m overburd - territorystories.nt.gov.au · a commercially available terrain conductivity meter...
TRANSCRIPT
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 1 of 22.
POWER AND WATER AUTHORITY N.,\TER RESOURCES DIVISION
TROUGHTON ISLAND GROUND\>ill.TER INVESTIGATION - JUNE 1990
FOR B H P PETROLEUN LTD
REPORT BY: ? ROWSTON D KARP
00lPR&DK
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 2 of 22.
1. INTRODUCTION
2. GEOLOGY AND SETTING
3. INS~'RUNENTATION AND TECHNIQUES
3.1 Resistivity
3.2 Electromagnetic
4. GEOPHYSICAL RESULTS
5. HYDROGEOLOGICAL IMPLICATIONS OF
GEOPHYSICAL RESULTS
6. RECOHHENDATIONS
DISTRIBUTION
B H P Petroleum Limited, Darwin
Wa ter ~.esources Library, Dar\-lin
11ater Resources Library, Alice Springs
Hydrology Branch P]'~WAi Dar'.vin
00lPR&-DK
2
1
1
2
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 3 of 22.
LIST OF TABLES
TABLE 1 Preferred inversion VES 2 and El' .. 1 forward modelling
TABLE 2 Preferred inversion YES 1 and El"! forfiard modelling
TABLE 3 Variation of ground conductivity with reasonable
variations of overburden and conductor resistivities: 8
m Overburden
TABLE 4 Variation of ground conductivity with reasonable
variations of overburden and conductor 1:"es i sti vi ties: 5
m Overburd.en
OOlPR&DK
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 4 of 22.
LIST OF' FIGURES
Figure 1 Vertical Electrical Sounding 1 and Interpre-ta tion
Figure 2 Vertical Electrical Sounding 2 and Interpretation
Figure 3 Ground Conductivity - 10 ill Vertical Dipole
Figure 4 Ground Conductivity - 10 m Horizontal Dipole
00lPR&DK
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 5 of 22.
SUM~L~.RY
A small geophysical/geological survey on Troughton Island was
conducted between the 5 and 7 June 1 9 90, Its purpose ....... as to
assess the potable water potential on the Island. The survey
indicates complete saltw'ater intrusion of the Lower Proterozoic
basalt/Tertiary laterite sec~ion. As a consequence a
pessimistic appraisal of the potable water potential of the
Island is given. Any future work should be done at convenience
and be of a direct sampling nat~re at 5i tes dicta ted by the
geophysical results.
OOl?R&DK
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 6 of 22.
1~ INTRODUCTION
During the per::'od 5 to 7 June 1990 a geophysical/geological
sur-vey °das completed on Troughton Island by staff of Water
Resources, NT Power and Water Authority.
The survey was undertaken as a result of a request by BHP
Petroleum Limited as to the likelihood of the occurrence of
potable water on the Island. The intention being to supplement
or yep lace the desalination of sea w-ater as the primary supply
of water during the majority of the year.
During a total field period of 2.5 days a crew" comprising one
geophysicist ar'~d one hydrcgeologist completed the following
operations:
1. More, than 4 kms of horizo!2tal coplanar and vertical coplana!."
coil electromagnetic profiles involving a total of some 582
observa t ions of apparent ground conductivi t:r·
2. Two Schlumberge!:' vertical electric soundings (VES} involving
40 apparent resistivity measurements.
3. Geological reconnaissance of the island.
00lPR&DK
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 7 of 22.
2. GEOLOGY AND SETTlNG
Trought.on Island lies the \Vest Australian Coast between
approximate AMG co-ordinates 8476950N 19120E and 8478350N
19900E. Little documentation of the islands geology exists.
Regional mapping suggests that the island is Tertiary laterised
Kimberley Basin Basalt of Early Proterozoic age.
The island has an elliptical shape with the major axis trending
north~ The island's edge is dellneated by a zone of beachrock
development behind ~,.;hich generally lies a well developed dune
system. The majority remainder of the island is outcropping
laterite.
Total relief of the lsland is approxima~ely 4 m with no obvious
drainage pattern. Vegetation consists of low grasses and rare
low trees with some mangroves
island in the tidal zones.
