Geoscience Reference
In-Depth Information
be calculated from time-distance relationships by taking the two-way travel time values at the maxi-
mum point over the reflection hyperbola (corresponding to
x
= 0, with the corresponding two-way
time
t
0
) and another point (e.g., any distance from the center of the hyperbola, call it
x
1
, and the corre-
sponding two-way time
t
1
) along the reflection hyperbola, and applying the following relationship:
−
( )
−
( )
2
xx
1
0
v
=
2
(7.2)
2
2
t
t
1
0
An analogous field computation of velocity can be obtained over a layer.
7.5
dISplAy And InteRpRetAtIon
7.5.1 d
i is P l a y
P
R o c e is is i in g
Processing and display are an integral part of being able to effectively interpret GPR data. GPR
data can rarely be interpreted without some type of processing to improve the resolution of coher-
ent signals that represent the targets that are the objective of the survey, and a display that enables
the interpreter to easily identify the anomalies that identify the time-space location of the targets.
Processing and display can be conducted in the field, but in many cases, it is more convenient to
process and display data at a later time in the office or laboratory.
Data display is a critical step toward providing an effective interpretation of GPR data. A poor
display masks anomalies, and a good display enhances the target anomalies above the noise and
coherent clutter. The spatial distribution of the field data determines the lateral resolution of the data.
Three-dimensional displays can only be produced if field data are measured on a two-dimensional
grid. The objective of GPR data presentation is to provide a display of the processed data which
closely approximates an image of the subsurface, with the anomalies that are associated with the
objects of interest located in their proper spatial positions. Data display is central to data interpreta-
tion, and an integral part of interpretation.
There are three types of displays of surface data, including a one-dimensional trace, a two-
dimensional cross section, and a three-dimensional display. A one-dimensional trace is not of very
much value until several traces are placed side by side to produce a two-dimensional cross section,
or placed in a three-dimensional block view.
7.5.1.1
two-dimensional displays
The wiggle trace (or scan) is the building block of all displays. A single trace can be used to detect
objects (and determine their depth) below a spot on the surface. By towing the antenna over the
surface and recording traces at a fixed spacing, a record section of traces is obtained. The horizontal
axis of the record section is surface position, and the vertical axis is round-trip travel time of the
electromagnetic wave. A GPR record section is similar to the display for an acoustic sonogram,
or the display for a fish finder. Two types of wiggle-trace cross sections of GPR traces are shown
in Figure 7.10. Wiggle-trace displays are a natural connection to other common displays used in
engineering (e.g., oscilloscope display), but it is often impractical to display the numerous traces
measured along a GPR transect in wiggle-trace form. Therefore, scan displays have become the
normal mode of two-dimensional data presentation for GPR data. A scan display is obtained by
simply assigning a color (or a variation of color intensity) to amplitude ranges on the trace, as shown
in Figure 7.11. Scan displays are generally used for GPR data because of the high data volume (large
number of traces/m).
