 0th February 957 about 70 tons of explosves were detonated south of Adamnaby New South Wales by the Snowy Mountans Hydro-electrc Authorty. Recordngs of sesmc waves generated by the explosons were obtaned from sesmographs operated by the Bureau of Mneral Resources at Melbourne Observatory the Warragamba dam ste n New South Wales and at a ste about to mles south-east of Whtfeld Vctora. Ths report ncludes a summary of thq data obtaned and subsequent prelmnary nterpretaton of the data. Mountanous count~j wth peaks up to 6000 feet surrounds the exploson ponts and extends to Warragamba and Whtfeld and to wthn 30 mles of Melbourne. 2. RESULTS Detals of the exploson tmes and postons are gven n Table and the staton postons n Table 2. Sesmc phases travel tmes and the epcentral dstance of each staton are shown n Table 3. A Benoff sesmograph was n operaton at the Melbourne Observatory and records of the two detonatons were obtaned by the short-perod sesmographs. The ampltude of the phases was not large but the magnfcaton could not be ncreased beyond about (at cs) because of vbratons due to traffc. The tmng accuracy of the frst recordng was consderably less than that for the second as tme checks were not made durng the weekend at the tme of the frst. Wllmore sesmographs were used at the statons at Warragamba and near Whtfeld. Magnfcaton of ths nstrument s about at 0. cs. 3.NTEBPRETATON OF RESULTS nterpretaton of the records s only tentatve as t s not possble to draw accurate tme/dstance charts from such lmted data. Fg. s a plot of all (mpetus or sharp) phases and of e (emerso) phases recorded on more than one component. The results of the two Melbourne records A and B (see Table 3) were combned. There was a dscrepancy of 0.3 seconds betvteen the tmes of the Pn phase on these records; the major part of the dscrepancy s attrbuted to record A as the tme mark accuracy on record B was known to be approxmately 0. sec. The Melbourne phases n Fg. are plotted at the mean dstance for the two explosons. The travel tmes are weghted means of the two travel tmes and an allowance s made for the small dfferences n dstance. When a phase was recorded for only one exploson the travel tme plotted ncorporates a small correcton for dstance. An examnaton of the very lmted data suggests that: (a) (b) The P g velocty s 5.9 lan/sec The Pn velocty s 8.0 lan/sec (c) The Sg velocty s 3.55 km/sec (d) The depth to the Mohorovc~c dscontnuty s probably about 35 km. The dscontn~ty may be deeper but s unlkely to be very muc? shallower. These nterpretatons may need modfcaton n future - partcularly the veloctes assumed for the grantc layer n whch wave veloctes are probably hgher at greater depths. /. 2.

6 -2-4. FUTURE PROGRAMME Other recordngs were made from these explosons by offcers of the Snowy Mountans Hydro-electrc Authorty and the Australan Natonal Unversty. These nclude recordngs at locatons near Benambra (epcentral dstance about 00 km) and Moss Vale (200 km). Results of these have not yet been publshed. These could be expected to record the Pg and Pn phases respectvely as frst arrvals. f any explosons are planned n the future t s recommended that the Vlllmore sesmograph should be set up at a pont approxmately half-way between Melbourne and Whtfeld say near Marysvlle at an epcentral dstance of about 300 km. Ths should pck up the Pn phase as a frst arrval and provde data for more accurate estmaton of the Pn velocty n the south-west drecton. f a prospectng-type sesmograph s avalable t should be set up near Mtta Mtta or Benambra n order to record a reflecton from the Mohorovcc dscontnuty_ Ths reflecton should be of large ampltude at such a dstance. Alternatve statons 0 mles east of Seymour or at Yarra Glen should also provde nterestng data and would be more convenent operatonally.

7 .Co: TABLE - EXPLOSON TMES AhD POSTONS EXPLOSON ~ LATTUDE LONGTUDE TME (E.S.T.) : A " 48 37' 43" 23rd Dec. 956 j B 36 07' 36" th Feb _ ~ TABLE 2 - STATON POSTONS \ STATON LATTUDE LONGTUDE RECORDED EXPLOSON Melbourne Observatory 37 49' ' 24" A and Near Whtfeld: ' B ~ 33 53' 30" Warragamba 20" B B j lable 3 -~SMC PHASES AND TRAVEL TMES STATON - Melbourne Observatory EPCENTRAL DSTANCE km. RECORD.~ Phase Travel Tme C N - S E Vertcal - W 55. e Pg 64.2 ; e 65.0

8 TABE 3 (cont.) - SESMC PHASES AND TRAVEL TME~ Phase Vertcal e 7.0 e 84.0 e Sn Sg 05.0 e 06.0 e e 6.0 STATON - Melbourne Observatory EtlCENTRAL DSTANCE lad RECORD B ~ Travel Tme N S E V " ; Phase Vertcal ~ Travel Tme ; N - S E - W Pn e Pg 64.4 e 03.3 e Sg 04.3 e 07.8 e e TABLE 3 (CONT.) - SESMC PHASES AND TRAVEL TMES STATON - Near Whtfeld - Vctora EPCENTRAL DSTANCE km ~ase TRAVEL TME ~ Vertcal Transverse Longtudnal Horzontal : Horzontal e Pn (pg) e 34.9 e e 5.2 REMARKS ;

9 TABE 3 (cont.) - SESMC PHASES AND TRAVEL TMES Phase TRAVEL TME REMARKS Vertcal Transverse Long tudnal Horzontal Horzontal e Tmng Sg accuracy appro xt rnately + _ 0.2 seconds. STATOl - Warragamba - New South Wales EPCENTRAL DSTANCE kn. - PHASE Travel Tme REMARKS VERmCAL COMPONENT e Pn Tmng 5.8 accuracy e 82.7 : 0.3 Sg 85.

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