055G46

RECORD 1977/12

PIJBMIt PUBLICATIONS COMPACTUS----- (LENDING SECTION)

DEPARTMENT OFNATIONAL RESOURCES

BUREAU OF MINERAL RESOURCES,GEOLOGY AND GEOPHYSICS

SEISMIC RETRACTION SURVEY IN THE AREA OF

BELCONNEN TOWN CENTRE, ACT, 1975

D.C. RAMSAY:

The information contained in this report has been obtained by the Department of National Resourcesac part of the policy of the Australian Government to assist in the exploration and development of

eral resources. It may not be published in any form or used in a company prospectus or statementlout the permission in writing of the Director. Bureau of Mineral Resources, Geology and Geophysics.

BMRRecord977/12

c.3

RECORD 1977/12

SEISMIC- REFRACTION SURVEY IN THE AREA OF

BELCONNEN TOWN CENTRE, ACT, 1975

by

D.C. RAMSAY

CONTENTS

SUMMARY

Page

1. INTRODUCTION

2. GEOLOGY 13. METHOD AND EQUIPMENT 14. RESULTS 2

5. CONCLUSIONS 46. ^REFERENCES 4

ILLUSTRATIONS

PLATE 1. Locality map and location of traverses

2. Seismic cross-sections of traverses 6 and 7

3. Seismic cross-sections of traverses 8 and 9

4. Seismic cross-sections of CCAE gossan, traverses A and B

SIMIMARY

The Bureau of Mineral Resources, Geology & Geophysics has carried

out a seismic refraction survey in an area adjacent to Belconnen town centre,

A.C.T. The work was requested by the National Capital Development Commission

to assist in foundation planning for proposed building development. The

results indicate the presence of several faults or shear zones where further

detailed investigation would be advisable before any large buildings are

sited.

1. INTRODUCTION .

This Record presents the results of a seismic refraction survey

' on either side of Lake Ginninderra, north of Belconnen town centre. (Fl. 1

Results of detailed geological and geophysical investigations in Belconnen-

town centre were previously reported by Briscoe & Ramsay (1975), and this

Record should be read in conjunction with the earlier one for a more complete

understanding of the results. The areas covered by the survey have been

designated for high-density residential and service industry development and

as such will involve the construction of medium-rise buildings. The aim of

the survey Was to furnish the building planners with information on the

- subsurface geology, as the area is known to be crossed by several faults

associated with and including the Deakin Fault. The field-work was carried

out by personnel from the Engineering Geophysics Group of the Bureau of

Mineral Resources (BMR) in June 1975.Also included in this Record are the results of a seismic investig-

ation of a gossan outcrop adjacent to the Canberra College of Advanced

Education (CCAE). (pl. 1). This work wasCarried out as a demonstration of

field technique for students of applied geophysics at the college.

2. GEOLOGY

To the west of Lake Ginninderra, which is also west of the Deakin

Fault, the most abundant rock type is a coarse-grained acid igneous rock

of the Upper Silurian Deakin Volcanics; there is also some minor sedimentary

rock of the Yarralumla Formation, which is of similar age. The area on the

eastern side of the lake is underlain by a very coarse-grained acid

intrusive rock, the Silurian to Devonian Glebe Farm Adamellite. More complete

information on the geology of the survey area is presented in the Record by

Briscoe & Ramsay (1975).

3. METHOD AND EQUIPMENT

An account of the seismic refraction method, the method of

interpretation, and the equipment used was given in the previous Record by

Briscoe & Ramsay, and will not be repeated here. Geophone spacings of 4 mwere used throughout this survey and long shots were fired at 60 or 70 m

beyond either end of each spread.

2.

RESULTS

Locations of traverses are shown in Plate 1 and interpreted

cross-sections based on the seismic recordings are given in Plates 2 to 4.

Traverse 6 (pl. 2) displays a normal weathering pattern with

fresh rock at a depth of 8 to 18 m. This is overlain by moderately

weathered rock having a seismic velocity of generally 2000 to 2800 m/s

though decreasing to 1550 m/s at the western end of the traverse. The top

of this layer is 1 to 2 m below the surface, except in a section about the

middle of the traverse, where a further layer having a seismic velocity of

1100 to 1300 m/s was detected. This lower-velocity layer, probably

representing highly weathered rock, may form a thin layer under the soil

cover right across the traverse although it was not detected in the records.

The soil and possible highly weathered rock down to a maximum depth of about

6 m should be readily removable if necessary, while the deeper layer with

seismic velocity in the range 2000 to 2800 m/s should provide adequate

foundation for the type of building envisaged in this area. A single

seismic spread recorded previously (Dolan, 1971) intersects this traverse at

its eastern end. The two interpretations agree in velocity of and depth to

bedrock, although the velocities of the intermediate layer differ; this

may indicate that the jointing in the weathered layer is predominantly east-

west.

