Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 319

New methods for displaying and interpreting EM data
are aimed at transforming the measured response into a
3D conductivity map of the subsurface, which can be
interpreted directly in terms of the geology. This is also a
convenient way of comparing data from the various EM
systems. However, there are limitations in deriving 3D
physical property information from EM data due to the
complexity of the mathematics describing the dispersion of
EM fields in the ground and the necessary simplifying
assumptions about the conductivity structure of the sub-
surface. Moreover, computing the 3D conductivity distri-
bution of the full survey area can take many hours and
sometimes days.

and possibly 3D conductivity block models, using LEI or
CDI (see Section 5.7.4.3 ) . Ideally LEI and CDI need to be
applied to both step and impulse response data (when
available) in order to resolve a wide range of conductivity
variations. Throughout the interpretation process the
interpreter needs to be alert to a number of common
interpretation pitfalls, as described in Section 5.7.6 .

5.7.5.2 Qualitative interpretation
Maps of decay channel amplitudes and the computed
conductivity data can be qualitatively interpreted like
any other geophysical dataset: seeking patterns in the data
defined by changes in amplitude and/or texture that can
be interpreted in terms of the geology. From LEI and
CDI products (see Section 5.7.4.3 ), obvious target
conductors can be identified, variations in the thickness
and conductivity of overburden can be resolved and areas
of

5.7.5.1 Interpretation procedure
Like most types of geophysical data, the interpretation of
EM survey data can be con ned to outlining conductivity

variations related to the various rock formations to assist
geological mapping, or identifying anomalous features
(targeting), or extended to detailed quantitative analysis
of target anomalies to obtain information about their loca-
tion, geometry and conductivity.
The most common interpretation approach involves
identifying various classes of conductors, possibly in the
following order: cultural conductors, topographic effects,
sur cial conductors, formational or regional conductors
and then the bedrock conductors. For most cases in mineral
exploration, all except the last category are forms of environ-
mental noise. Surficial conductors, most commonly conduct-
ive overburden, are usually large in area, producing broad
and strong responses (see Section 5.7.6.1 ) . Formational or
regional conductors are associated with rock units and rock
formations. They typically have large strike extent and a wide
range of conductance and produce a wide range of anomaly
amplitude. They include conductive faults and shears, rock
units such as shales and graphitic zones, and the contacts
between rocks with contrasting conductivities across which
the background response changes. Bedrock conductors are
localised and possibly also steeply dipping with a wide range
of conductivity producing a wide range in anomaly ampli-
tude. They may have large strike extent and may be associ-
ated with formational conductors, whose stronger responses
can mask that of the bedrock conductor. The different types
of conductor can be resolved using a variety of data process-
ing techniques. A summary of the response characteristics
of some forms of conductors is given in Section 5.7.2.6 .
The

ed (where
penetration into the bedrock will be limited). Features
with large horizontal extent, such as formational conduct-
ors, major structures and contacts in the host rocks,
should be reasonably well resolved in the parasections
and depth-slice images. Pro le displays of the multichan-
nel data provide the highest resolution of responses from
compact sources.
The interpreter needs to be aware always that resolution
of the data is strongly dependent on the parameters of the
EM system. If delay times are small enough then it should
be possible to resolve the surface overburden response (see
Late-time measurements in Section 5.7.2.3 and Section
5.7.6.1 ) , and possibly variations within it. Resolution of
smaller features is dependent on the number of channels
recorded and their delay times.
In the case of EM systems where the receiver is separ-
ated from the transmitter (see Moving-loop mode in
Section 5.7.3.2 , and see Section 5.9.2.1 ) , the response of a
conductor is not only asymmetric, but also depends on
which side of the transmitter the receiver is located, relative
to the conductor. Furthermore, for a dipping conductor
the response will be different for each survey direction.
Ideally, adjacent survey lines should be surveyed in the
same direction, but this is not always practical. Where
adjacent lines have opposite directions, anomalies of the
same conductor will be laterally offset across the lines, and
will be affected by the dip direction, creating the herring-
bone effect in image and contour displays. It is most
obvious with laterally continuous conductors. Sykes and
Das ( 2000 ) present some examples.

thick conductive overburden identi

rst step in interpretation is to produce

conductivity - depth parasections and depth-slice images,