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  1. 1. Student Name: Benjamin Seive Student Number : 17672137 Date submitted: 22/09/2104 1
  2. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 52 References  Al-Garni, M. A., H. M. El-Kaliouby, 2009, Delineation of saline groundwater and sea water intrusion zones using transient electromagnetic (TEM) method, Wadi Thuwal area, Saudi Arabia: Arab Journal of Geosciences, 4, 655-658  Department of Water, 2009, Perth Regional Aquifer Modelling System (PRAMS), model development: Application of the vertical flux model: Hydrogeological record series, report HG 27, Government of Australia  Curtin University field trip, 2014, GPR Data provided  Everett, M. E., 2013, Near Surface Applied Geophysics: Cambridge University Press, First Edition  Fitterman, D. V., 2014, Mapping saltwater intrusion in the Biscayne Aquifer, Miami Dade County, Florida using Transient Electromagnetic Sounding: JEEG, 19(1), 33-43  Fitterman, D. V., M. Deszcz-Pan, 2004, Characterization of saltwater intrusion in south Florida using Electromagnetic geophysical methods  http://archive.thecalifornian.com/article/99999999/NEWS11/301100055/With-many-California-aquifers-declining-calls-grow-more-oversight-groundwater  http://en.wikipedia.org/wiki/Saltwater_intrusion  http://hephaestus.teikav.edu.gr/index.php/ct-menu-item-7/petroleum-engineering/222-amt-caamt  http://prod-http-80-800498448.us-east-1.elb.amazonaws.com/w/images/e/e1/Schlumberger.jpg  http://pubs.usgs.gov/pp/1273d/report.pdf (Meju, M.)  http://seisweb.usask.ca/classes/GEOL335/2013/Lectures/PDF/11-GPR.pdf  http://www.cas.umt.edu/geosciences/faculty/sheriff/Equipment_Techniques_and_Cheats/GPR/Processing%20Ramac%20GPR%20Data%20with%20Reflex.pdf  http://www.eos.ubc.ca/ubcgif/iag/foundations/properties/resistivity.htm  http://www.geometrics.com/geometrics-products/geometrics-electro-magnetic-products/stratagem/  http://www.impact-structures.com/geophysics-of-impact-structures-2/geoelectric-surveys/  http://www.interpex.com/ix1dv3/ix1dv3_version.htm  http://www.moombarriga.com.au/default.asp?ContentID=26  http://www.onr.navy.mil/focus/ocean/water/salinity1.htm  http://www.phoenix-geophysics.com/products/receivers/mtu/  http://www.solinst.com/resources/papers/101c4salt.php  http://www.uni-muenster.de/Umweltforschung/medis/restricted/d2d3final.pdf  http://www.water.wa.gov.au/PublicationStore/first/95313.pdf (hydrological assessment of the Superficial Aquifers, Perth)  http://www.water.wa.gov.au/tools/water+education+tools/resources/wer+107+gnangara+ground+water+system/default.aspx  http://zonge.com/instruments-home/software/data-processing/tem-avg/  https://www.watercorporation.com.au/home/faqs/saving-water/how-many-litres-of-water-does-the-average-perth-household-use  Jansen, J. R, 2011, Geophysical methods to map brackish and saline water in aquifers: Proceedings of the 2011 Georgia Water Resources Conference, held April 11-13, 2011, University of Georgia  Jol, H. M., 2008, Ground Penetrating Radar Theory Applications: Elsevier  Kepic, A., 2014, Excel Spreadsheet to Forward model a simple 3 layers resistivity model: Curtin University Lectures  Kirsch, R., 2008, Groundwater Geophysics, A tool for Hydrogeology: Springer, Second Edition  Lowrie W., 1997, Fundamental of Geophysics, First Edition, Cambridge University Press  Lowrie, W., 1997, Fundamentals of Geophysics: Cambridge University Press, 1st edition  Milson, J. J., A. Eriksen, 2011, Field Geophysics: Wiley, 4th edition  Nabighian, M. N., J. D. Corbett, 1988, Electromagnetic methods in applied geophysics: Society of Exploration Geophysics.  Schon, J. H., 2011, Physical Properties of rocks  Takam, E. Time Domain Responses, Curtin University Electromagnetics Lecture. Bentley. August, 2014  Than, K., 2014, A new way to see saltwater intrusions into Groundwater: Stanford University: https://woods.stanford.edu/news-events/news/new-way-see-saltwater-intrusion- groundwater