~,.

COMMONWEALTH OF AUSTRALIA

DEPARTMENT OF NATIONAL DEVELOPMENT

BUREAU OF MINERAL RESOURCES,

GEOLOGY AND GEOPHYSICS

'RECORDS 1956,

GEOPHYSICAL SURVEY OF THE

DEE TUNNE'L, LAKE ECHO

POWER DEVELOPMENT SCHEME, TAS:MANIA

by

W. A. WIEBENGA, D. F. DYSON £! L. V. HAWKINS

.,' "

~ .

...

COMMONWEALTH OF AUSTRALIA

DEPARTMENT OF NATIONAL. DEVELOPMENT

BUREAU OF MINERAL RESOURCES,

GEOLOGY AND GEOPHYSICS

RECORDS 19 56, N°~' l::Ll.-----:::1 , BUREAU OF UINERAL RESOURCE~:

, GEOf'!.Jy~'f' f: I l\fRl, qy ~~ , '~n \.:.\\J:,l_ U' It '

, Ref ....... ······ ·· ................. "" . GEOPHYSICAL SURVEY ·OF~TAE-~· ~ #--·~~ ·

DEE TUNNEL, LAKE ECHO

POWER DEVELOPMENT SCHEME" . 'TASMANIA

by

W. A. WI£B£NGA, D. F. DYSON c.e L. V. HAWKINS

....

"

....

CONTENTS

Page

ABSTRACT (iii)

1. INTRODUCTION 1

2. GEOLOGY 1

3. METHODS 2 n General 2 b Resistivity .2 c Seismic refraction 2 d Magnetic 3

4. RESULTS 3 n General 3 b Resistivity 4 c . Seismic refraction 5 d Magnetic 5

5. DRILLING RESULTS 6

6. ACCURACY 6

7 . CONCLUSIONS 6

8. RZFznENCES 6

ILLUSTRATIONS

Plate 1. Fig. 1. Locality mal'. Fig. 2.

Fig. 3.

2. Fig. 1.

Fig. 2.

Fig. 3.

Geophysical traverses and position of tunnel line and drill holes.

Section along tunnel line,

Topographic and seismic profiles (Traverse A).

Magnetic and resistivity profiles (Traverse A).

Expanding electrode profiles -graphical interpretation.

3. Magnetic and apparent resistivity profiles (Traverse B).

4. Contours of vertical magn·::tic intensity.

(ii)

'"

..

ABSTRAC.T

The geophysical exploration of the Dee Tunnel,

which is part of the Lake Echo power development scheme,

was made in September and October, 1954, at the request

of the Tasmanian Hydro-Electric Commission.

Electrical resistivity, seismic refraction and

magnetic methods were used, and results from the three

methods were in fairly good agreement with each other.

Zones of probable shearing and/or weathering at tunnel

level are indicated.

The magnetic survey disclosed a high, sharp

anomaly for which no definite explanation is apparent.

( iii)

..

I., INTRODUCTION

The Dee Tunnel is part of the Tasmanian Hydro­Electric Commission's Lake Echo power development scheme. It is ap~)roximately two miles in len~th and will connect Dee Lagoon and Brady's Lake (Plate 1).

A geophysical survey was made at the request of the Commission (after tunnelling operations had already commenced) , to investigate possible variations in rock type and to locate zones of shearing or weathering which might extend down to tunnel level and thereby cause difficult tunnelling conditions.

(i)

(ii)

(iii)

The geophysical methods used were:-

Resistivity.

Seismic refraction.

Magnetic.

Resistivity and magnetic methods v~eI'e used along the ~unnel line (traverse A), which runs approximately Nl12 E, and alonG a parallel line 500 feet to the north-east (traverse B). The seismic refraction survey was limited to the major part of traverse A.

Diamond drilling has been done by the Commission on traverse A ncar the two portals and also south of traverse A (see Plate 1). -

The field work was carried out in September and October, 1954. The geophysical party consisted of D.F. Dyson, party leader, L.V. Hawkins, geophysicist, and six field assistants provided by the Coramission.

