Resistivity Method

Question 1

Resistivity Survey

Answer 1

• Uses Man-made (active) electric field which is applied to
  the ground via a battery
• Measures current passing through soil
• Resistivity of metals = very low because electrons move
  easily through metal
• Resistivity can distinguish between metals
• Not effected by nearby cars
• Cheap and Easy to use
• Can be used to detect a body buried in a moorland
• Sample interval of 1m is unlikely to be enough to be
  capable of defining a grave effectively

Question 2

What is Resistivity and the Resistivity Survey?

Answer 2

• A property of the surface layers of the soil
• This is an active method, it requires a man-made electric
  field to be created.
• Used to look for archaeological structure
• Anomalies can indicate recent burial
• Accuracy in position = 50cm

Question 3

Resistance (R)

Answer 3

• Can be measured by applying a current (I) - in amps and
  measuring voltage (V) - in volts
  R = V / I
• Units = Ω (ohms)
• Depends on size of area being investigated

Question 4

Resistivity (ρ)

Answer 4

- Calculated from values of resistance (R)
           ρ = (R * area) / L
- Units = Ω.m   (ohm meters)
- Conductors = 0
- Insulators = unlimited
- Human tissue = 2Ωm
- Clay/soil = 1-10Ωm
- Sea water = 0.3Ωm
- Porous rocks =  100-1000 Ωm
- Non-porous rocks = up to 106 Ωm
- Copper = 1.7 x10^-8 Ωm

Question 5

Changes in resistivity can be due to …

Answer 5

• Soil type (Infill due to burial)
• Soil density (excavation due to burials)
• Moisture content (decaying bodies)
• Chemical content of water

Question 6

Effects on resistivity in changes in soil

Answer 6

• Decreased density variable ( air and water)
• Increased air pockets increases
• Increased water content decreases
• Decreased salt content increases
• Presence of large rocks increases

Question 7

Measurements in Resistivity Surveys

Answer 7

• Two electrodes distance (L) apart and power supply
  generates current (I)
• V is measured with a voltmeter
• I is measured with an ammeter

Question 8

Four terminal method

Wenner Array

Answer 8

• Common in Archaeology
• V from inner terminals and I from outer terminals
• Better at averaging out variations due to soil
• Simplest of arrays

Question 9

Two terminal method

Answer 9

• Both V and I from same terminals
• Faster
• Can measure on smaller grids

Question 10

The Maths

Answer 10

• A current of 20mA is applied over a distance of 1m and
  gives a voltage of 10V.
  Cross sectional area of 2cm^2

RESISTANCE:
F = 20mA
V = 10 V
R = 10V / 20 x10^-3 A
The resistance (R)  R = V / I = 500Ω

RESISTIVITY:
ρ = (500Ω x 2m^2 ) / 1 m
The Resistivity (ρ) = 1000Ωm

Question 11

Factors that effect the resistivity survey

Answer 11

• Walk electrode arrangement to successive positions
  along grid lines
  = Easy, Cheap, Non-invasive
• Distance L determines the smallest features in survey
  map.
  = 0.5 m grid spacing is necessary in order to detect
  grave-sized anomalies
  = for small value of L, the resistivity is of soil close the
  surface
• Effects due to burial and soil
  = Results different if L is aligned parallel or
  perpendicular to a grave
  = vertical features like pits are less noticeable than
  horizontal features like ditches
  = Sources of noise - variable soil, wires, rough terrain
  = Difficult to detect burials

Question 12

Interpreting the images produced

Answer 12

• White = Low resistance

- Black = High resistance

Question 13

Tomography

Answer 13

• Recent advances in this direction
• Readings are acquired from a large number of electrode
  positions simultaneously
• Computer programs can be used to unscramble the
  contribution of each element
• Greater information results in a 2-dimensional image