Geoscience Reference
In-Depth Information
5.7.2.5
Step and impulse responses
The step and impulse responses (see
Section 5.7.1.7
)have
the amplitude of the step response (
Eq. (5.24)
)
decreases
with decreasing time constant (
power-law decays (the late-stage decay in the case of
the half-space).
The decay rate is diagnostic of the half-space and con-
ductive layer responses.
•
), i.e. for lower-quality
conductors, for all delay times. It is fairly constant for a wide
range of conductance but it is strongly determined by con-
ductor geometry and size, i.e. large bodies produce strong
responses and small bodies produce weak responses. So
step response data are generally quite intuitive to interpret,
as the response amplitude for targets of a given size and
depth can be predicted. They provide diagnostic informa-
tion about a conductor for a wide range of conductivity.
In contrast, the amplitude of the impulse response
(
Eq. (5.25)
) at early delay times is also strongly determined
by the inverse of
τ
•
For the conductive layer and half-space, the horizontal
(X and Y) components of the
field decay faster than the
vertical (Z) component, a signi
cant diagnostic param-
eter for interpretation.
In resistive environments, confined conductors exhibit a
late-time exponential decay with time constant (
•
τ
).
The value of
depends upon conductor quality, i.e. the
conductivity, shape and size of the conductor.
τ
•
The response of a conductive overburden layer is strong
and decays quickly in early times.
•
The slower late-stage response of the half-space is
observed after the overburden response has diminished.
•
τ
, the amplitude being stronger for bodies
with low
increases. The reverse
occurs at late times where the response is higher for higher
τ
and decreasing quickly as
τ
The slower and weaker exponential decays of con
ned
conductors are observed at late times, after the half-
space response has diminished.
•
.
The impulse response is complexly dependent upon both
conductivity and geometry, making the analysis of impulse
response data more complex. It responds only to conductors
of poor to moderate quality and, significantly, conductors of
very high quality are
τ
The response of a con
ned conductor in a conductive
environment is in
uenced by current channelling which
produces power-law decay.
•
to the impulse response. The
signal-to-noise ratio for good conductors is greater in the
step response than in the impulse response, and for poor
conductors it is greater in the impulse response.
Step response data are required for detecting and char-
acterising high-quality conductors, e.g. large highly con-
ductive zones like massive sulphides, in particular nickel
sulphides. Step response systems can see good conductors
at greater depth, and at greater distance in DHEM, than
impulse response systems. Also, the stronger step response
of good conductors at early times allows greater discrimin-
ation of targets located under weakly conductive overbur-
den. The impulse response complements the step response
by increasing the resolution of conductors of low conduct-
ivity and/or small size.
Note that, amongst other parameters, the ability to dif-
ferentiate bodies of different conductivity is also strongly
determined by the shape of the transmitted pulse and the
system base frequency (see
Section 5.7.3.1
).
'
invisible
'
•
The step response is more indicative of conductor size
than the impulse response.
•
The impulse response is strongly inversely determined
by the time constant.
•
The impulse response only responds to conductors of
poor to moderate quality with the response of
'
conductors being stronger, and greater than that of the
step response.
'
poor
The step response can
good conductors, i.e. those
with very large time constants (
'
see
'
•
τ
), which are
'
invisible
'
to
the impulse response.
Measurements over a wide range of delay time provide
greater ability to discriminate between the various
classes of conductors, and between poor and good
conductors.
•
5.7.3
Acquisition of EM data
Conductors with different electrical properties, different
shapes and orientations, at different depths, occurring in
isolation or near other conductors, and in host rocks with
different conductivities, with or without a conductive
overburden, are optimally detected in different ways by
EM surveys. The ultimate aim of the survey (i.e. whether
it is detection of a discrete conductor, such as an orebody,
or mapping spatial variations in conductivity, such as a
5.7.2.6
Summary of subsurface EM responses
The key characteristics of TDEM responses are summar-
ised as follows:
•
The conductive layer and the homogeneous half-space
are unconfined conductors whose responses exhibit
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