It is desired to acknowledge the ready co-operation of the ~ersonnel of the Resident Engineer's offices at Bronte Park ,and Wayatinah, and the Invostigations Branch of the Co~nission at Hobart.

2. GEOLOGY

. The tunnel connec~ing the Dee Lagoon (inlet) and Brady's Le.ke (outlet ~, is be ing driven through a flat-topped hill. Marshy conditions exist between stations 4498A and 7198A, i.e. along about one-quarter of the lenGth of Traverse A.

The following rock types are present:

( a) Dolerite

This is amedium-grained to fine-grained crystalline , rock which crops out on the hill near the flanks. The, hill slopes arc covered with dolerite scrce, and boulders fre- ' quently occur within the soil of the marsh area. Evidence from drill coros and tunnelling shows that jointing is a comrtlon feature: of the: dolerite.

Thu dolerite is intrusive into sandstone. Drill holes DH7002, 7003, 7004, 7005 and 7006 (Plate 1) show that along tho tunnel line! dolel~i te may be expectod at tunnel level •

""~

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- 2 -

(b) Sandstone

Tho sandstone is · considered to be of Triassic age and is "fJrosont at both inlc.t and outlet Dortals and also in Drill Hole 7014. -

A high :=mglo s o.ncls-cono-dolc:i:'i te contact occurs within thd tunn(;)l at the Brady's L3,ke end. In Drill Hole 7014 ncar tho ~)or-Gal at Dec; Lagoon, sandstono occurs above tho dolerite.

3. METHODS

(a) General

The exploration of tho area through which the proposed tunnel is to bo drivon was :;Jrimarily conc..:rned with:

(i) The location of any shoared and/or w0utherod zones extending down to tunnel level.

(ii) Tho rock types through which the tunnel is to be driven.

(b) Resistivity

Tho resistivity method is frequently used in dete~· m~n~ng shear zones, formation boundaries and horizontal dis­continuities at depth,

In this survoy th~ 'Tionne:r configuration was used for all resistivity work. E10ctrodo spacings of 100 feot and 200 f00t wore used for continuous )rofiling, readings being talwn OV8rJT 50 feet and 100 feot rcspoctivcly. The dopth penetration of tho measured resistivities is approximately equal to the olectrode s ~)aGing in the: YfGnner configuration,

"Depth probing", using expanding electrode spacings, was dono in three places for comparison with seismic results.

A Megger Earth Tostor 9 which yields speedy and reliable rosults, was used for continuous profiling.· A more accurate resistivity moter, designed and built by the Buroau of Mineral RGsourcos 9 was usod for the expanding electrode method, .

(c) Seismic refraction

The "method of differences" was used in this survey. For a general d(;)scription of tho method and its application, see Heiland (1946, p. 548) and :Gdge; and Laby (1931, p.339).

A goophono interval of 50 feet was used for normal geophone spreads, giving a spread coverage of 500 feot. Shot distances of 50 feet and 200 fout from both ends of the spre;ad were usod. "Weathorin '? s1)rc:ads", with U'eol)hones at o .. 0 J,;

intervals of 10 fout, wera shot ovory 1,000 feot to give the velocity of tho weathl;red layors and the thickness of the soil cover.

Tho seismic refraction method enables a quantitative estimate to b\) made of tho dGPth to a high velocity, modium which corr(!sponds to umvcatlLrod rock. It also gives the . volocity of a seismic wave in thu rock through which it travols 9 thoroby 0nabling an \;stimato to bo mado of tho rock type and th~ dograe of waath~ ring. Ovur broad shoar zones

.-

- 3 - .

in unvwathcred dol,;ri te for cJ:arnple, the soismic velocity is lowor than normo.lly observed oV.Jr · that rock.

The seismic surv~y was made along Trav~rse A as a chock on the interpretation of the resistivity rosults. A Contur.y l2-channel portable refraction unit, model 506, was used.

(d) Magnetic

This survey consistod of measuring the change in the vertical component of the earth's magnetic field from point to point.

For a doscription of the m0-choc~ and its application the roador is referred to any of tho stcmdard geophysical text books (Jakosky, 1940, pp 111-131).

