1. Student Name: Benjamin Seive
Student Number : 17672137
Date submitted: 22/09/2104
1

2.  For websites, all information presented here were still
online on 22/09/2014 @10.00am
 Other references (authors) are listed in the source
section at the bottom of each slides . Details are
presented at the end of the ppt.
2
Notes about references

3. Theoretical Background – Saltwater Intrusion
Sources: http://www.solinst.com/resources/papers/101c4salt.php //
http://en.wikipedia.org/wiki/Saltwater_intrusion
Salt water intrusion is the movement of saline
water into freshwater aquifers in coastal areas
3

4. Theoretical Background – Saltwater Intrusion
Sources: http://water.usgs.gov/ogw/gwrp/saltwater/salt.html
Normally the pressure head of the recharge
inland creates a seaward movement of the
fresh water that “pushes” on the salt water and
prevents intrusion
4

5. Theoretical Background – Saltwater Intrusion
Sources: http://kanat.jsc.vsc.edu/student/spatafora/setup.htm // Jansen, 2011
Keep in mind:
The transition between Fresh and Salt water is a
dispersion zone (i.e. not sharp contrast)
Saltwater is denser than fresh water and tends to create
a wedge that move underneath the fresh water when
encroaching in land
Some places are more prone to water intrusion due to
topography, channels, geology and permeability that
facilitate inland progression of saltwater
Salt Water intrusion is a 3D problem
5

6. Theoretical Background – Saltwater Intrusion
Sources: http://kanat.jsc.vsc.edu/student/spatafora/setup.htm //
http://pubs.usgs.gov/fs/2002/fs030-02/
The problem:
If the hydraulic head of the
fresh water decreases, then
the saline water “wedge”
move inland
This can happen because of
excessive pumping
The problem is exacerbated
if there is little recharge of
fresh water and predispose
topography effects
6

7. Theoretical Background – Petrophysics
Sources: http://www.onr.navy.mil/focus/ocean/water/salinity1.htm //
Everett, 2013 // Kirsch, 2008
Characteristics of fresh water Vs. Saltwater:
Different chemistry: more salt dissolve in salt water 
approximately 35 ppt (part per thousand; Sodium Chloride) for
salt water compare to <0.5ppt for fresh
More salt dissolve in salt water implies :
 Higher density of salt water
For geo-electric methods :DC resistivity
 More conductive
For EM induction: Frequency and Time domain EM
 More conductive
For EM wave propagation: GPR
 Same electric permittivity but much greater adsorption
7

8. Theoretical Background – Petrophysics
Let’s have a look at each of them in more details:
 DC resistivity
 Ground Penetrating Radar (GPR)
 Time Domain EM (TEM)
 Magnetotelluric (AMT)
8

9.  Electrical conductivity in
most soil/rock is electrolytic
conduction  ions in pore
fluids are the charge carriers
 When salinity increases,
more charge carriers are
present conductivity
increases
 When porosity increases
more paths are present 
conductivity increases
(when saturated)
9
DC resistivity –Electrolytic conduction and
porosity, salinity
Sources: Everett, 2013

10. 10
DC resistivity –Archie’s law for sand/sandstone
 For porous material, typically sand, gravels, sandstones,
conductivity can be linked to the “geometrical”
structures of the pores Archie’s law
𝜎 = 𝑎. 𝜎 𝑤. 𝑆 𝑤
𝑛
. ∅ 𝑚
0.4 < a < 2
Function of: pore cementation,
tortuosity, grain size, shape,
clay content
0.3< 𝝈 𝒘< 1.0 S/m
Conductivity of the water
0 < 𝑺 𝒘
𝒏
< 1.0 with 𝒏~𝟐
Saturation
1.2 < m < 2.3 for sandstone
Function of grain shape
Porosity
!!! Archie’s law note “designed” for clays !!!
Sources: Everett, 2013

11. 11
DC resistivity –Archie’s law for sand/sandstone
Sources: Schon, 2011
 Some typical values of m parameter can be found in the
literature:

12. 12
DC resistivity –Apparent Resistivity
• In a VES a simple electrical circuit is created with
the earth acting as the “resistance”
• Current is injected in the earth by two current
electrodes (source and sink)
• And a voltage is measured generally in between
the current electrodes by two potential
electrodes connected to a voltmeter
• From the geometry and measurement of I (amp)
and V (volts) an apparent resistivity can be
calculated
Example of one
streamline of electric
current between
point source and
point sink
Sand dry 80 Ohm.m
Sand +Fresh Water 30 Ohm.m
Sand +Ocean Water 5 Ohm.m
Apparent resistivity because refer
to the resistivity of the “volume of
earth” that was sampled by the
streamline of electric current.

