Assessing geophysical methods for mapping the saline water interface along Perth coastal and river margins

8/19/2019 Assessing geophysical methods for mapping the saline water interface along Perth coastal and river margins

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Assessment to be handed in as a Power Point PresentationStudent Name: Sutthisrisaarng Pholpark

Student Number: 17682974

Date submitted: 9 October 20 14


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Geophysics for defining the saline

water interface Theoretical background

Saline water is defined by a concentration of chloride

exceeding 250 mg/l (Grube et al. 2000, quote in Kirsch2006, 423).

The main source of saline groundwater are seawaterintrusion in coastal areas (the problem of this study) ,

salt domes, high concentration of minerals in ground water under arid conditions (Kirsch 2006, 423).

When aquifers have hydraulic contact with seawater,seawater intrusions happen as a consequence of higherdensity of saltwater (Kirsch 2006, 423).


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Theoretical background (continue)

Balanced (top) and disturbed (below) salt/freshwater interface in a

coastal area (after Keller 1988, quoted in Kirsch 2006, 424 )

Disturbing the equilibrium between freshwater and saltwater by high pumpingrate for water supple decreases depth of salt/freshwater interface and

influences saltwater intrusion (Kirsch 2006, 424).


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Petrophysics

Radar and moisture content

The propagation speed of magnetic wave used for time-depth calculation in GPR can be calculated from:

C is a speed of light in vacuum C = 3 x 10^8 m/s

Electric permittivity ε depends on polarisation propertiesof material (Kirsch 2006, 16).

Typical values for ε are: water = 80, saturated sand = 20 –30, and air = 1 (Kirsch 2006, 16).

ε of material increases with water (moisture) content in amaterial because ε of water is as high as 80.

From GPR travel time equation, increasing moisturecontent -> increases ε ->reduces speed of GRP propagation.

=


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Petrophysics

=

Water contentElectric permittivity

GPR velocity

Permittivity of glacial sediments from Finland and Wisconsin (USA) in

relation to water content (after Sutinen 1992, quoted in Kirsch 2006, 17 )


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Radar and moisture content (continue)

Topp’s relationship

Topp’s relationship is a relationship between apparentelectrical permittivity εand volumetric soil water contentθ(m^3 m^(-3)) proposed by Topp et al. 1980 (Huisman etal. 2003, 478).

The apparent permittivity used in the equation obtainedfrom measurement of electromagnetic propagation

velocity in the soil.

We can use GPR apparent velocity from GPR dataprocessing to calculate apparent permittivity from the

velocity equation and then use the calculated apparentpermittivity to obtain soil water content in Topp’sequation.

Petrophysics

=-5.3× 10−

+2.92× 10−

5.5 × 10−

+ 4 . 3 × 1 0−6


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Radar and water chemistry: Impact of saline water

on velocity In saline water, ε is reduced because of dissolved salt

ions reduce polarisability of water (Kirsch 2006, 17).

ε is reduced by degrees of salinity.

From GPR velocity :

Salinity ε

GPR velocity

GPR velocity increases with salinity.

=


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Radar and water chemistry: Impact of saline wateron amplitude.

As EM wave (GPR) propagates through a conductormaterial, its amplitude is absorbed (Musset and Khan2000, 218).

The skin depth equation is used to determine depth of

EM wave penetration when its amplitude drops to 1/eof the amplitude at depth = 0 (Musset and Khan 2000,218).

Depth of penetration increases with resistivity.

Resistivity of water decreases with its salinity.

So that the degrees of amplitude attenuation increases

with a salinity of water.

ℎ = 503

• Salinity • Resistivity • Amplitude attenuation


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Salinity a measure of the concentration of total dissolved solids

in water (Department of Water 2009).

0–500 mg/L; fresh

500–1500 mg/L; fresh to marginal 1500–3000 mg/L; brackish

>3000 mg/L; saline (Department of Water 2009).

Relates to water resistivity which can be explained by Cl

ion content.


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DC resistivity

DC resistivity and conductivity of water rely heavily onChloride (Cl) ion content(Kirsch 2006, 424).