Variations in magnetic intensity aro caused largely by difforences in tho magnetite content of thu rocks. Such changcs may be due to:-

(i) Change of rock type, e.g. at a sandstone/dolorite contact.

( ii)

(iii)

Differontial woath~ring~ . Magnotite and most other magnotic iilino mls lose tho ir magnetic lJrOperties when weathered, and variations in the; thickness of weath,:;ring ovcr a magnotic body lJroduce changes · in the magnotic intensity.

Conccmtl"ations of magnotic minJrals as in some ore bodiGS and mineralised shoars.

(iv) Primary difference in magnetite content.

Readings 'Nore taken at 50-foot intervals on traverses A and Bandon tho dot~ilod traverses shown on Plate 4.

The instrument used was a Watts vertical force variomoter with a sensitivity of 42 gammas per scale division.

4. RZSULTS

Good agreement exists behveen the results of the three methods along the tunnel line (Traverse A) (See Plate 2). Zones of shearinG and deep weathering on the seismic profile co]:'respond to minima on the apparent resistivity and magnetic jrofiles. The exccytion is a :?rominent magnetiC maximum over ·,Jurt of travel"se A (See Plates 2 and 4). The cause of this me.gn ; tic L.1axiIIium is not reflected in either the seismic or resistivity results.

Places where sheared or weathered rock is thought to extGnd to the tuni1el level are shown on Plate 2, Fig. 1.

The }ror::;(~nCQ 0 :': 8~!,nc1::d; ono 'rY2,S not indica·l;odalong tho tun1l81 lin;) c;:l; -elK! Durfacc. Tho drilling r0}sul ts indicD:tc that dolGri·l;o ox-t; ,mds to ·GW11!. ~;1 l ovel (8 ,,".:0 "Goo loGY"). The mn.;S'notic ''i.nc1 r.:.siotivi-;:;~T ;Jrofilus alon; traverse B over tho "" ! nu.'C' J·on{'/'~ol""··~l·J- ··:, C·)-1

J · .., C"- '."110·'''''·'c' ,:, ""lar~) cOll+rnst 'D"tHln ·"n ~} t,;, ." IJ LJ v \.....l _ 4 V .L lJ I..;; 1,. J._ lJ ... ,.., l.J '_' •• . \ \,., • \...., to..::) 1. : v c.. 1....;,;, _ \..0

·""1'·' "" .. "·n·-, ···l .. c l' n+ -'n81· t,·- "nG." r"'"'l' c·t ·L· Vl' ·L.'T 0 ·1.-::' J-}l· ' J·"IO .£:'or~Iii1":lJ-;ons II l. v "!1e-',s ~ l; u ~ • .1 u. v ,.>.J. l".;. l" \, l" . 1. . c. l" ....

Uio ·::: Plato 3).

,-.....

...

- 4 -(b) Resistivity

(i) Continuous profiling

The resistivity of a rock depends mainly on its porosity, the degree of saturation and the salinity of the solutions. Loose dry gravel or sail and fresh crystalline roclm have a high re sist i vi ty. Moist, weathered zones have a low resistivity.

The marsh shows up as an area of low resistivity. This is due to the ground being saturated with water and the probable presence of weak organic acids. High resistivity values occur over the dolerite out-crops near the flanks of the hil19 where fr~sh rock is shallower and drainage is good. These conditions control the general pattern of the resistivity profile.

Sheared and/or weathered zones are indicated by marked minima on the resistivity profile, supqrimposed on the general pattern.

Traverse A'(see Plate 2, F'ig.2)

The sharp rise in the profiles near station l0398A indicates a steeply dipping transition from weathered dolerite on the Dee portal side to fresh dolerite. This corresponds to the conditions found in the tunnel 9 where a transition to fresh jointed dolerite caused partial flooding of the tunnel.

The drop in the profiles near station 8498A is not as sharp. It represents deeper general weathering and indicates a probable transition into morc weathered rock at tunnel level.

Marked minima occur near stations 2298A, 3l98Al 3498A-3798A, 4098A, 4498A 9 5398A, 7l98A (edge of march), 759bA, 7898A, 8698A and l0498A (See Plate 2, Fig. 2).

