Specifications

University of Pretoria etd – Combrinck, M (2006)
inside the transmitter loop. It is impossible to see this behaviour in a conductive half space
or layered earth environment and it therefore implies either an IP effect or extensive lateral
variations (including two- or three-dimensional conductors) in the subsurface.
Table 3-1 Summary of late time approximations and behaviour for four general models.
Model Late time behaviour Decay plot properties
1. Conductive half space
law)(Power
2
5
−
∝
∂
∂
t
t
H
z
Log (time)
m-2.5
=
Log (emf)
2. Thin, conductive layer
law)(Power
4−
∝
∂
∂
t
t
H
z
Log (time)
m-4
=
Log (emf)
3. Confined conductor in
resistive host rock.
al)(Exponenti
τ
t
z
e
t
H
−
∝
∂
∂
tim
e
m-1/
=
τ
Log (emf)
4. Conductor in
conductive host rock.
High contrast:
al)(Exponenti
τ
t
z
e
t
H
−
∝
∂
∂
Low contrast:
law)(Power
v
z
t
t
H
−
∝
∂
∂
Log (time)
Low contrast:
m-v (2.5)
=
High contrast:
emf ∝
e
-t/τ
Log (emf)
Decay curve analysis as described here, although not yielding the complete solution, can be
applied without any user input and therefore has the potential to be fully automated. It is
also very fast compared to inversion and can be applied to data in real time.
23