m Overburd - territorystories.nt.gov.au · a commercially available terrain conductivity meter...

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



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



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



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



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



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



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



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



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



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

-



.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~ -.



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=~



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



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



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



~ I •

!

I

j

./ /

FIG. 3

/ ! Ri

~~ ~n

<s)-1 ..... 3< . .... ~2

... 0.., ,.. .., o

•



,.....c I 3

I I -'0 r"o

" ,--~ \ '

j ," .... () cO 3

f I r

i ,"-)

:c :'-'

~l: .r Ivl , , , ,

""-'-

FIG. 4

j



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


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



,

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


10

LAYER

1 2

3

COIL SPACING (}lETRE)

10

LAYER

1 2 3


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



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



,

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



•

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

Date post:	07-Feb-2020
Category:	Documents
Upload:	others
View:	0 times
Download:	0 times

m Overburd - territorystories.nt.gov.au · a commercially available terrain conductivity meter...

Documents