Traverse 7 (pl. 2), on the eastern side of Lake Ginninderra, also

shows that the rocks have a normal weathering pattern over most of the

length of the traverse: soil overlies moderately weathered rock, which

overlies fresh rock. This cross-section again may include a thin layer of

highly weathered material under the soil, though it is not apparent in the

records. An anomalous zone about 80 m wide near the eastern end of the

traverse is shown in the cross-section, where the intermediate layer has a -

markedly reduced seismic velocity, but the bedrock appears to be homogeneous.

Whether this is due to a band of rock with different weathering characteris-

tics, or to some form of jointing or shearing which has encouraged the

weathering process, or even to a remnant stream channel or alluvial fan

containing unconsolidated sediment, is unknown; further detailed

investigations would be advisable before any buildings of substantial Size

are sited at this location.

The short tnverse8 (Pl. 3) intersects the Deakin Fault Zone

at the eastern end and indicates a dramatic deepening of the weathered

layer at its western end. In the middle there is a normal weathering

sequence grading from surface soil to fairly fresh rock at a depth of about

3 .

9. to 10 m. Unfortunately, the construction of Coulter Drive prevented the .

traverse being extended farther to the west to clarify the subsurface

sequence, at this end. This traverse is also located close to the position

of a single seismic spread recorded previously (Dolan, 1971). .Although

apparently within the Deakin Fault Zone, as delineclted by traverse 8 and

by the results of other investigations to the south (Briscoe & Ramsay, 1975),

Dolan's single spread records the boundary of a bedrock refractor having . a

seismic velocity of 3800 m/s at a depth of 6 to 17 m, which conflicts with

the record at the eastern end of traverse 8. This apparent discrepancy is

. probably due to the fact that the single spread was more closely in line

with the strike of the sheared fault zone, thus allowing the propagation of

energy at a given velocity at a lesser depth than in a direction across the

shearing.

Traverse 9 (Pl. 3), the longest, indicates the most 'complex

- cross-section. Major points of interest are a sheared zone with reduced

bedrock velocity at about chainage 200 m, possibly the Deakin Fault, and . •

another wide, deeply weathered zone from chainage 310 to about .450 m. .Apart

from these two zones, bedrock has a velocity generally between 4000 and 5300

m/s and occurs at depths of 10 to 20 m. From chainage 460 to 670 m, the

layer with seismic velocity 2200 m/s is not readily apparent in the records -

but is thought to be continuous with the layer (seismic velocity 1800-2000

m/s) recorded farther to the west. Apart from the two highly weathered

zones discussed at the beginning of this . paragraph, which would require

some further investigation before any substantial buildings were placed

there, there are no other hazards shown by the seismic survey.

The two short traversescrossing the CCAE gossan (Pl. 4) indicate

a shear zone 12 M . wide in the bedrock beneath the outcropping gossan. This

is interpreted as the Winslade Fault, with which the mineralization in the

goesan is associated. Extremely weathered material occurs for at least 70 m

in a direction transverse to the fault and reaches a maximum depth of about

30 m. This is an area with deep gradually increasing weathering - that is,

gradually increasing velocity with depth - and the retractors selected Will

depend to some extent on the geophone spacings and long offset distances

used. Different geophone spacings and offsets were used on the two

traverses, and the interpretations indicate different velocity refractors

mostly for this reason.

4 .

Apart from the previous seismic work by Dolan (1971) there are

also two intersecting traverses southwest of Lake Ginninderra (Hill, 1971);

perusal of this material should give the reader a better insight into the

geology in the area as expressed by the seismic properties.

CONCLUSIONS

Seismic refraction work in the area of Belconnen town centre

has confirmed the presence of several fault zones on both sides of Lake

Ginninderra, and indicated an anomalous zone within the weathered layer on

the eastern side. Apart from these areas which would require further

detailed investigation before large buildings are sited, no other major

hazards have been revealed by the seismic method.

6. REFERENCES

BRISCOE, G., & RAMSAY, D.C., 1975 - Geological and geophysical investiga-

tions at Emu Bank Belconnen town centre, A.C.T., 1975. Bur. Miner. Resour. Aust. Rec. 1975/179 (unpubl.).

DOLAN, B.H., 1971 - Seismic refraction survey on the site for relocation

of the Belconnen trunk sewer, A.C.T., 1971. Bur. Miner. Resour.

Aust. Rec. 1971/91 (unpubl.).

HILL, P.J., 1971 - Belconnen (Sections 43 and 50) seismic refraction

surveys, A.C.T., 1970. Bur. Miner. Resour. Aust. Rec. 1971/16

(unpubl.).