13. 13
DC resistivity –Cations Exchange Capacity
 Clay minerals add other high
conducting paths for current
conduction  conductivity
increases when clay particles are
present
 Due to the double layers of
exchange cations [3]:
Loss in the crystal lattice attract
cations in the electrolyte  one
layers is bounded but another one
has charges free to move under an E
field surface conductivity
Sources: Ward, 2004 // Everett, 2013 // Reynolds, 2011
Typical Values:
Clay 1-20 Ohm.m
Sand wet to moist 20-200 Ohm.m

14. 14
DC–What to expect for salt water intrusion
 For Vertical sounding, we expect a sharp decrease of apparent
resistivity from Soil->Fresh water->Salt water:
Sources: http://www.uni-muenster.de/Umweltforschung/medis/restricted/d2d3final.pdf

15. DC–What to expect for salt water intrusion
Sources: Modified from Than,2014 //
http://www.uni-muenster.de/Umweltforschung/medis/restricted/d2d3final.pdf
 For ERT, a highly conductive wedge encroaching inland:
15
Inland  Offshore
Highly conductive
Wedge
 Typical resistivity values: :

16. GPR and moisture content
Sources: Everett, 2013 // Lowrie, 1997
GPR velocity Vs. water saturation:
 To first order , between 10Mhz and 2GHz, velocity is
independent of frequency and conductivity and only
function of 𝜀 𝑟:
𝑽 𝒈𝒑𝒓 =
𝑪
𝜺 𝒓
 Because 𝜀 𝑟 = 81 in water is much greater than in dry
materials 3<𝜀 𝑟<5 for most soil/rocks velocity decrease
with water saturation
 Dielectric constant of an unsaturated to saturated rock/soil is
a composite of the dry 𝜀 𝑟 of the matrix and the volume of
water present in the pores at 𝜀 𝑟 = 81 Topp’s relation
With 𝜀 𝑟 𝑑𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 and C speed of light
16

17. GPR and moisture content
Sources: Topp cited in Everett, 2013
GPR velocity Vs. water saturation: Topp’s Relation
 Variation of 𝜺 𝒓 with saturation is given empirically by the
Topp’s relationship (similar to Archie’s law but for EM):
𝜀 𝑟 = 3.03 + 9.3. 𝜃 𝑤 + 146. 𝜃 𝑤
2
-76.7.𝜃 𝑤
3
with 𝜃 𝑤 = 𝑣𝑜𝑙𝑢𝑚𝑒𝑡𝑟𝑖𝑐 𝑤𝑎𝑡𝑒𝑟 𝑐𝑜𝑛𝑡𝑒𝑛𝑡
Topp’s relation ok in clays and loams, no so good in organic rich soil
Start at 3 And increase with
water content
𝜃 𝑤 =
𝑉 𝑤
𝑉𝑡𝑜𝑡
0 < 𝜃 𝑤<1
17

18. GPR and Water Chemistry
Sources: http://www.wellog.com/resistivity.htm //
Hizem et al., 2008 cited in Everett, 2008 // Hippel, 1954
Impact of saline water on velocity and amplitude
 Saline water in pores increases conductivity 𝜎
Examples: sand with fresh water 𝜌 = 80 𝑡𝑜 120 𝑜ℎ𝑚. 𝑚
sand with saline water: 𝜌 = 2 𝑡𝑜 20 𝑜ℎ𝑚. 𝑚
 Dielectric constant decreases with salinity
Examples: 𝜀 𝑟 can go down to 60 for saline water at 100ppt
 Attenuation of radar wave is function of 𝜎and 𝜀 𝑟:
𝛼~1690.
𝜎
𝜀 𝑟
[dB/m]
• Therefore the amplitude of the EM wave decreases
sharply when in saline water
18