Cl ion content can be used to define water type (Kirsch2006, 424). Fresh water: Cl ions < 150 mg/l or ppm

brackish water: Cl ions are between 150 - 10000 mg/l Saltwater: Cl ions >10000 mg/l

The more degree of Cl content, the more conductivity ofthe water (low resistivity).

Petrophysics


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Petrophysics

The conductivity of saltwater (at 18ºC) in relation to the Cl- content

(after TNO 1976, quoted in Kirsch 2006, 427)

∝ −

DC resistivity


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DC resistivity and sand/sandstone resistivity Archie’s law for the resistivity of water saturated clay-free

material can be described as

Formation factor

m depends on “cementation” of the grains, influence bypores geometry, the compaction, the mineral compositions(Ransom 1984, quoted in Kirsch 2006, 9)

a is related to influence of mineral grains on current flow,a=1 for perfect insulator grains and reduces according to thedegree of conductivity (Kirsch 2006, 9).

∅ is porosity of the material which is the fraction ofsentiment that is pore space (Musset and Khan 2000, 184).

Overall, Arhie’s Law describes that the resistivity of aquiferis not only related to salinity of water content but alsocementation, degrees of conductivity of material andporosity.

Petrophysics

=

∙

= ∙ ∅−


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DC resistivity and Clay content/type In clay content material, traditional Archie’s law is no

longer valid since its fine particles trap a film of electrolytearound them (Musset and Khan 2000, 184) . Clayey material is considered as a low resistive material

ranging from 5 – 60 ohm*m due to it has high surfaceconductivity (Kirsch 2006, 12).

Electrical charge at the clay mineral surface is negativebecause cations ions in clay minerals are replaced bycations of higher valence (Kirsch 2006, 12).

Cation Exchange capacity (CEC) is a quantification ofcompensating the negative charges by the concentration of

cation ions in the pore water adjacent to the mineralsurface (Kirsch 2006, 13). CEC strictly depends on mineral contents at a particular

area (Kirsch 2006, 14) and salinity of water content.

Petrophysics


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DC resistivity and Clay content/type (continue)

Resistivity decreases proportionally to CEC under the

same temperature, porosity and salinity conditions(Ussher et al. 2000).

From the table, determining from CEC average value,smectite clay should have the lowest resistivitycompared to other clay minerals.

CEC -> Resistivity

Typical CEC values for clays ( after Grim 1953, quoted in Ussher et al. 2000, 1917)


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AMT: Audio-magnetotellurics

AMT is one of electromagnetic sounding method

which employs ratio between electric field andmagnetic field to calculate lateral electricalresistivity of the subsurface.

AMT source: natural EM sources from natural

variations of Earth’s magnetic field cause by solarenergy or lightening.

When magnetic field travels from very far field,its orientation almost parallel Earth’s subsurface.

Since a very high resistivity contrast between airand the subsurface, after the wave hit subsurface,it travels down through the subsurface verticallyand generates current flow horizontally in the

subsurface while travelling.

Petrophysics


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AMT: Audio-magnetotellurics (Continue)

AMT is more sensitive to a conductive unit e.g.

groundwater because current flows horizontally and it triesto avoid travelling through a resistive unit.

The frequency range of AMT is from 5 Hz – 100,000 Hz, with penetration depth from several meters up to 100 km+

which relatively shallow compared to MT (10 m – 500 km),however, it gives higher resolution than MT.

Penetration depth of AMT depends on frequency of aparticular wave and resistivity of the subsurface at aparticular area (use skin depth principle to explain since

ATM the source is one of electromagnetic wave).

AMT method is not suitable to perform t in city areasbecause there are too much artificial electromagnetic wavesources e.g. mobile signal, power lines (50Hz for Australia)

which disturb the natural EM source.

