Geophysics Lecture 8- SP and IP Flashcards
Self potential (SP)
Passive electrical technique
What does SP measure
Measurement of naturally occurring potential differences in the ground
What does SP exploit
The ability of some large
subsurface bodies (e.g. metal ores)
to build up and store charge in
response to its electrical potential
Anomalies- SP
Seek potential “anomalies” at
surface to locate such bodies
Self Potential (SP)
(a.k.a. Spontaneous Polarisation)
All terrestrial objects
Sit within an electrical field
Electrical charge
Causes potential difference to develop
between opposite faces, and the object to store electrical charge
Storage of charge
Capacitance
Direction in which charge flows
Defines the polarity if the potential
Detectable sources
Groundwater. Objects that straddle the water table can build up strong electrical fields due to electrolytic properties of water
Main use of SP
Secondary tool in base metal exploration- best at detecting massive ore bodies
Perks of SP
Uncomplication, low cost
To measure the ground and not the electrode’s potential
Requires non-polarising (porous pot)
electrodes, e.g. not affected by
current passing through them
Base of non polarising electrodes material:
Unglazed pottery,
unvarnished wood, plaster of Paris
Copper rod immersed in saturated
copper sulphate (electrolytic)
solution
Makes ionic contact
with the ground through porous base
Simply monitor the potential (mV), either as:
The difference from a fixed (reference) point.
The difference between electrodes at a fixed separation.
Increasing distance between electrodes:
Measures V as a function of distance, x.
Fixed separation between electrodes:
Measures ΔV for a fixed Δx, essentially the gradient (derivative) of V.
Leap frog method
Saves time when measuring potential difference between pairs of equally-spaced electrodes
Leap one electrode over the other, then reverse the sign of the signal measured
Measured anomalies
Negative in sign – top of
body acts as the negative
pole of the battery
Typical size of anomalies
Up to ~1V, but typically a few hundred mV
Depth to surface of
causative body:
~half-width of anomaly
Ground water- anomalies
Groundwater tends to
produce a small positive
anomaly (10 mV)
Induced Polarisation (IP)
Active electrical technique
What is IP
Measurement of subsurface
potential response to artificially
induced polarisation of
subsurface electrical field
What are IP techniques useful for
Detecting dispersed metal deposits
Negatively-charged clay particles
Attract positively-charged ions in
electrolyte
Mineral grain fills pore and blocks ion flow.
Charge of opposite sign builds up on opposite faces of grain and attracts ions from electrolyte. Effect occurs with dispersed mineral grains.
Unimpeded movement of ions when no obstacle
Normal current flow (e.g., what we measure with DC resistivity).
Usual IP surveying mode
*Dipole-dipole array with porous pot electrodes to
measure potential
* Fix spacing, a, between current and potential
electrode pairs
* Increase spacing between the current and potential
dipoles to sample deeper, i.e. n = 1, 2, 3..
Environmental uses of IP
*Groundwater exploration
* Groundwater and soil contamination
* Detection of metal pipes and cables
* Cross-borehole imaging e.g. for hydrocarbon contamination