19. GPR and Clays –Impact on salt water mapping
Sources:
http://www.epa.gov/esd/cmb/GeophysicsWebsite/pages/reference/methods/Surface_Geophysical_Meth
ods/Electromagnetic_Methods/Ground-Penetrating_Radar.htm
Impact of clays on amplitude:
 Clays in pores increases conductivity 𝜎 due to the CEC
double layer effects
 Therefore increase attenuation of the radar wave in similar
way as saline water will do  possible misinterpretation of
data
 But overall effect of velocity and amplitude attenuation is
more pronounced on the salt water
19
Vs.

20. GPR -Summary
Sources: Everett, 2013
 Overall, velocity of EM wave decreases with water saturation
because 𝜀 𝑟increases
 Overall, amplitude of EM wave decrease with water
saturation because 𝜎 increases
 Overall, amplitude of EM wave decrease with water salinity
because 𝜀 𝑟 decreases and 𝜎 increases
20
Example with antenna 100Mhz :
Wet clay rich soils->1-2m penetration
Dry Clean Sand and gravels->10-20m penetration

21. GPR –What to expect for salt water intrusion
Sources: Everett, 2013 //
http://www.sensoft.ca/Resources/Case-Studies/Geotech---Environment/Saltwater-Infiltration.aspx
 A progressive velocity and amplitude reduction in the unsaturated
zone toward the fresh water
 When in fresh water , a velocity mainly govern by the fresh water
dielectric constant
 A second and very sharp velocity and amplitude reduction in the
salt water intrusion (signal might even completely vanished)
21

22. 22
Time Domain EM - Principles
Sources: McNeil, 1990 cited in Takam,2014 Everett, 2013
http://www.epa.gov/esd/cmb/GeophysicsWebsite/pages/reference/methods/Surface_Geophysical_Meth
ods/Electromagnetic_Methods/Time-Domain_Electromagnetic_Methods.htm
 Sharpy turning off the current at Tx generate an electromotrice force in
the ground (Faraday’s law) EM induction process
 Currents propagates downward and to the side as a “smoke ring” pattern
diffusion of the image of Tx into the conductive medium
 The smoke ring decay rapidly and generate a secondary magnetic field
measured at the receiver Rx
Tx
Rx

23. 23
Time Domain EM – Apparent Resistivity
 Measurements at successive later time measurements at successive
depth
 From the decay curves (using the late stage) apparent resistivity at
different depths can be calculated
~
DC Vertical electrical Sounding
Sources: Takam,2014 //
http://www.epa.gov/esd/cmb/GeophysicsWebsite/pages/reference/methods/Surface_Geophysical_Meth
ods/Electromagnetic_Methods/Time-Domain_Electromagnetic_Methods.htm

24. 24
TEM–What to expect for salt water intrusion
 Same as DC resistivity : a highly conductive wedge encroaching
inland
 The differences is that TEM:
• Is more sensitive to small changes in conductivity (vary with 𝜎3/2 )
• But is longer to perform on site
• Generally used when can’t do resistivity sounding due to high surface resistance
• Is well suited to detect conductive targets such salt water (sensible to conductive target )
Sources: modified from Al-Garni et al., 2009 // Fitterman, 2014
Highly
conductive
Wedge
2D TEM section built form adjacent TEM soundings

25. 25
AMT–Principles
 Audio Magneto telluric = Far Field frequency domain EM = Tx emits
continuously but the Tx and Rx are separated by a large distance primary
field doesn’t swamp the signal at the RX
 In AMT, the Tx source is a “Natural Passive Source” coming from distant
thunderstorms in the tropical rain belts
 AMT fq range : above 1Hz to about 20Khz but focused on sferics (4-
1000Hz)  depth of exploration from hundreds to kilometres depths
Sources: Milson and Eriksen, 2011 // Nabighian and Corbett, 1988
http://www.geometrics.com/geometrics-products/geometrics-electro-magnetic-products/stratagem/
http://hephaestus.teikav.edu.gr/index.php/ct-menu-item-7/petroleum-engineering/222-amt-caamt
Tx field coming from far away as a plane wave at
grazing incidence with horizontal magnetic field
At Rx measurements are
made of :
• Ex, Ey (porous pots)
• Bx, By (Coils)
Set up need to be
aligned with True North
Direction at the site