..:::kir ..... -ina ...... -- '-- -' the perimeter of the
Nean annua1 rainfall for tbe period 1956 to 1972· "\"las 787 mill.
Average dai l~l maximum and minimum tempera tures for the Same
period were 32.9 and 27.1 degrees for December and 27.8 and 22.0
a"eg~-eQ -_~or ·'uly. .... <::: >W v A~~nough no wi~d speed data could be round J
the period of the ~nvestigation the w~nd blew
consistently at 10 ;:0 15 knots &.nc. it is believed that this
situation is the norm.
00 LPR&DK
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 8 of 22.
3. INScrRUNE~lTATION AND TECHNIQUES
3~1 Resistiyitv
A brief specification of tr.e instrumentation employed for the
resist~vity soundings at Troug~ton Island is as follows:
TransntJ. tter DC to DC oonverter capable of up ':0 300 Watts
output at selectable voltage levels to greater that': 1500 Volts
and output currents to 2 Amps.
Receiver digital millivoltmeter with a resolution of 10
micro?olt.s and proV'isio!: for automatically offsettin.g up to 200
millivolts of DC noise voltages.
All resistivity sounding readings were made using an expanding
Schlumberger a2:'ray with a mini:i.um half current electrode spacing
of 2.5 m to a ~ax~mum of 100 m. I<leasurements -;.rere made
accord~ng to a scheme which results in 10 aeoarerlt resistivity
observations per decade of electrode expansion.
elect:!:ode a minimum of 0.5 m to a maximum of
10 m, the expansion occur~ing in two steps.
The VES i:1te:.::-pretations vl€!:'e obtai::ed using CSIRO program GRENDL
running on ~he Po~,.;er and Nater Authority's V_:;X 11/750 computer.
It is stressed that the interpretations presenLed represent
minimum layered, best fit I ' \ In a least squares sense) solutions
to the field data. Nhile the true geoelectric layering is
ambiguous to a greater Ol:" lesser extent dependent: on the deg=ee
of field observation error, no effort bas been ma~e to resolve
this ambiguity' by constraining the solutions wir:h geological
information, .'3. S no direct subsurface information exists.
However the range 0= the ambiguity (consistent with an es~imated
5% relative error in the field data and a 68% confidence level
for the estimated solution parameters) is available in the
extreme pa~ameter S€ts presented.
00lPR&DK
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 9 of 22.
Electromaanetic
The elect~omagnetic instrumentation used at Troughton Island was
a commercially available terrain conductivity meter (G80NICS
El·134) . This instrument involves a dual coil arrangement in
either horizontal or vertical coplanar mode wit!1. three coil
separation/frequency combinations. The systems operating
frequency is linked to the users selection of coil separation to
ensure operation and measurement at low induction numbers over a
wide range of earth resistivities. For such operations the
apparent ground conductivity is definable from the instrument1g
observed quadrature response, and is read directly from the
:cecei'ling unit. The in-phase response is used to co-ordinate a
uniform intercoil spacing.
Forward modelling using assumed geolectric sections
perforr.led using the United States Geological Sur~ley f s program
Er434 . The program upon input of a given earth model, outputs
the ground cond~ctivity as measured by the 2M34 in any of its
operating modes, assuming:
1. The measurement was made on the surface of the
2. The Earth ca n .be :nodelled as a space consisting of
several " " nc-:::-l.zonta.l. the ~esistivity of each
being constant.
OG1PR&DK
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 10 of 22.
4. GEOPHYSICAL RESULTS
The t~?O VES attempted have both been quantitatively interpreted
us ing GRENDL. The results are shown in Figu=es 1 and 2. In
general the quality of the field daca is quite good, consider1ng
large initial offsets at i1N (potential elec1:rode) change over
points can be expected where the slopes of the descending
branches are steep. The large mean errors be'tW'een field and
solution curves-/ 13.7% anc. 10.9% respectively can be attributed
to the extremely l:igh, aooo: 1 and 700: 1, resistivitv contrasts
present between ferricrete/mottled zone (laterite profile 20nes
A and B) and the seawater sa~urated host. These high contrasts
produce erroneous oscillations in solution curves.
Nevertheless the interpretations presented provide geologically
reasonable results.
