Environmental Engineering Reference
In-Depth Information
Boring
V
0
= 300 m/s
650
Final
roadway
grade
V
1
= 1200 m/s
625
V
1
= 1200 m/s
V
2
= 3000 m/s
V
2
= 2500 m/s
V
3
= 4500 m/s
V
3
= 5000 m/s
Legend:
V
0
= Residual soil above groundwater level
V
1
= Residual soil below groundwater level
V
2
= Moderately weathered and fractured rock
V
3
= Fresh to slightly weathered rock
600
575
500
510
520
530
Station number
FIGURE 2.26
Example of subsurface section prepared from a continuous refraction profile. Velocities are correlated with
rippability as shown in
Table 3.7.
Downshole survey:
The energy source is located on the surface and the detectors incor-
porated in a sonde which is raised or lowered in the borehole, to give either a continuous
or intermittent log of adjacent materials.
Crosshole survey
: The energy source is located in a center test boring and the detectors
are placed at the same depth as the energy source in a number of surrounding boreholes.
Advantages over Refraction Surveys
In the uphole and downhole methods, the influence of reflection and refraction from the
layers surrounding the layer of interest is substantially reduced.
In the crosshole method, the influence of surrounding layers is eliminated (unless they
are dipping steeply) and the seismic velocities are measured directly for a particular stra-
tum.
Crosshole
is the dominant and most useful technique.
Seismic Reflection Method
Application
Seismic reflection surveys obtain a schematic representation of the subsurface in terms of
time and, because of the very rapid accumulation of data over large areas are used in engi-
neering studies, primarily offshore. In recent years, as technology has improved, the
method is being used with increasing frequency to pictorialize stratigraphy in land areas.
The results for a landslide study are shown in
Figure 2.28.
The sliding mass was 3500 ft in
length and the survey was to identify wedge boundaries in the failure zone.
For marine surveys, reflection methods are much more rapid than refraction surveys and,
when obtained from a moving vessel, provide a continuous image of subseafloor conditions.
Operational Procedures
Continuous marine profiling
is usually performed with an electric-sonic energy source gen-
erating continuous short-duration pulses, while towed behind a vessel, in conjunction
with a hydrophone that detects the original pulses and reflected echo signals. The output