26. 26
AMT–Apparent Resistivity
 From the measurement of perpendicular fields Ex (t) and Hy
(t)FFT apparent resistivity can be calculated for different fq:
𝜌 𝑤 =
𝐸 𝑥
𝐻 𝑦
2
.
1
𝑤. 𝜇
 With increasing frequency increasing depths  AMT gives
information about a large range of depths
 Study of the phase shift between Ex and Hy give information on the
resistivity variation with depth:
𝐸 𝑥
𝐻 𝑦
= 𝑤. 𝜇. 𝜌. 𝑒 𝑖𝜃
 Examples:
 Phase shift is 45° for homogeneous medium
 Phase shift is > 45° if resistivity decrease with depth  salt water
intrusion
Sources: http://www.uni-muenster.de/Umweltforschung/medis/restricted/d2d3final.pdf

27. 27
Sources:
http://www.water.wa.gov.au/tools/water+education+tools/resources/wer+107+gnangara+ground+water+
system/default.aspx
Field work background-Perth Basin Aquifers
 Perth Basin aquifers
are from top to
bottom:
1. The superficial =
Gnangara Mound
2. The Mirrabooka
(semi confined, up
to 160m deep)
3. The Leederville
(confined, up to
500m deep)
4. The Yarragadee
(confined, up to
1000m deep)

28. 28
Sources: http://www.water.wa.gov.au/PublicationStore/first/95313.pdf (Hydrogeology Superficial) //
https://www.watercorporation.com.au/home/faqs/saving-water/how-many-litres-of-water-does-the-
average-perth-household-use
Field work background-The Superficial Aquifer
 20km³ of fresh water
 Primary fresh water
supply for Perth City
(60%)
 Also heavily used by:
agriculture, forestry,
market gardens, local
government and
private bore users
 Average person
consumption in Perth
is 132,000L/annum
 But not enough
recharge (less rainfall
since 1960 (dry period
average of
732mm/annum)

29. 29
Field work background-The Superficial Aquifer
Sources: http://www.water.wa.gov.au/PublicationStore/first/95313.pdf (Hydrogeology Superficial)
 Late tertiary to Quaternary sediments up to 90 m thick but
laterally and vertically variable
 Consist of from east to west: Clayey material (Guildford Clay) to
sandy succession (Safety Bay sand, Becher Sand, Bassendean
Sand, Gnangarra Sand, Yoganup formation and Ascot formation)
to Tamala Limestone near the coastline
 Sequence of sand, limestone, silt and clay
 On the coast typically calcareous marine sands and coastal
limestone (Tamala Limestone and safety Bay sands)

30. 30
Sources: http://ga2.er.usgs.gov/coastal/contaminationsc.cfm
http://pubs.usgs.gov/wsp/2331/report.pdf Fitterman and Deszcz-Pan, 2004
Comparable Salt Water intrusion – Hilton Head Island, South
Carolina
Floridan Aquifer System
 Well documented salt water
intrusions
 Surficial aquifers
(Floridan) Superficial
Aquifers (Perth)
 Surficial made of sandy
materials with unconfined
water overlying limestone
 Large pressure head drop
has developped due to over-
pumpingsalt water
intrusions
 TEM used successfully to
map saltwater intrusions
(Fitterman and Deszcz-Pan,
2004)

31. 31
Sources: http://archive.thecalifornian.com/article/99999999/NEWS11/301100055/With-many-California-
aquifers-declining-calls-grow-more-oversight-groundwater //
http://www.seasidebasinwatermaster.org/Other/2011%20Seawater%20Intrusion%20Analysis%20Report.
pdf // http://info.ngwa.org/gwol/pdf/882946800.PDF
Comparable Salt Water intrusion – Salinas Valley, California
 4 aquifers: the “perched”, the
“180 ft”, the “400 ft” and the “900
ft”
 The 180ft is of similar thickness
of the superficial: 50m to 70m
thick
 Time lapsed map since 1944 
show that it is 3D and that
“preferential paths” can exist
 Principally analysed with wells,
water samples and electric
induction logs
 TDEM successfully mapped the
500ppm isochlore interface in
1986 (Mills et al., 1986)