VES 2 indicates a depth to saltwater satu~ated rock, layer 3, of
5.1 ill with a 68% co~fidence li~it (see section 2.1) error bound
of 4.34 m to 6.05 m, ~ is in~erpreted to be the late~ite
lltopsoiln only developed In this part of the island; :-lhilst
interpreted to the compact clays of the so called
pallid zone (Layer Z of the laterite p=ofile) or perhaps
s -p ........ , ~"'e CL ..... v ......... I.,. • For4a!:'d modelling of the E!'134 {s response to thi s
section produces resul~s which are enti~ely consistent with the
actual measured conduc~ivitiesJ see Table 1.
"IES 1 indicates a d.eeth to saltr,.,rate!.' saturated rock, again laye!.""
3, of 8.2 ffi. Unfortunately the ultra-high resis~ivity contrast
of 8000: 1 bet",yeen layers 1 and 3 means that on2.y high error
solutions to the fielc. data ~,.;as possible ar:.d as a result no
lIerrcr bounded!! depth to layer 3 provic.ed. The
interpretation provided in Figure 1 ~'I~as cnose:l because it was
the only inversion solution which when for"ward. modelled ?roc.uced .... ' ., ~ .... neoretJ..ca.L EN34 responses consistent with actual measured
conducti vi ties, see Table 2 (NB. suggested presence in data of
surficial layer of approximately 1700 ohm metres cannot be
consistently (in".rersion and E!ot fo::-~.vard mOdelled) modelled, all
resulting inversions failing to match measured conductivities.
001PRSDK
-
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 11 of 22.
.sCHt.'JM8ERC€:!'( €LECTRICAL SCliNDLJ.iG
AT":::.:
V£.5. 1
... '--~--" .. :.:.
.' ~. -.: -~ '.~-: ::---
'-~.--. . .... ;
=.
:-:.c::;: . ,. ---.- ----
----.-.,._--- ,~- .
3
r."'--.................. --".~'-~ -_ .. _--',_ ..... - ....
--- -
.. -
... _--
t-
.- .
- .
1+..2/2 CI1E'tRES)---,.-
---
joe
FIG. -1
~. r"_._~_. ___ _
..... ----
---._- -- .
-- -
ICC
>-f...
> -f.-" '" (!J
"" - f. ~
" iO '!
~.
"""
, A \OCO~ -.
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 12 of 22.
I
2-
.3
+
V£S.2
667 .!-b . 7 , I
/-06
778
FIG. (2
[ ....... =~~ ... '-' .. I .........
'-,:..-_"''''.:''''''~C''''' _ •• _ .... 4 ...............
~ --0'~!)
.46,8 ,
I _____ .. _____ .. -L ____ ---1 _____________ . ________________ ..
----
,
;
. _.
i
10
---.
-_:: _~: ">0 .... :.:.~:~ .
lOO
-, ,
-
2
,
s
.~ I ,
, , ~ I!
IOCO=~
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 13 of 22.
TABLE 1
PREFERRED INVERSION YES 2 AND EM FORWARD MODELLTNG
EARTH MODEL BASED ON YES 2 I}NERSION
LAYER
1 2 3 4
COIL SPACING (1.!ETRES)
THICKNESS (,'!ETRES)
1.8 3.3
46.8 10000.0
H - HORIZONTAL, COPLANAR COILS (VERTICAL DIPOLE) V - VERTICAL, COPLANAR COILS (HORIZONTAL DIPOLE)
EARTH MODEL EM34 FORWARD MODELLING
10 H v
RES~STIVITY
(OE:4 METRES)
667 47
1 998
APPARENT CONDUCTIVITY (M~!HO' S! ,,!ETRE )
66.9 '0 0 c ... ,,..
FIELD APPARENT CONDUCTIVITIES (AVERAGE 3 NEARES STATIONS)
10
OOlPR&DK
'" .. v
68.5 71.2
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 14 of 22.
TABLE 2
I?REFERRED INVERSION VES 1 AND EM FQRNARD MODELLING
EARTH !'ODEL BASED ON YES 1 INv"'"ERSION
LAYERS
1 2 3 ., ..
