Geoscience Reference
In-Depth Information
An important implication with respect to the issues of investigation depth and feature resolution
(detection) is to use equipment with the proper setup that provides an investigation depth similar to
the investigation depth of interest. Using an equipment setup with an investigation depth substan-
tially greater than the investigation depth of interest results in the minimal size for feature resolution
being increased over what would be the case if the equipment investigation depth coincided with
the investigation depth of interest. Additionally, by using an equipment setup with an investiga-
tion depth substantially greater than the investigation depth of interest, a problem could arise of
not being able to determine whether a detected feature is located within the investigation depth
of interest or at a deeper level. An equipment setup investigation depth substantially less than the
investigation depth of interest means that features positioned between the equipment setup investi-
gation depth and the depth of interest will not be detected. For example, when a resistivity survey
is employed to map lateral changes within a well-developed soil profile, a specific electrode array
length might be chosen to provide an approximate 2 m investigation depth. Significantly shorter or
longer electrode array lengths than that selected for a 2 m investigation depth would respectively
produce investigation depths much less or much greater than 2 m, thereby producing information
that does not include the entire soil profile (short electrode array length problem) or information
where it is difficult to determine whether resistivity changes occurred within the soil profile or at a
greater depth (long electrode array problem). Finally, there are instances where small, deeply buried
features are unlikely to be detected, and therefore, time and expense should not be wasted conduct-
ing a geophysical survey.
1.3.3 f
i e l d
o
P e R a t i o n s
: s
t a t i of n
i
n t e R v a l
, s
t a c k i n g
, s
u R v e y
l
i n e
/g
R i d
s
e t u P
,
a n d
g
l o b a l
P
o s i t i o n i n g
s
y s t e M
(gPs) i
n t e g R a t i o n
The distance is usually fixed or at least fairly consistent from one geophysical measurement location
to the next along a transect, and this distance between measurement locations is referred to as the
station interval. A short station interval provides a better chance for finding the smaller features that
are capable of being resolved with the geophysical equipment used. Reducing the station interval
has the downside of increasing the time needed to conduct a geophysical survey. Consequently, it
makes sense to use the shortest station interval possible that still allows the geophysical survey to
be carried out in the time allotted.
Often, several measurements are collected at each measurement location and then are added or
averaged. This overall process is called stacking. Unwanted signals referred to as noise tend to be
random and can thus be cancelled out by adding or averaging multiple geophysical measurements
obtained at the same location. Although data quality is improved, increased stacking can slow
the geophysical survey. Data collection procedures should be optimized to provide the greatest
amount of stacking possible within the time frame during which the geophysical survey needs to
be conducted.
For a larger subsurface feature where the general directional trend is known, a sufficient num-
ber of geophysical measurement transects should be oriented perpendicular to the feature's trend
so as to better delineate the feature. A measurement transect parallel to a linear subsurface feature,
but offset from it by sufficient distance, will in all probability not detect the feature. A geophysical
survey grid covering a study area is commonly composed of either one set of parallel measurement
transects or two sets of parallel measurement transects oriented perpendicular to one another. Set-
ting up a geophysical survey grid composed of two sets of parallel measurement transects oriented
perpendicular to one another reduces the risk of not finding long, narrow subsurface features, such
as agricultural drainage pipes, whose trend prior to the survey is unknown. The spacing distance
between adjacent transects is usually fixed at some constant or fairly consistent value for a particu-
lar set of parallel transects. This spacing distance should be set small enough, within reasonable
limits, to avoid missing important features.