32. 32
The PRAMS Groundwater Model
Sources: Department of Water, 2009
 PRAMS = Hydrogeological
model of the Perth region
aquifers= a vertical flux model
(VFM) + a 13 layer saturated
groundwater model run on the
Modflow software
 VFM = boundary condition
and initial conditions taking
into account the recharge and
usage of water
 Modflow constantly updated
with new information from
VFM and can run “prevision”
scenario  integrated risk
based tool to manage our
water resources

33.  Site is located in the Ern Halliday Reserve, 200m east of
the Indian ocean, in an open land located 1km north of
Hillarys Harbour
33
Field Location
Site
location
Sources: Google Earth, 2014

34. 34
 The site is convenient for geophysics:
 Cleared of vegetation and flat  easier to deploy
geophysical tools (cables, drag the GPR cart, …)
 The site is convenient relative to the hydrogeological
purpose of detecting saltwater interface:
 Situated in a low ground area and not far way from the
coast closer to the targeted salt water intrusion
 The small lake nearby could be an expression of the
Superficial aquifer
 Subsurface certainly made of sand (outcrops of dunes
along the coast)water bearing formation
Field Location

35.  Evaluate various geophysical methods to map salt water
interface along the coastline of Perth (target is the
Superficial Aquifer)  average depth 50m
 Evaluate each methods (advantages and disadvantages of
each method):
 Penetration depth with respect to geology and other
environmental factors such as noise
 Ability to do repetitive survey (time lapse)
 Speed of measurements
 Resolution of each methods
 Ability to map in 3D
35
Objectives
Sources: http://www.water.wa.gov.au/Understanding+water/Groundwater/default.aspx

36.  Acquisition at one location with a square In Loop system
 Tx loop 10x10m and Rx 4x4mloop  target depth
approximately 10m
 As fast as possible turn off ramp to try to get early channels
 information about shallow depth and stronger S/N ratio
 But very noisy site (fences, overhead cable, car engine..)
ramp time automatically calculated by the nanotem was
not good at all  TEM can’t be used here
36
Field Work: TEM - Acquisition
Tx: Early time Transient EM
(Zonge nanotem NT-20)
Rx: Zonge receiver GDP 32-II
Sources: Takam, 2014 // Curtin University Field Trip, 2014

37. 37
Field Work: TEM - Processing
 Extract the raw data from the Rx unit (GDP 32-II)
 Use quality control software to visualize the raw data and clean the
data form outliers. Use Transient plots with B field or
𝑑𝐵
𝑑𝑡
 Example with TEMAVGW from Zonge:
37
Sources: http://zonge.com/instruments-home/software/data-processing/tem-avg/

38. 38
Field Work: TEM - Processing
 1D inversion of TEM data is not trivial  need to use complicated
algorithm and software
 Example with IXID V3 from Interpex:
38
Sources: Meju, 1998
http://www.interpex.com/ix1dv3/ix1dv3_version.htm
1.Import the data
2. Define the
numbers of layers
for the algorithm
3. Run the
inversion for
multiple layers
4. The software
calculate a best fit
apparent resistivity
model to the
collected data

39. 39
Field Work: TEM - Interpretation
39
Sources: Fitterman, 2014
 Since TEM inversion provide an apparent resistivity 1d depth
section similar to DC resistivity interpretation
 At the fresh to salt water interface we expect a sudden decrease in
resistivity values
Decrease in
Resistivity= salt
water interface

40.  One Mala 250 MHz shielded antenna (+receiver) mounted on a sledge
was dragged along the site = Tx and Rx at the same location
 Every data collected is synchronized with a DGPS measurement and a
reading of the wheel odometer
1 data point= 1 EM trace + 1 DGPS location + 1 Odometer reading
 Real time data display (unprocessed) on a portable screen (laptop)
40
Field Work: Radar - Acquisition
DGPS Base Station
Shielded antenna
and receiver
Wheel Odometer
DGPS and Real
time data display
Sources: Curtin University Field Trip, 2014