COIL APPAREN-T SPACING CONDUCTIVITY (HETRES) (HNHO' S l~lETRE)
THICKNESS (l·!ETRES)
" " ~.~
3.0 2D.2
10000.0
H - HORIZONTAL, COPLANAR COILS (VERTICAL DIPOLE) V - VERTICAL, COPLAN!'.R COILS (HORIZONTAL DIPOLE)
EARTH MODEL EM34 FORWARD MODELLING
10 H V
RESISTIVITY (OHMJ.!ETRES)
4110 19
1 4990
41.0 31. 1
FIELD APPARENT CONDUCTIVITIES (AVERAGE 3 NEAREST STATIONS)
10
00lPR&DK
H V
40.8 30.5
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 15 of 22.
E.ight EM34 ground conductivity profiles (lines 1 to 3) were
completed using the smallest available loop separation of 10 m
and the results cor:toured and presented in Figures 3 and 4.
Generally, w"hen resistivity mapping ~· .. ith frequency domain E?y!
methods the vertical dipole is looked ~o to provide information
on horizontal conductors whilst the horizontal dipole can be
expected to map vertical conductors ° I~ the Troughton Island
data set, \...;hi 1e both measurements camp lement each other:', the
horizontal dipole data appears to provide both a higher dynamic
range and more consistent results. The higher dyr.amic range can
be explained by the horizontal dipoles shallow Udepth of
The noise in the vertical dipole (horizontal
1000) data particula:::-ly near the edge of the island. can be
accounted for by ~he inconsistency of the coupling bet:ween
horizontal loops because of the elevation differences bet',.,een ';'" 1 ~ • , t 1:H9 _oops at cnese pOln So
The contoured results are essentially a function of depth to
salt--:,.;ater as the vast- majority of signal return is at'tributable
to the saltwateo:: sat°clrated rock, see Tables 3 and 4 ° TC1e
maximum variation bet~,.;een conductivity data pairs is consistent
with a change depth to the conductive layer of between 4 and
5 m. As this is apprOXimately the variation in elevation of the
island above mean sea level, it ca~ be assumed that there exists
no deepening of the saltwater interface and thus the saltwater
can be represented as a planar horizontal conductive layer. The
conductivity contours only depart from the
elevation contours in the dunes at the north-easterly tip of the
island,
OOlPR&OK
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 16 of 22.
~ I •
!
I
j
./ /
FIG. 3
/ ! Ri
~~ ~n
<s)-1 ..... 3< . .... ~2
... 0.., ,.. .., o
•
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 17 of 22.
,.....c I 3
I I -'0 r"o
" ,--~ \ '
j ," .... () cO 3
f I r
i ,"-)
:c :'-'
~l: .r Ivl , , , ,
""-'-
FIG. 4
j
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 18 of 22.
TABLE 3
yariat'on of ground conductivity with reasonable ~J'ariations of overburden
and conductor resistivities! 8 m Qy-erburden
LAYER
1 2 3
COIL SPACING (METRE)
10
LAYER
1 2 3
COIL SPACING (HETREi
10
LAYER
1 2 3
COIL SPACING (METRE)
10
LAYER
1 2 3
COIL SPACING (HETRE)
10
OOlPR&DK
THICKNESS (>IE'rRES)
S.O 30.0
10000.0
COIL SETUP*
a V
THICKNESS (NETtlES)
8.0 30.0
10000.0
COIL SETUP*
H v
THICKNESS (METRES)
COIL SETUp'!,
H V
THICKNESS (HETRES)
8.0 30.0
10000.0
COIL SETU?;'~
H V
RESISTIVITY (OHH HETRES)
5000 I
1000
APPARENT CONDUCTIVITY (HHHO'SJHETRE)
39. 1 29.9
RESISTIVITY {OHM HETRES)
500 1
1000
AP?ARENT CO~DUCTIVITY
(i1HHO'SiHETRE)
L;O. 1 30.9
RESISTIVITY (OHM HETRES)
5000 2
1000
APPARENT CONDUCTIVITY (I-!:1HO'S/;lETRE)
37.4 26.0
RESISTIVITY (Offi1 NETRE)
500 2
iDOO
APPARENT CONDUCTIVITY (H!-fHO ' S /METRE )
38.5 27.0
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 19 of 22.
