Regolith Geophysics Outside the Square

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Regolith GeophysicsOutside the Square

• Graham Heinson
• Wilsons Promontory RGG, 21st February 2007
• School of Earth and Environmental Sciences,
  University of Adelaide

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Puzzle 1 Ants on a Stick

• 100 ants are dropped on a 1 m stick. Each ant
  travels either to left or right at speed of 1
  m/min. When two ants meet, they bounce off each
  other and reverse direction. When an ant reaches
  an end of the stick, it falls off
• Question over ALL possible initial
  configurations, what is the longest amount of
  time that you would need to wait to guarantee
  that the stick has no more ants?

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The cause is hidden. The effect is visible to
all Ovid Roman poet (43 BC - 17 AD)

• Non-invasive and non- destructive
• Rapid spatial
• Bulk properties or layer boundaries
• (Relatively) low cost
• Value adding

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Hydrogeophysics
Angas Bremer Plains
Conductivity grid 15-20 m interval (CRC LEME
Open File Report 176 A. Fitzpatrick)
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Hydraulic Conductivity
Grain size r
Water density rw, viscosity m
?P
Porosity f
Tortuosity T
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Hydraulic and Electrical Properties

• Saturated hydraulic conductivity K
• Kozeny-Carman equiv. channel equation
• Inverse of hydraulic radius is specific surface
  area per unit volume (Spor)

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Electrical Conductivity?
Somewhat similar but
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Hydraulic and Electrical Properties

• Measured electrical conductivity
• s s (total energy loss term - conduction)
  is (total energy storage term - polarisation)
• s measured with DC resistivity
• s measured with induced polarisation (IP)

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Electrical Measurements
Conduction (s) depend largely on pore-volume
conduction AND pore-surface conduction
effects (PhD students Mike Hatch, Tania Dhu,
Honours students Regis Neroni, Gwendal Fily)
DC


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AC

Induced polarisation polarisation (s) depends
on pore-surface effects (frequency
dependent) (PhD student Lachlan Gibbins)
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Electrical and Hydraulic Scaling
DC
Connectivity of hydraulic and electrical
properties depends on the scale over which it is
measured (fractal dimension) (PhD student Tania
Dhu)
DC
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Hydraulic and Electrical Properties

• For DC resistivity (s) Purvance and Andricevic
  2000) show
• Where pore-volume conduction is most important
  (such as sands), b is positive
• Where pore-surface conduction is most important
  (such as fine-grained clays), b is negative

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Puzzle 2 Monk Mountain Walk

1. A monk walks A ? B at variable speed over 24 hrs
   (midnight to midnight). The monk then returns B ?
   A (midnight to midnight). What is probability
   that there is one unique location at which the
   monk will be in the same place at exactly the
   same time of day?

B
A
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Case Study 1 The Fractal Dimension of Landscapes
500 m
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Fractal Dimension
Conductivity Map
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Discrete Samples
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Case Study 2 Fractured Rock Aquifers
2 m
1 m
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Zones of Influence
Observation well
Electrodes
Pump well
Observation well
Pump well
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Zones of Influence
Drawdown
Observation well
Electrodes
Pump well
Observation well
Pump well
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Electrokinetic Potential

• Electrolyte (Na and Cl- ions)
• Grain boundary double layer
• Excess of Cl- ions in mobile fluid
• Movement of fluid movement of ions

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Electrokinetic method
?V
Conductive electric current
?P ? ?V
Cl-
Cl-
?P
Cl-
Cl-
PhD student Sukhyoun Kim
Advective electric current
Cl-
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Governing Equations
Helmholz-Smoluchovsky Eq.
Darcys Law
?V Electric potential gradient (V/m) ?P
Pressure gradient (Pa/m) ?H
Hydraulic gradient (m/m) ?
dielectric constant (F/m) ? ?
(zeta) potential (V) ? Fluid
viscosity (Pa s) ? Fluid
electrical conductivity (S/m) ? Fluid
density (kg/m3) g Specific
gravity (m/s2) C Streaming
potential coefficient (V/Pa) C Streaming
potential coefficient (V/m)
Q Flow (m3/s) A Area
(m2) K Hydraulic conductivity (m/s) ?H
Hydraulic gradient (m/m)
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Laboratory Apparatus
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Determination of z and C
Sample Q1B (fine sandstone) (Wendouree)
H 1
H 2
H 3
H 4
H 5
? - 40mV C -1.8 mV/m for ? 1470 ?S/m
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Fieldwork Pump Tests
Pump (Bore 4)
Bore 1
Bore 2
Bore 3
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6
Bore 3
5
Bore 1
4
Drawdown (m)
Bore 2
Bore 5
0
50
100
150
200
250
300
350
400
450
500
Time (minutes after start of pumping)
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SP Measurements
Porous pot electrode
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Logarithmic drawdown according to Thiems equation
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Sowhat has this talk been all about?

• Well, the main message is to understand something
  about the Earth we must think laterally,
  sometimes turn problems around!
• Measuring geophysical data to produce a map is
  not always sufficient to interpret these maps we
  have to think how the process reflects the data
  rather than how the geophysical data reflects the
  process.