41.  Typical processing flow with ReflexW
include:
41
Field Work: Radar - Processing
 Similar Process to zero offset
seismic data (be careful though:
not exactly the same; not same
physical principles)
 Be consistent and systematic:
keep the same flow and if
possible the same processing
parameters across the whole
dataset of a common site
Sources: Jol, 2008 // Curtin University Filed Trip , 2014
http://www.cas.umt.edu/geosciences/faculty/sheriff/Equipment_Techniques_and_Cheats/GPR/Processin
g%20Ramac%20GPR%20Data%20with%20Reflex.pdf
 Result of processing is a zero
offset time section <-> two
way travel time for an EM
wave in displacement mode
Dewow the data
Static Correction
Move Startime
Band Pass Filter
Equidistant Traces
Gain Correction (AGC)
Correct 3d topo
Kirschoff Migration
(diffraction or another)

42. 42
Sources: Curtin University Filed trip, 2014
Field Work: Radar - Interpretation
 The zero offset
processed section is an
image of the different
dielectric constant
contrasts of the
subsurface  as
mentioned before we
expect a sudden loss of
signal in the salt water
 In our field trip, the
GPR did not reveal
any sudden loss of
amplitude and
velocity  no salt
water interface
Well marked
reflector, possible
fresh water interface
Zone of hyperbolic
reflectors possible
buried structures

43. 43
Field Work: AMT - Acquisition
Sources: http://www.moombarriga.com.au/default.asp?ContentID=26 // http://www.phoenix-
geophysics.com/products/receivers/mtu/ // Curtin University Field Trip, 2014
 AMT measures time variations in the natural EM fields at the surface of the
earth need to measure both E field and H field in all three directions
 Ex, Ez measure with porous pot electrodes
 Hx, Hy measure with coils
Fluxgate Magnetometers
(magnetic induction coils)
to measure H
Pb/PbCl non
polarisable
electrode to
measure E
Data Loggers
(voltmeter)
Field set up need to
be aligned with true
north

44. 44
Field Work: AMT - Processing
Sources: http://www.moombarriga.com.au/default.asp?ContentID=26
 The time domain recorded E field and H field is FFT
 The ratio of two orthogonal components of the E field and H field gives
information about apparent resistivity at different frequencies <-> at different
depths
 The phase shift between two orthogonal components of E and H also gives
information about the location of conductive bodies
FFT
What is recorded is a time series of all
components of H and E (except Ez
not recorded)

45. 45
Field Work: AMT - Interpretation
Sources: http://www.moombarriga.com.au/default.asp?ContentID=26 //
http://www.impact-structures.com/geophysics-of-impact-structures-2/geoelectric-surveys/
 Calculated apparent resistivity in function of frequencies can be inverted to
produce 1D sounding of the earth resistivity <-> similar to a DC VES
 Resistivity vs. depth can be interpreted to find salt water interface when there
is a decrease of resistivity and typical values of resistivity of salt water
saturated rock/soil

46. 46
 Because we try to record a weak signal (amplitude varying
with strength of events), Rx can be swamped by nearby noise
in city environment (car engines, power line, man made
conductors, …) need to apply filters during acquisition, for
instance 50Hz notch for power lines and to wait for period of
good signal (transient AMT method)
AMT–Problems in city and What to expect
 For detection of shallow salt water intrusion (Perth scenario)
AMT is not a method of choice:
 Depth of investigation with AMT is too deep
 Cultural noise is too intense
 Measurement are very long to perform <-> not practical
 Measuring footprint can be big <-> not practical in heavily
built areas

47. 47
Field Work: DC resistivity - Acquisition
Sources: http://prod-http-80-800498448.us-east-1.elb.amazonaws.com/w/images/e/e1/Schlumberger.jpg
Curtin University Field Trip, 2014
 Schlumberger array collected with
AB/2 = [2,3,4.5,6.5,9,12,15,20,30,45m]
approximately penetration depth of 15m
(rule of thumb)
DC generator with
Ampere meter and
Voltmeter for
measurements