Petrophysics

h


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GPR versus EM versus DC resistivity the main differences

Source

GPR: High frequency electromagnetic field 1 kHz – 100 MHz EM:

FEM: AC current (continuous, multi frequencies) 100 – 50,000 Hz TEM: low frequency square wave 1 ms – 1 sec on time

DC resistivity: DC Current

Measure GPR: Travel time of EM reflection EM:

FEM: Primary and secondary electromagnetic field (In-phase and quadrature response) TEM: Decay curve of EM field after current has been just turned off (transient response)

DC resistivity: potential different of the subsurface

Depth of investigation GPR: 1m – 1 km

EM: FEM: 1 – 100 m TEM: 10m – 30+ km

DC resistivity : 1 – 1000 m

Purpose GPR: Imaging near surface structure EM: mapping subsurface resistivity DC resistivity : mapping subsurface resistivity

Petrophysics


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Location The Ern Halliday Recreation Camp is located on

Whitfords Avenue, Hillarys. In a view of hydrogeological, the site is selected

due to its location is between saline water source(Perth coast) and fresh water source (artificial

lake) with height 1 - 2 m below the sea level. Inaddition, groundwater in superficial aquifer f lowsfrom the crest of the Gnangara Mound toward thecoast and increases its salinity in the direction of

flow and with depth (Department of Water WA2009). We assume that there is a hydraulic contactbetween these sources at superficial aquifer, hencethere is a high potential to observe a salinityinterface at the site.


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The survey areahttps://www.google.com.au/maps

Location (continue)

In a view of geophysical, we expected to observe physical variations e.g. resistivity and electrical permittivity of thesubsurface along the survey transverse due to salinitygradient from Perth basin towards the survey site (inversedirection of groundwater flow).


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Field Work Background Perth Basin Aquifers

Can be explained by Gnangara groundwater system which is composed of superficial aquifer,Leederville aquifer, Yarragadee aquifer (major) and Connectivity of aquifers

(minor)(Department of Water WA 2009). Highest groundwater 75 m above the sea level locates at the crest of the Gnangara Mound and

water flows away from this high point towards the Indian Ocean, the Swan River, Ellenbrookand Gingin Brook (Department of Water WA 2009).

Gnangara system hydrogeological cross-section(after Gnangara Sustainability Strategy Situation statement, department of water WA 2009)


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Perth Basin Aquifer: Superficial aquifer

A major unconfined aquifer consist of the Quaternary –Tertiary sediments of the coastal plain: Safety Bay Sand,Tamala Limestone, Bassendean Sand, Gnangara Sand,Guildford clay (Department of Water WA 2009).

Maximum thickness 75 m, with average 45 m.

Groundwater in the Superficial aquifer is generally fresh,flows from the crest of the Gnangara Mound (less than 250milligrams per litre total dissolved solids (TDS)) toward thecoast and increases its salinity with flow direction anddepth (Department of Water WA 2009).


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Perth Basin Aquifer: Leederville aquifer

A major confined aquifer consist of the Henley sandstonemember of the Osborne Formation and the Pinjar, Wanneroo and Mariginiup Members of the LeedervilleFormation (Department of Water WA 2009).

Ranges in thickness up to 500 m (Department of Water WA2009).

The groundwater salinity ranges from less than 500milligrams per litre TDS to in excess of 3000 milligrams perlitre TDS (Department of Water WA 2009).

Restricted to public water supply for supplying Perth withdrinking water (Department of Water WA 2009).


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Perth Basin Aquifer: Yarragadee aquifer

A major confined aquifer comprising; the Cretaceous GageFormation; the Parmelia Formation; and the Jurassic Yarragadee Formation (Department of Water WA 2009).

Thickness in Perth region excess 2,000 m (Department of

Water WA 2009). Salinity ranges from less than 500 milligrams per litre TDS

to in excess of 3000 milligrams per litre TDS (Departmentof Water WA 2009).


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Field Work Background Comparable Saline Water Intrusions / Interface studies Fitzpatrick et al. (2007), propose technics to map salt-loads at the Murray

River, Australia by using airborne and in-river electromagnetic methods.

Both airborne EM and in river EM successfully investigate fine scale variationsin water flow direction and showing alternation between losing and gaining

groundwater in a river.

