Geology Reference
In-Depth Information
5
ELECTRIC CURRENT METHODS:
GENERAL CONSIDERATIONS
Many geophysical methods rely on measurements of the voltages or mag-
netic fields associated with electric currents flowing in the ground. Some of
these currents are natural, being sustained by oxidation-reduction reactions
in the subsurface (self-potential methods) or by variations in ionospheric and
atmospheric magnetic fields (magnetotellurics). The currents used in resis-
tivity, induced polarisation (IP) and most electromagnetic (EM) methods are
generated artificially.
Currents can be made to flow in the ground by direct injection, by
capacitive coupling or by electromagnetic induction. The term 'galvanic'
is used where currents are either capacitively coupled or are injected via
electrodes. Methods using this second approach are usually referred to as
direct current, or DC, even though in practice the current is reversed at
intervals of between 1 and 0.05 seconds (1 to 20 Hz) to cancel some forms
of natural background noise. In capacitively coupled resistivity (CCR), the
currents, typically with frequencies of between 10 and 30 kHz, are sourced
from coaxial cables that rest on, but are insulated from, the ground. In EM
surveys the currents are driven inductively by time-varying magnetic fields
generated by coils or long wires that are not in contact with the ground.
Because all electrical methods respond to the same intrinsic properties of
materials (resistivity, conductivity and chargeability), and because of over-
laps between them (and because what is noise in one type of survey may be
signal in another), the basic electrical concepts are all introduced in this chap-
ter. DC and CCR methods are then discussed in Chapter 6. Natural potential
(self-potential or
SP
)and
induced polarisation
(
IP
) methods are covered in
Chapter 7, and Chapters 8 and 9 deal with EM surveys using, respectively,
local and remote (often natural) sources. Many modern transmitters and
receivers can be used interchangeably for DC, IP and EM surveys.
5.1 Resistivity and Conductivity
Metals and most metallic sulphides conduct electricity efficiently by flow
of electrons. Electrical methods are therefore important in the search for
