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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 It may not be published in any form or used in a company prospectus or statementlout the permission in writing of the Director.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