Conductivity mapping (a) In-river nanoTEM and (b) RESOLVE airborne EM(after Fitzpatrick et al. 2007 )


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Field Work Background The Perth Water Supply and the “PRAMS” Ground water model

Conceptual model of PRAMS(after Perth Regional Aquifer Modelling System (PRAMS) scenario modelling for the Gnangara

Sustainability Strategy , department of water WA 2009)


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Objectives To employ various geophysical methods to investigate the saline water interface

along Perth’s coastal and river margins. To understand the overview of Perth’s hydrogeological system which

contributes to supportive information in selecting geophysical methods toinvestigate the saline water interface.

To understand water chemistry effects in physical properties of water which are

the objects of geophysical investigation. To understand limits of geophysical methods in water chemistry assessment.

To provide recommendations on geophysical methods that could be deployedat a large scale to better understand groundwater chemistry distribution onlyPerth's coastal and river margins (Environmental Geophysics Blackboard).

Since a saltwater interface to the freshwater aquifers is present along the coastand adjacent to the Swan River estuary, abstraction from areas near river alongthe coast influences saltwater intrusion to fresh water aquifer which results inthe deterioration of water quality in the freshwater aquifers (Department of water 2009). Employing geophysical methods in water chemistry assessmenthelps to detect the problem in the early stage.


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TEM data Acquisition

Transmitter section Power source

Typically, the power source for TEM is DC power supply e.g. battery.

Transmitter instrument

The transmitter operate by using power from DC power source as and

input and sending low frequency square wave signal as an output totransmitter loop. The transmitted signal parameters e.g. rise time,ramp, initial delay, duty cycle and data listen time can be adjusted asuser desires. In a very conductive target, signal frequency can be as lowas 1 Hz. Increasing number of stack, increases signal to noise ratio,however, it also increases measurement time.

TEM transmission waveform

(after Pethick 2014)


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TEM data (continue)

Transmitter loop The rectangular electrical wire transmitter loop, controlled by

transmitter instrument. Transmitter area can be adjustedaccording to the purpose of a survey.

Receiver section

Receiver instrument The receiver instrument is used to record atransient response after transmitted current is turned off for aninterpretation. Since the response is varied by time, the recording isdivided into channel which is time window for each recording. Theearly channels have smaller bin width than late channels due to the

field variation in the early time is higher than the late time. Receiver loop

The receiver loop receives induced voltage (emf = dB/dT)generated by variation of induction current.


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TEM data Processing

Outliers removal: remove bad data and bad channel (too low or toohigh value, out of trend value, negative number).

Interpretation Each decay curve of measured data can be calculated and plot in

apparent resistivity curve. To investigate the saline water interface, we expect to the resistivity

curve to reduce from the survey area towards the coastal direction. This apparent resistivity curve can also be use to construct layered earth

model. Geological knowledge of a survey area is used to determinenumber of layers and layers resistivity for a start model.

TEM method is not suitable for a build up area. After current is turnedoff, current caused by an abruptly change of electromagnetic fieldtravel downward and outwards through like a smoke ring. What wemeasure is the decay of an electromagnetic caused by that currentsmoke ring. Since the electromagnetic field magnitude is small,disturbance electromagnetic signal from artificial sources e.g. radiosignal, power line can mask the received signal and ruin themeasurement, consequently the measured data can not be interpretedaccurately.


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Surface Radar

Acquisition Equipment

Antenna sending a very short pulse wave, contains frequenciesin the range 25 – 1000 MHz. The shorter pulse the higherfrequency.

The pulse creates electromagnetic field propagates throughthe subsurface and propagates back to the atmospherereceived by receiver.

This system require an accuracy GPS to sense the surveylocation.

The system can be carried around the survey transvers. Since travel time of each pulse lasts only a few nanoseconds,

Radar can cover the survey area very fast.


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Surface Radar

ProcessingSmoothing data

Assigned geometry

Amplitude recovery to enhance weak reflectors

Reduce effects of diffraction whicobscure true reflection

Use average velocity to correct topographic

Convert travel time to depth

General processing flow for the large-scale GPR dataset

(after Strobach et al. 2010)

Remove inherent and nonlinear noisesassociated with the antenna characteristic


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Surface Radar Processing (continue)

Radar processing method is very familiar with seismic dataprocessing.

