Prior to conducting any survey, the user should evaluate the objective(s) of the investigation and the
survey site conditions. The survey objective should be carefully considered, as this will determine the
frequency(ies) of operation, the mode(s) of instrument operation and the spatial sampling density
required. The local surficial geology should be investigated in detail, as the on-site geologic conditions
will be a principal factor in determining the effectiveness of the induction method. Site conditions such as
topography, the presence and proximity of surface cultural features and surface vegetation should all be
evaluated. These will affect survey logistics, data quality, and ultimately the survey results.
Survey planning can be broken down into two phases. The first phase involves collecting information on
the site geology, hydrogeology, and the site history, as well as evaluating the physical conditions at the
site and its suitability for conducting an EM survey. Any site conditions which may present obstacles to
the efficient collection of high quality data must be taken into consideration, or dealt with directly prior to
moving on to the second phase, survey grid setup.
EM surveys often have multiple objectives. These may include the detection of buried objects such as
metallic tanks, drums or buried foundations, the detection and delineation of waste burial pits or trenches,
the mapping of the surficial soils and/or overburden materials, the detection and delineation of
contamination plumes in the soil and groundwater or mapping the location and extent of geologic or
archaeological features.
If there are multiple survey objectives, it is generally desirable and more cost effective to plan the survey
coverage so that it will be effective in addressing the most difficult of the survey objectives. The required
level of survey coverage that will provide adequate information to meet the most difficult survey
objective will generally be more than sufficient to meet the requirements of other secondary goals. The
frequency(ies) of operation, spatial sampling density, the mode of operation (i.e., the instrument
geometry) and, in some cases, the speed at which the data is collected, determine the level of survey
coverage and measurement precision.
As we have indicated above, the first phase in survey planning is to acquire as much information as
possible on the geologic and hydrogeologic conditions of the site. This information should be collected
from as many sources as possible. These sources could include maps of the surficial geology of the area
(surficial geology maps of most areas of the US are available from the USGS), as well as data from local
or on-site well logs, test borings and/or test pits. It is also extremely useful to obtain any available reports
of previous geologic or geophysical investigations that may have been conducted at the site. Publications
on the application of EM for similar types of surveys should be carefully reviewed. These studies can
provide valuable insight on data collection methods, data processing and data interpretation.
In urban and semi-urban areas, collecting data on the land-use history of the site (if possible) is also
extremely important.
The review of all available geologic information enables the survey planner to assess the on-site geologic
conditions and their possible affects on the overall effectiveness of the method, to make estimates of the
magnitude of the conductivity contrasts that may be encountered and to make estimates of measurement
sensitivity and probable depths of exploration (DOE).
The land-use history data can provide the survey planner with information on the approximate location
and distribution of extant or historical man-made features. When evaluated within the context of the