,
TABLE 4
variation of grnqnd conductivity with rf':8.sonable yari{Jt;ioos of overburden
and ccnduetQ[ resistivities: 8 m Oyprhurden
LAYER
1 2 3
COIL SPACING (METRE)
10
LAYER
1 2
3
COIL SPACING (}lETRE)
10
LAYER
1 2 3
COIL SPACING (METRE)
10
LAYER
1 2 3
COIL SPACING (METEr:)
10
OOlPR&DK
THICKNESS (HETRES)
5.0 30.0
10000.0
COIL SETUP"
H V
THICXNESS (HETRES)
5.0 30.0
10000.0
COIL SETUP*
H
V
THICKNESS (METRES)
5.0 30.0
1000.0
COIL SETUP*
E v
'THICKNESS (aETRES)
5.0 30.0
10000.0
COIL SETUP"
RESISTIVITY (OP.1-! HETRES)
5000 1
1000
APPARENT CONDUCTIVITY (!-1MHO 1 S /METRES )
65,9 69.S
RESISTIVITY (OID! ,lETHES)
500 1
1000
APPARENT CONDUCTIVITY (!-lNHO f S/HETRE)
66.6 70,2
RESISTIVITY (OH,l HETRES)
5000 2
1000
APPARENT CO)l"D[]CTIVITY (t1NEO' S /NETRE)
64,9 55,7
RES I STIVTTY (OE~l ~!ET;:(ES)
500 2
1000
APPAREN'f CONDUCT1VITY (flMEO'S/:1ETRE)
65,7 56.5
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 20 of 22.
5. HYDROGEOLOG:::C.>,L IHPLICATIONS OF GEOPHYSICAL RESULTS
Based on previous Water Resource Division experience in similar
hydrogeological environments it is expected that any store of
freshwater on the island would exist as a thin lens of
fresh';later sitting atop brackish and then saltwater ~vithin the
laterised Proterozoic section~
It has been st:ated in section 4 that all g eoohvsical . . results
imply that Troughton Island is absolutely invaded by seawater at
a level approximately equal to mean sea level, al thou-gh the
condu.ctivity contours do depart from the elevation contours
significantly on the northeastern pen~nsula, indicating a
The scale of dipping of the saltwater interface at this point.
this dune environment hoy..~ever precludes it as a site for any
signi£ icant \'later supply. ~he othe:z:wise absolute invasion of
the sea would then imply that either:
1. A oe:cmeab~e . - ane. transmissivE zone exists at a depth
cor=esponding to sea level} OR
2. permeability enhancement has occurred during late~isation
with invasion occur~ing before induration, with 2 being
COI'_S idered very unlikely.
In any case either contention is unfavourable to development of
a potable supply rl'li thin ~ .. -l,.n1.S zone as bo th ,.;ou Id prevent
exploication at any desirable rate because of:
1. Salt inflow
2. impermeab:Llity
The base of the laterite/mottled zone ~hen is the only zone of
potential on
representing
the
the
island
greatest
proper. The
thickness of
island's high pOints
uninvaded rock and
therefore the greatest: storage capacity with the possibility of
a pallid zone aquifer~ Although the permeability of laterite is
ex~rernely low it has been the water Resources DivisionJs
experience tha.t aquifers do occur within laterised profiles 1
given suitable conditions.
OOlPR&DK
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 21 of 22.
,
Unfortunately, the total
relative impermeability
depth
of
to salt-saturated rock, the
laterite and the climatic
considerations of low rainfall and high winds force a
pessimistic appraisal on the likelihood of significant supplies
of potable water on Troughton Island.
00lPR&DK
Technical Report WRD90035
Viewed at 15:07:56 on 29/07/2010 Page 22 of 22.
•
6. RECOt~NENDATIONS
It is recommended that any future work be of a direct nature,
and be done at BHPP Limited's convenience when appropriate
equipment (eg Earthmoving) is available during any future period
of construction or upgrading. It is suggested that any such
work concentrate on the high points of the island where greater
rock thicknesses exist. The 30 mmho/metre horizontal dipole and
40 mmho/metre vertical dipole coincident contours delineate at
least 2 appropriate '~ 5.1.1.-85, namely 20 ill west of both the windsock
and dangerous goods shed .
001PR&DK