Interpretation Radar is effective method to mapping a water table depth because a

water table has high electrical permittivity cause a high electricalpermittivity contrast between a water table and the upper layer,

consequently water table interface is a strong radar reflector andcan clearly recognize in radar data. Radar can propagate faster in high electrical permittivity material.

Saline water has higher electrical permittivity than fresh water,hence radar propagate faster as degrees of salinity.

In radar survey, radar image is better (clearly see the subsurface

structure) when survey in unsaturated zone because it has higherresistivity than saturated zone, so radar energy is less attenuated while it propagates. As a result of less attenuation in unsaturatedzone, higher energy reflect back to a receiver so we can see radarimage clearer.

Radar image is hard to interpret when conduct a survey in high

saturated zone, due to high energy absorption by low resistivitymaterial and low energy reflect back to a receiver.


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Surface Radar (continue) Interpretation

Rainfall recharge reduces salinity of water content in the subsurface.

Radar travels slower in the less degrees of salinity. Hence, rainfallrecharge affects radar in reducing velocity of propagation.

However, in dry area, rainfall recharge reduces radar depth ofpenetration because it increases moisture content in the subsurface,hence, decrease resistivity.

While radar propagates in low conductivity materials, it is highlyattenuated by energy absorption in these material. As saline waterhas low resistivity, radar can travel in a shorter distance compared totraveling in water with less degrees of salinity.

To deploy Radar in a particular area with different survey time, thedepth of investigation may varied. Since the moisture content is

changed by the annual rainfall cycle, it changes the subsurfaceresistivity of that area, and subsequently changes radar depth ofpenetration.

Since radar in one of EM methods, it does not work well in build upareas because there are to much artificial EM sources e.g. power

line, radio signal which could mask received radar signal and ruinsurvey data.


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AMT Acquisition

Equipment : 2 coils for recording natural magnetic field in x and y plains. The

orientation of coils need to precise to recover frequency of thesignal. If need to record magnetic field in Z component (3DEarth), 1 more coil need to be added (z plain).

4 non-polarise electrodes (EW and NS) for record electric fieldin EW and NS direction.

GPS for record the survey position, and synchronize timesbetween these equipment.

A recording unit for recording amplitude and phase of electricfield and magnetic field.

Data acquisition durations for AMT can take from 1 hour up to 24hours depends on desires frequency content in the data which isdirectly related to the depth of investigation. The higherfrequency, the shorter penetration depth (skin depth relation).


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AMT Acquisition (continue)

AMT Survey layout (after Moombarring geophysics 2014)

http://www.moombarriga.com.au/?ContentID=26


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AMT

Type equation here.Processing Measure two to three components of the magnetic field and two components of the

electric field. Time synchronize is done by GPS (Takam 2013).

Apparent resistivity :

Correction for Static shift (vertical displacements of the apparent resistivitysounding curves, between adjacent sites due to boundary charges on surficial

inhomogeneties) (Takam 2013). Noise removal (Ground motion, power lines, radio transmitters, electric current

from irrigation pumps, electric fence) (Takam 2013).

=

=

= 2 = 4 × 10−7 · −


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AMT

Interpretation:

The calculated apparent resistivity can be plotted as in asounding curve in resistivity method.

To investigate the saline water interface, we expect to theresistivity curve to reduce from the survey area towards the

coastal direction. This sounding curve can also be use to construct layered

earth model. Geological knowledge of a survey area is usedto determine number of layers and layers resistivity for a

start model. The data cannot be interpreted correctly in a build up area

due interference signals source e.g. power lines, radio canmask measured natural magnetic signal which as small as

few thousands of nano Tesla. The collected data from thissurvey is unusable.


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DC – Resistivity

Acquisition Schlumberger array is deployed in the survey.

Schlumberger array is suitable for vertical electrical sounding formodelling layered-earth.

As current electrode moving further (to decrease depth of current

penetration), measured voltage difference is increased. Thentransmission current is increased to compensate this effect.


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DC – Resistivity (continue)

Processing Apparent resistivity in Schlumberger array can be computed as the

equation below.