The profile of uniaeathered dolerite obtained from the seismic survey shows that, in general, a thickening of weathered dolerite corresponds to a resistivity minimum. Corresponding magnetic minima could also confirm deeper weathering.

Traverse B (See Plate 3)

On tho resistivity profile of traverse B, the contact between scondstonc and dolerite is shown by the abrupt change in rosistivity values near 698B. The.;position of the contact is confirmed by an abrupt change in the magnetic profile.

Botween 698B and 2698B the high resistivity values indicate that unweathered dolerite is very close to the surface.

In the central portion of the traverse the marsh pro­duces relatively low resistivity values and tends to mask tho variations of resistivity within the dolerite.

The fairly broad minimum between 9098B and l0898B and several. sharper EJ.inima elsewhere along the traverse indicate deeply weathered shear zones. It is not possible to establish a satisfactory Gorrelation botwoen the resistivity minima on tho two traversos A and B, or to determine the strike of the shear zones from tho resistivity profiles •

..

oio. .

- 5 -(ii) Depth probing (Plato 2, Fig. 3)

Depth determinations by the expanding electrode techni~ue were made at stations 5698A, 6398A, and 7748A, giving depths to the hit;;h resistivity layer of 54 feet, 14 feet and 33 fect respectively. The corr~spondin~ values found by the seismic method were 55 feet, 33 feet and 45 feet respectively.

Depth determinations obtainod by the resistivity method are not as accurate as those obtained by tho seismic method and should be considored as qualitative only.

(c) Seismic refraction

The seismic survey extended along traverse A from station 2898A to station 8848A (Plate 2, Fig. 1).

The observed seismic velocitios in the weathered dolerite rang0 from 3,000 ft/sec. toward Dee Lagoon portal and 4,500 ft/soc. · over the marsh to 7,500 ft/soc. towards Brady's Lake portal. The 7,500 ft/sec. velocity probably represents jointed dolerite, extensively weatherod along the joints.

The unweathered dolerite has a velocity of 17,000 to 18,000 ft/soc. A velocity of 15,900 ft/sec. ncar station 5798A may ropresent a shear. It is near the centre of a large, positive magnetic anomaly (see below).

Indicat~ons of woathered zones from the seismic and resistivity results are in good agreement.

(d) Magnetic

The magnetic survey was made along traverses A and B and also along some detailed traverses shown on Plate 4.

Minima on the magnetic profiles may indicate deeper weathering, and in the present survey such indications are in agreement with those obtaintJd by tho other methods..

A prominent, positive magnetic anomaly with a maximum value of morc than 4000 gamrllas oxists betY'lCC!n stations 4998A and 6498A (see Plate 2, Fig. 2 and the:; contour map on Plate 4). The anomaly takes tho form of a double-poakcrd curve with the sharper gradiont and highest values on the north-western side. The strike is approximately N25 0E. The centre of the anomaly corresponds approximately with a lower seismic velocity of the unweathered rock of 15~900 ft/soc., and with two resistivity minima at stations 539~A and 56g8A.

The anomaly may be due to a concentration'of magnotic minerals in a body striking approximately betweon N250 E and north and dipping fairly steeply to the east.

The steepness of the magnetic gradients indicates that the cause of the anomaly is probably clos8 to the surface. The above-mentioned lower seismic velocity indicates a broad zone of shoarin~. The two close reSistivity minima and the double-peaked magnetic profile may indicate two main shears within tho gneral shear zone. On the basis of this interpre­tation it was predicted that difficult tunnelling conditions could bo encountered between stations 5198A and 5898A.

Tho position of zones of shearing and/or weathering deduced from tho above rosults is shown on Fig. 1 of Plate 2.

Aftar tho tunnel had entered the zone indicated by tho magnetiC anomaly it was noted that the dolerite was of a different type. This change was very sudden along a plano

-

•

....

- 6 -

. with strike as IDuntioned above and dipping approximately 500

cast.

