Understanding Resistivity and Electrical Resistance in Geologic Surveying, Slides of Geology

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Resistivity Surveying

Electrical Methods

Geologic Resistivity 101

• Resistivity surveying investigates variations of electrical

resistance, by causing an electrical current to flow

through the subsurface using wires (electrodes)

connected to the ground.

• Resistivity = 1 / Conductivity

A multi electrode resistivity survey (^) A close-up of an electrode

But what exactly is “Resistivity?”…

Resistance, Voltage, & Current

• The amount of potential difference required to push a given

current is directly proportional to the “ Resistance ”

• Ohm’s Law:
  • Resistance [=] Ohms (symbol = Ω)
  • But this chapter is about resistivity, not resistance…
  • Resistance, R ≠ Resistivity, ρ (rho)
    • They are related, but are fundamentally different things…

I

V

Resistance = R =

How do we measure resistance? Why does this work?

Resistivity…Finally

• Resistance depends on:
  • The material properties
    • i.e. the resistivity, ρ (so, yes, ρ is a material property!)
  • The shape of the material that has current flowing through it.
    • R = Resistance, a = cross sectional area, l = length
  • Therefore…
    • Resistance is higher when current is forced through a:
      • Small area
      • Long length

a

l

R = ρ

l

a

Or… ρ = R

Resistors in Series

• A “series” circuit has more than one resistor in series

(one after the other)

• Series: all current must travel the same path
• Two or more resistors in series behave like one resistor

with an equivalent resistance, Req of…

Req = R 1 (^) + R 2 Req Rn i

n

=

∑ 1

Or in general…

This rule does not apply to all electrical devices. E.g., capacitors are different