If the computed resistivity too low or too high it need to be removed fromthe data for interpretation accuracy.


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DC – Resistivity Interpretation

Schlumberger array is suitable for electrical vertical sounding andit’s result can be used to created layered-earth model.

Each layer is characterized by layer resistivity.

Since each material has a specific resistivity, we can deduce materialtype in each model layer from it resistivity.

This method can be used to map water table. Future work:

For saline water interface, dipole-dipole array is a suitable array forapparent resistivity profiling because we can obtain apparentresistivity at a particular station along a survey line from this array.

If there is saline water interface in the area, we expect to observeapparent resistivity increase towards Perth coastal.


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Conclusions TEM, AMT and DC resistivity (dipole-dipole array) can be used to investigate

water chemistry variation. Since salinity changes water resistivity andconductivity, these methods can be deployed to map distribution of the salinewater interface.

Radar is suitable from mapping high resolution surface structure which can beuse to investigate a depth of the water table. In addition, radar propagationvelocity is related to salinity, as it velocity increases with salinity, it may be use

to map distribution of the saline water interface. The survey area is considered as urban area, there are a number of artificial

electromagnetic sources e.g. power line, radio signal, as a result of theseinterference signals, all range of EM methods (TEM, AMT and radar) fail to yield

good survey results

The most suitable method for mapping distribution of the saline water interfacein Hilary area is DC resistivity method with dipole-dipole array. It can be used tomap apparent resistivity profile, hence it can investigate the distribution fromvarying in apparent resistivity.

AMT theatrically yields the highest depth of penetration. However, distributionof the saline water interface in superficial aquifer which the average depth is 45

m, so all of these method satisfies minimum depth investigation key point.


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Recommendations

Seismic reflection methods can be used to map water table which contributeunderstanding to water flow path and determine possible location of saltwater interface.

Moving groundwater containing salt cause potential different at the subsurface and itdistribution can be investigated by self-potential method.

Several drilling holes may be used to collect water sample and test it salinity at lab toobserve salinity trend from hole locations.

Downhole EM can be use to investigate resistivity variation a particular depth to observesalinity variation from resistivity.

Wireline logging can be used as the same principle as downhole EM.

Numerical flow and solute transport modelling can be used determine potential saltwaterinterface from water flow path. It is a valuable information to determine appropriatedrilling hole locations to observe saltwater interface. In addition, it is a supportiveinformation to select potential location of saltwater interface for geophysicalinvestigation.


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References Musset, Alan E, and M Aftab Khan. 2000. Looking Into the Earth. New York:

Cambridge UniversityPress Strobach, E. Harris, B.D. and Dupuis, J.C. and Kepic, A.W. and Martin, M.W. 2010.

“GPR for largescale estimation of groundwater recharge distribution.” 13thInternational Conference on Ground Penetrating Radar (GPR), Jun 21 2010, pp. 1-6. Lecce, Italy: IEEE.

Ussher, Greg, Colin Harvey, Roy Johnstone and Errol Anderson. 2000.

“Understanding The Resistivities Observed In Geothermal Systems.” ProceedingsWorld Geothermal Congress 2000 Kyushu - Tohoku, Japan, May 28 - June 10, 2000

Huisman, J A, S. S. Hubbard, J. D. Redman and A. P. Annan. 2003. “Measuring SoilWater Content with Ground Penetrating Radar: A Review.” VADOSE ZONE J (2):476-490

Department of Water WA. 2009. Gnangara Sustainability Strategy Situationstatement 2009. Perth, Gnungara Sustainability Strategy: Department of Water.

Pethick, Andrew. 2014. “Time Domain EM : Profiling.” PowerPoint lecture notes.

Takam, Eric. 2013, “Magnetotellurics” PowerPoint lecture notes.

A. Fitzpatrick, T. J. Munday, V. Berens, M. A. Hatch, A. L. Telfer

Symposium. 2007. “Application of Geophysics to Engineering and EnvironmentalProblems.” 410-41.

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Assessing geophysical methods for mapping the saline water interface along Perth coastal and river margins