Geophysics Lecture 8- SP and IP

Question 1

Self potential (SP)

Answer 1

Passive electrical technique

Question 2

What does SP measure

Answer 2

Measurement of naturally occurring potential differences in the ground

Question 3

What does SP exploit

Answer 3

The ability of some large
subsurface bodies (e.g. metal ores)
to build up and store charge in
response to its electrical potential

Question 4

Anomalies- SP

Answer 4

Seek potential “anomalies” at
surface to locate such bodies
Self Potential (SP)
(a.k.a. Spontaneous Polarisation)

Question 5

All terrestrial objects

Answer 5

Sit within an electrical field

Question 6

Electrical charge

Answer 6

Causes potential difference to develop
between opposite faces, and the object to store electrical charge

Question 7

Storage of charge

Answer 7

Capacitance

Question 8

Direction in which charge flows

Answer 8

Defines the polarity if the potential

Question 9

Detectable sources

Answer 9

Groundwater. Objects that straddle the water table can build up strong electrical fields due to electrolytic properties of water

Question 10

Main use of SP

Answer 10

Secondary tool in base metal exploration- best at detecting massive ore bodies

Question 11

Perks of SP

Answer 11

Uncomplication, low cost

Question 12

To measure the ground and not the electrode’s potential

Answer 12

Requires non-polarising (porous pot)
electrodes, e.g. not affected by
current passing through them

Question 13

Base of non polarising electrodes material:

Answer 13

Unglazed pottery,
unvarnished wood, plaster of Paris

Question 14

Copper rod immersed in saturated
copper sulphate (electrolytic)
solution

Answer 14

Makes ionic contact
with the ground through porous base

Question 15

Simply monitor the potential (mV), either as:

Answer 15

The difference from a fixed (reference) point.
The difference between electrodes at a fixed separation.

Question 15

Increasing distance between electrodes:

Answer 15

Measures V as a function of distance, x.

Question 16

Fixed separation between electrodes:

Answer 16

Measures ΔV for a fixed Δx, essentially the gradient (derivative) of V.

Question 17

Leap frog method

Answer 17

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

Question 18

Measured anomalies

Answer 18

Negative in sign – top of
body acts as the negative
pole of the battery

Question 19

Typical size of anomalies

Answer 19

Up to ~1V, but typically a few hundred mV

Question 20

Depth to surface of
causative body:

Answer 20

~half-width of anomaly

Question 21

Ground water- anomalies

Answer 21

Groundwater tends to
produce a small positive
anomaly (10 mV)

Question 22

Induced Polarisation (IP)

Answer 22

Active electrical technique

Question 23

What is IP

Answer 23

Measurement of subsurface
potential response to artificially
induced polarisation of
subsurface electrical field

Question 25

What are IP techniques useful for

Answer 25

Detecting dispersed metal deposits

Question 26

Negatively-charged clay particles

Answer 26

Attract positively-charged ions in electrolyte

Question 27

Mineral grain fills pore and blocks ion flow.

Answer 27

Charge of opposite sign builds up on opposite faces of grain and attracts ions from electrolyte. Effect occurs with dispersed mineral grains.

Question 28

Unimpeded movement of ions when no obstacle

Answer 28

Normal current flow (e.g., what we measure with DC resistivity).

Question 29

Usual IP surveying mode

Answer 29

*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..

Question 30

Environmental uses of IP

Answer 30

*Groundwater exploration * Groundwater and soil contamination * Detection of metal pipes and cables * Cross-borehole imaging e.g. for hydrocarbon contamination