Environmental Engineering Reference
In-Depth Information
TABLE 2.10
Typical
P
-Wave Velocities of Weathered and Fractured Igneous and Metamorphic Rocks
Material
Grade
a
V
p
(m/sec)
Fresh, sound rock
F
5000
Slightly weathered or widely spaced fractures
WS
5000-4000
Moderately weathered or moderately close fractures
WM
4000-3000
Strongly weathered or close fractures
WH
3000-2000
Very strongly weathered (saprolite) or crushed
WC
2000-1200
b
Residual soil (unstructured saprolite), strong
RS
1200-600
b
Residual soil, weak, dry
RS
600-300
b
a
See
Table 5.21.
b
V
p
(water)
≈
1500 m/sec.
V
p
(min) of saturated soil
≈
900 m/sec.
Geophones on surface
for uphole survey
Downhole
shotpoint
S - shothole
R - recording hole
R
Partially
reflected and
refracted
paths
Geophone: downhole
or crosshole
Direct wave
path
Geophone for
crosshole
or downhole
Shot point for uphole
or crosshole
FIGURE 2.27
Direct seismic methods to measure dynamic properties of soils and rocks, and assess rock mass quality.
A single borehole is used in the uphole or downhole survey; array of usually four borings is used in the
crosshole survey. In uphole surveys, the geophones should be set on rock. If possible, to obtain measurements
of rock quality.
“Sparkers” and “airguns” provide penetrations to depths of 50 to 1000 m or
more. They generally operate in water depths from 10 to 600 m with resolution
capabilities of about 15 to 25 m.
●
Interpretive Information Obtained
A pictorial section beneath the seafloor, or other large water body, is obtained, showing the
general stratigraphy, and such features as slumps and gas pockets when the resolution is
high.
Figure 2.29
is a Bubble Pulser image below a reservoir bottom in about 100 ft of
water. The study purpose was to locate possible rupture zones at the toe of a large land-
slide.
Figure 2.9
is a side-scan sonar image of the area.
Deeper penetration methods, such as sparker surveys, can indicate major geologic
structures such as faults and salt domes (
Figure 2.30)
and massive submarine slides
(Figure 9.57).
