Geoscience Reference
In-Depth Information
Overhead
structures
Other
equipment
Buildings
Vehicles
Walls, hedgerows,
or fences
Trees
Powerlines
Re-bar, or wire mesh
under pavement
fIGURe 6.11
Field features that can interfere with electromagnetic measurements.
Furthermore, structures containing metal are always more problematic than nonmetallic objects;
a building constructed from some types of stone may not create any noticeable interference, but a
building constructed with a lot of steel-reinforcing bar or from rocks containing a lot of magnetite
may cause serious interference for any EM system. A practical guideline is to avoid making mea-
surements any closer to an object than five times the height of the object. A simple field test consist-
ing of running a profile line perpendicular to the potentially interfering feature can determine the
limit of influence of overhead structures.
6.5 pRoCeSSInG, dISplAy, And InteRpRetAtIon
With the exception of metal detectors used for locating buried utilities, EM data must be displayed
and interpreted. The level of sophistication varies from visual identification of anomalies on the
data to more complex modeling and inversion to extract specific features from a buried object of
interest. The following outlines these approaches to data preparation and interpretation.
6.5.1 d
a t a
P
R e P a R a t i o n
a n d
P
R of c e is is i in g
Assuming the interpreter has chosen the type of data to be displayed (e.g., phase, amplitude, ellip-
ticity, etc.), EM data can be displayed effectively as profile lines or two-dimensional maps. A criti-
cal stage in the analysis and processing of geophysical data is their presentation in a manner that
permits the analyst to obtain a comprehensive, integrated view of the data set. The use of contour
maps is based on experience that indicates they are very useful in presenting the spatial and ampli-
tude character of geophysical fields and subsurface features and their relationship to one another.
Two-dimensional displays permit rapid visual identification of patterns, trends, gradients, and other
characteristics of a three-dimensional field. In a similar manner, data observed in one dimension,
along a linear or curvilinear transect, are displayed as profiles with the geophysical variable as the
vertical component and the relative spatial position of the observation site as the horizontal axis.
Machine processing and presentation of digital data, which is the norm, have opened up numerous