A hand specimen from the tunnel below tho area indicated by the magnetic anomaly showed coarse pyroxenes and fine-grained dark minerals which may bo magnotite and pyrrhotite. A crude test with a hand magnet showed that the specimen was extremely magnetic, whereas the adjoining dolerite (which ~xhibits froe quartz) showed no magnetic reaction in a similartost.

5. DRIL:;~ING R2SULTS

The r0sults obtained from diamond drilling on a line offset to the south of traverse A (soo Plato 1) ~d prior'~ the geophysical survey yielded insufficient information to make roliablo lJredictions concerning tho naturn of the rock.

Tho results of the drilling appeared to indicate a fairly steady increase in depth to unweathered dolerite from Brady's Lake to Dee Lagoon portals. This is not in agreemont with the geophysical results.

6. ACCURACY

The accuracy of tho quantitative results, such as tho depth to unwG8.th;.;rcd rock, is r.::lativcly unimportant, as the proposed tunnel is considerably deeper than tho unweathorod rock profile.

The accuracy of the seismic depth determinations is about ± 15 per cent.

The error in magnetic readings is estimated as less than 10 gammas. This is inSignificant when comparod with tho magnitude of the anomalios observod.

Poor olectrode/0arth contacts in dry rocky ground tend to .give erratic values of a~parent rosistivity. This will have affected only the values obtained over tho outcroyping dolerite near tIl(; flanks of the hill. The results arc quali ta­tivc and errors involved ara small comuared with the lateral vui~i~. -

7. CONCLUSIONS ·

The goophysical results indicate that the unweathered rock along th0 tunnel line (oxcopt for the portals) consists of dolerite and not of sandstone.

In general, the bottom of tho weathered zone does not extend to tunnol level but some shear zones (Fig. 1, Plate 2), with attendant deep weathering, may do so.

8. nI:FERENCES

Edge, A.B. and Laby,T.H., 1931

Hoiland, C.A., 1946 -

Jakosky, J.J., 1940 -

THZ PRINCIPL::S i~D PRACTICE OF G::OPHYSICAL 2ROSPECTING, Cambridge UniverSity Press. GEOPHYSICAL EXPLORATION, Prentice Hall, Now York. ZXPLORATION GEOPHYSICS, Times-Mirror Press, Los Arigoles, Calif •

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LEGEND

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PLATE 2

FIG.3 ' EX PANDING ELECTRODE PROFILE S,

WENNER CONFIGURATION GRAPHICAL INTERPfHTAT ION.

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GEOPHYSICAL SURVEY OF DEE TUNNEL, LAKE ECHO POWER DEVELOPMENT SCHEME,

TASMAN IA.

400

I

TRAVERSE A STATION NUMBERS ARE INDICATED AS DISTANCES IN FEET

FROM SURVEY PEG'OO' NEAR BRADY'S LAKE PORTAL.

o

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PLATE 3

•

\ LEGEND

I . Iii I < 0 !-1400 ------~I~.~~~~+_--------------+_+_----~--~----4_-----+--------------------------------------------~--------------~r_-+------------------~--~~----------~{~-----+--+---------~----~--------------------------~--------~~--------------·~~----o

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400 o 1-

MAGNETIC PROFILE

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RESISTIVITY' PROFILE.

20rJ'SPACiNG. Jl'ifN!J[R CONFIGuRATION

RESISTIVITY PROFILE.

HORIZONTAL SCALE IN fEET

400 800 1m . __

VeRTICAL SCALES AS SHOWN

1200 IbOO ---- ---~

GEOPHYSICAL SURVEY OF DEE TUNNEL,

" I I I , I

II I I

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LAKE ECHO POWER DEVELOPMENT SCHEME. TASMANIA. 400

I I I ' , •• I !

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o

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MAGNETIC AND APPARENT RESISTIVITY PROFILE S,

(TRAVERSE B) G 162-6

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PLATE 4

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GEOPHYSICAL SURVEY OF DEE TUNNEL. LAKE ECHO POWER DEVELOPMENT SCHEME, TASMAN IA

CONTOURS OF VERTICAL MAGNETIC INTENSITY

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