Geoscience Reference
In-Depth Information
of very shallow features would be less distinct. The center-band frequency of the antenna should be
computed so that the wavelength is smaller than one half the size of the smallest target. However,
this does not mean that you will necessarily miss objects that are smaller than twice the size of the
wavelength, because scattering depends upon a number of factors in addition to target size.
The size of the site, the access to the survey area, and the nature of surface features at the site
influence the location and spacing of measurements. A large site area may necessitate the use of
a vehicle to tow the antennas and preclude the possibility of making fixed-mode measurements.
Conversely, site access problems (e.g., confined space) may make it necessary to make fixed-mode
measurements. Surface metallic objects (e.g., fences), buildings, overhead utilities, and underground
objects may influence both the frequency of the antenna that is selected and the location and spacing
of measurements. For example, a small confined area containing a lot of cultural features above the
surface may require the use of a high-frequency antenna that can be shielded from scattered energy
above the surface.
Finally, the desired detail of the output display should strongly influence the survey design. If a
three-dimensional pseudo-image is the desired output, then it is necessary to make measurements at
a very close spacing. However, if the objective is to simply detect a large object, then measurements
at a wide spacing may be adequate.
7.4.2 f
i l t e R i n g
a n d
a
M P l i f i c at i o n
Filtering GPR raw field data is a necessary step to obtaining good data, and all field recording sys-
tems involve some type of analog and digital filtering. This is clearly illustrated by the three traces
shown in Figure 7.7. The first trace is the raw field data, the second trace is the trace with a low-cut
filter, the third trace has had a band-pass filter applied, and the fourth trace is that with band-pass
filtering and amplification. The raw field data (Figure 7.7) do not even look like they contain any
useful information. The low-cut frequency filter, which removes the low-frequency components,
improves the appearance of the trace so that it looks like a GPR trace with positive and negative
polarities situated properly above and below the zero line. The process of applying a low-cut filter in
the early stages of processing is call “de-wowing” by GPR processing practitioners. The final trace
filtering step removes the externally generated high-frequency noise that may be present on the data,
and removes more of the low-frequency components beyond the basic de-wow filtering. Energy that
occurs on a trace at later times can be enhanced by applying amplification to the trace that increases
with time. This amplitude increase as a function of time is called gain. The output trace after apply-
ing these two filtering stages and gain is data that can be used for display and interpretation.
7.4.3 s
t a t i c
c
of R R e c t i of n s
Static corrections consist of applying a time correction to data measured at a different distance from
the object, when the difference in distance is caused by changes in elevation on the surface. The
static correction simply consists of subtracting the travel time for each trace, using the elevation and
velocity of the surface material, as follows:
t
s
= 2
d
/
v
, where
t
s
is the two-way static time correction
and
d
is the elevation between the baseline and the elevation where the trace is measured, as defined
in Figure 7.8. Figure 7.8 also illustrates applying static corrections to field data where the elevation
is changing.
7.4.4 v
e l of c i t y
a
n a l y s i s
a n d
t
w o
-d
i M e n is i o n a l
f
i l t e R i n g
One of the primary features of GPR is the fact that data can be displayed, processed, and interpreted
in two and three spatial dimensions. Data are commonly measured along a line to create a cross
section that is analogous to a seismic cross section. The vertical axis is the two-way travel time of
the pulse that was transmitted and received at each position along the line. If the velocity of the host