PLATE I

'94

TRAVERSE 9

TRAVERSE 8

41/

Lake

TRAVERSE 6 Ginninderro

TRAVERSE 7

Hall

SURVEY AREA ^CANBERRA

iOueanbeyan

A.C.T.

N. ^\^ L-17^'■E^\\̂ \ \ \

COLLEGE STREET ^

1111h

\\TRAVERSE ATRAVERSE B \ r

\-‘4

100 0 100 200 300 400 500 metresI^I^I^I^I^I^I

LOCALITY MAPBELCONNEN AREA

Record No /977//2^ I55/B5-335 A

0 0

CCAE

PLATE 2

TRAVERSE 6

WEST EAST

~600

1-570

f--595

~565

1-575

1-590

1-585Edge of Lake ~Ginninderra J i:::

~I- 580 4.J

~~

(2500)

Location of MolongloInterceptor sewer.

1

(4600)

(2550)

-------------,

(2550)

(4500)

(2300)

1'(300),

EAST

(2800)(ZOOO)(2500)

(4900)(4900)

(2400)(2500)(2600)(2450)(2000)

(1750)(1650)

-------------....---- ---............... (2500) (.200)

-- - - _---__ 2 _--------- _---- ----- -- - - --- - ------ -- -- --- - - ----- --------- -------- ~--

TRAVERSE 7

WEST

r-------

595-1-- - =-==-=-(.(4;;;o?;o7:)1-----------(-4-o-o-)--=::-:=-==-=:=:::-:=-~;o:C._=:=:~=-=-===:-::=-(4,?0'===:----------f40;:,!-------------;c=-,-- -~=----------~~,,-----------:=_=4"'OLc:-=-=-=-=-=::-:=-=-=-:i'-=~':-:=_===~---t:""~(400) ,(300)

(4OD) J300) • A300)590- -!400) J400i (400)_ __ __ _ __ (300L" ~-.::(1550) (1100) (1200) _----___ ,(300)

- - - --- --- --- - - - (1300) - - - ~ -=-::-..--:-=--------.---

575-

600-

l ~:~ ,--_---_--------:=~---------__::~- ~(5:/~00~)~ ~:;;-_---_------------ -.;:ro -;l;(~5~0~0~0~) ---------------------;;;6---------------------------0.;:--------------------------~ro I __~570- (4800) (5100) r 000

I 100mI 200I 300I 400I 500700 600

575-

580-

590-

SEISMIC CROSS-SECTIONS OF TRAVERSES 6 AND 750m

I40

I30

1

20I

Approximote position of bedroCK

Appl'oximote postlion of boundarybetween /oyel's of differentseIsmic velocities.

Seismic velocity(m/s)

LEGEND

Seismic discontinuity

10I

(2100)

oI I

10I I

.590

585

1-580 --...,§.~

'- 575 ~h:

~li.J

-570 ~

565

f-560

----

(2100)

d400)f

(4200)

(1500)

/(400),.---------

(1300)

d400)I

(4500)(5000)

(40C!.4.(400h

\--... '--- (400) ./" ---;

(400)\---,

"7

(2200)(2400)

(1800)(2500)

(.,

(4800)

(400,\

(2200)(2000)

(400),

----- ,(400) _---

(2000)

(4400)

------ ~----- -- (/" ---- --------------- -- ..---- -- -- (,--- ------ --~--------- --------~------------

575-

560-l ~----------------------------__2=__----------- -··-----------------;0c;----------~-------------------;~'io-----------------------------~4~~~0~m;:__-------------------------~I 300" 200000 100m

565-

570-

585- Edge of

~ 580-1 r'" GiMiOd=------­

~ ~i:::~li.Jl;j

Record No 1977/I? 155/B5 -336

SOUTHEASTTRAVERSE A

655 NORTHWEST

TRAVERSE BNORTHWEST

660 —SOUTHEAST

(300)

(650)

(900) (650)

(300)

(5000P) ol approx 30m depth

(650)

(2000?) al depth (5000P) ot opprox 30m depth

650

3_645

1E640 —LLI

L&J

635 —

630

(2000)

(/600

(300)

650 —

k.

11.1645 —

III

(500) (700)^(900)

(2700)

(800)^(3000)

(/300)^S^(/000)asool

(3000)

(300)

LEGEND

Seismic discontinuity

(/300) Seismic velocity(m/s)

Approximate position of bedrock

. Approximate position of boundarybetween layers of differentseismic velocities.

SEISMIC CROSS-SECTIONS OF CCAEGOSSAN,TRAVERSES A AND B

640 —

635 —

655 —

10m