48. 48
Field Work: DC resistivity - Processing
Data collected:
• B constant for this survey
• At every A intervals: ∆𝑉𝑓𝑜𝑟𝑤𝑎𝑟𝑑 𝑉𝑜𝑙𝑡𝑠 , ∆𝑉𝑟𝑒𝑣𝑒𝑟𝑠𝑒 𝑉𝑜𝑙𝑡𝑠 , 𝐼 𝑎𝑚𝑝 is measured and
𝜌 𝑎calculated:
10.00
100.00
1 10 100
Log(ApparentResistivityin
Ohm.m)
Log (Current Electrode Spacing in meters/2)
VES with Schlumberger array at Ern Halliday
Reserve
Schlumberger Array pot spacing 1m

49. 1
10
100
1 10 100
AppRes
AB/2
Calculated Resitivity vs Data
49
Field Work: DC resistivity - Interpretation
Points with less
confidence
 We can see in the data a constant decrease of apparent resistivity with depths
followed by a sharp increase :
1. Water table very close to the surface and conductivity increase from saturated
soil with fresh water to saturated soil with salt water (zone of dispersion , refer to
slide 4)
2. Then current lines reach a saturated rock  resistivity increase
Forward Model
that “match” the
data collected
Topsoil
Sand +
Fresh
water
Sand +
Salt water
Sandstone
PossibleInterpretation
Model Resistivity p Thickness h
Layer 1 30 0.7
Layer 2 15 6.5
Layer 3 4 8
Substratum 2000
Sources: Forward Modelled with Excel Spreadsheet from Kepic,
2014

50. 50
Conclusions
 A geophysical program to detect saline water interface along Perth Coastal and River
margins should be based on the pros and cons of each methods:
Practicality Depth of Penetration Resolution Conclusion Choice
GPR
Fast to deploy. Real time display.
Overall not too affected by
ambient noise (powerlines…). Can
be dragged on rough terrain
(dunes).
Depends on the Tx fq and geological
condicitons but shallower than the
other methods (first 10 metres).
Very good resolution. The salt
water will be well defined by a
sudden loss of signal strength
Best choice to deploy at first
stage of the investigation near
the coast line to confirm the
presence of a salt water intrusion
prior to make a decision whether
a deeper sounding/profilling
method (DC or TEM) will be
necessary
1
TEM
Not easy to set up in a city
environment (need space) and is
disturbed by surrounding
environment that can act as
antennas
Depends on the frequency recorded
by the Rx, but (in general) larger
than with GPR (10's of metres).
Good Resolution and will give
value of conductivity that can
be correlated to salt water
content
As a second stage of investigation
to map conductivity values
2
DC
Need a relatively long straight line
to be able to map at depth (rule of
thumb array length 3x times depth
of penetration. Could be affected
by surrounding environment.
Depends on the DC current
generator strenght, gelogy and array
length, but (in general) larger than
GPR and less than TEM.
Good Resolution and will give
value of conductivity that can
be correlated to salt water
content
As a second stage of investigation
to map conductivity values and
even possibly used ERT system to
get 2D and even 3D geometry of
the intrusion
2
AMT
Not easy to set up in a city
environment (need space and need
to burry the H receivers) and is
disturbed by surrounding
environment that can act as
antennas. Measurenement are slow
to perform (no good coverage per
day)
Very low frequency recorded. Can
go very deep. (100's of metres)
Average resolution but will
map very deep structures. Also
give a value of conductivity
that can be correlated to salt
water content
Can be employed if the previous
methods did not work and we
suspect the interface to be very
deep
4

51. 51
Recommendations
 Salt water interface can be mapped along the Perth Coastline with
geophysical tools
 A program of investigation should consist of different “stages”:
1. Confirm the presence of a salt water intrusion and a rough delineation
of its extent  GPR or FEM. Choice dependant on geological condition,
depth requirement and if down in urban settings or not
2. Better characterise the salinity content by measuring conductivity
properties with DC method or TEM method
3. Possibly use ERT survey to map the intrusion in 2D or 3D
4. Based on the results of geophysics, organize a drilling program to
monitor the salt water intrusion (water samples , saline water interface
water level, wire line logging) with time and possibly collect
permeability measurements to assist with the creation of a flow
modelling to estimate future scenarios of movement of the intrusion
5. If large movement of the intrusion and after flow modelling and on-site
monitoring, necessary repeat geophysical survey to redefine the drilling
program

52. 52
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