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Depth of Investigation with IP
Kim FrankcombeExploreGeo
Modern 3D-IP surveying. Practical techniquesand short cuts.
ASEG Conference - Perth 2015
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Can we have both deeppenetration and resolution?
DepthResolution
Signal/NoiseArrayOptimisation
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Signal to NoiseSignal into target below cover
1 10 100 1000 10000
Current Dipole Size vs Depth to Layer 2
0
20
40
60
80
100
Scintrex
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Signal at the target50m of 10 Ohm m overburden over 1,000 Ohm m basement
1 10 100 1000 10000
Current Dipole Size vs Depth to Layer 2
0
20
40
60
80
100
We need a 10,000m currentdipole to get 50% of our
current to basement
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Signal at the target50m of 10 Ohm m overburden over 1,000 Ohm m basement
1 10 100 1000 10000
Current Dipole Size vs Depth to Layer 2
0
20
40
60
80
100
For a 500m current dipoleonly 5% of our current
reaches basement
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Improving Signal to NoiseAssume noise is geological and environmental (e.g. lightning)and not something we can easily change. Not entirely true as
pot selection and planting can effect noise level
PStack longer. Improvement .%&# stacks
PStack smarter
PPut more current into the ground< Reduce the circuit resistance< Increase the Voltage
P Increase voltage resolution of receiver
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NoiseIf the data look like this increasing the voltage
resolution at the receiver should help
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NoiseBut not if the data look like this
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Signal vs NoiseSmarter signal processing?
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Signal vs NoiseTime series from previous slide - 3 different groups - 3 different answers
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Signal vs Noise
PWe can generally afford to stack longer.
PWe definitely need to stack smarter
PWe may be able to reduce our circuitresistance
PWe CAN increase the voltage and powerof our transmitters
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Transmitter Power and VoltageIf only 5% of your current is going into bedrock you want to start with
a big number! Size does matter!
200 kVA, Vmax 10 kV50 kVA, Vmax 4 kV
20 MVA, Vmax 100 kV, 10,000A into a 5kmdipole
2.5 kVA, Vmax 1 kV
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Transmitter Power and Voltage
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Operating limits for real systems - Assuming 100% e fficiency in all cases
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ResolutionResolution is controlled by the electrode placement
Model the response of different arrays to the sametarget using Res3Dmodx64 and Res3Dinv.
Noise added to forward model and inversion data setclipped to remove low Vp readings prior to inversion
Target at a depth of 1300m -potential electrodespacing at surface 100m.
Host 10 mV/V, target 30 mV/V, imagesindependently scaled, contours at 0.5 mV/V
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Forward Modelling Details
Forward modelling was undertaken using Res3Dmodx64 routine developed by Meng Heng Loke
The mesh consisted of 50m x 50m voxels over a 21000m x 2000m areawith 13 layers increasing logarithmically in thickness from 50m to 2162m toachieve a maximum depth of 6487m. All electrodes (current and potential)
were included in the mesh. The mesh size was 420 x 40 x 13
8000 9000 10000 11000 12000
-6000
-5000
-4000
-3000
-2000
-1000
0
Continues at50m spacingto 21000m
Continues at50m spacing
to 0m
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Inversion Details
Noise was added to the forward modelled primaryvoltage as follows:
If Vp > 0.1 mV
Vp(noise) = Vp + RN*Vp*0.1
If Vp # 0.1 mV
Vp(noise) = Vp + RN*0.1
RN = Random number between -0.5 and +0.5
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Inversion Details
The apparent resistivity was then recalculated.
The chargeability had noise added proportionally tothe noise in the primary voltage.
m(noise)= m * Vp(noise) / Vp
The noise adjusted apparent resistivity andchargeability were input to Res3DinvX64 afterclipping the data to Vp(noise) > 0.1 mV
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Specs
Expanding gradient - 100m potential electrodespacing, 6500m to 21000m Tx dipoles.
3D Pole-dipole - 200m potential electrode spacingwith active current electrode in the centre of a 2dipole x 2 dipole square and remote at 10 km.
Offset Dipole-Dipole - 100m Rx dipoles, 200mTx dipoles
All sections are along line 1000 over the top of thebody and use linear colour stretch applied to the
section being displayed with 0.5 mV/V linearcontours
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Expanding Gradient Original Orion Pole-Dipole
DODD Max N=8 DODD Max N=27
None of the above could be considered a success - perhaps not surprisinggiven 100m dipoles and 1300m to top of target!
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DODD Max N=62 New Orion Pole-Dipole
Pole - Pole
Maybe in with a chance by reading to very long offsets but still probably notdetectable in real data. However of the arrays tested long offset DODDappears to offer the best chance.
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Use “Multipoles” to improvepenetration and resolution
From 100m spaced electrodes acquire multipledipole sizes - preferably simultaneously.
The examples below use 100m electrodes toacquire 100m, 200m, 300m and 400m dipolesfor the same array
Expanding Gradient MP1234 DODD MP1234