Environmental Engineering Reference
In-Depth Information
Table 5.1.
Application of surface based geophysical methods (adapted from Fell et al., 2000 and
McGuffey et al., 1996).
Type of survey
Applications
Limitations
Seismic
Determines depth to strata and their
May be unreliable unless strata are thicker
refraction
characteristic “P” wave seismic
than a minimum thickness, velocities increase
velocities
with depth and boundaries are regular
Information represents average values
Self-potential
Locates seepage areas and low density
Accuracy affected by presence of ferrous
(SP) profiling
materials. The only technique that
objects. Needs to be accompanied by a
directly indicates subsurface
magnetic survey. Not available “off-the-
water flow
shelf” No “textbook” manuals
Electrical
Locates boundaries between clean
Difficult to interpret and subject to
resistivity
granular and clay strata; assessment
correctness of the assumed subsurface
of degree of saturation, position of
conditions; does not provide any indication
water table and soil-rock interface
of engineering strength properties.
Resistivity imaging may locate
Accuracy affected by presence of ferrous
cavities
objects
Electromagnetic Locates boundaries between clean
Difficult to interpret and subject to
conductivity
granular and clay strata; assessment
correctness of the assumed subsurface
profiling
of degree of saturation, position
conditions; does not provide any indication
of water table and soil-rock interface.
of engineering strength properties. Accuracy
More rapid reconnaissance than
affected by presence of ferrous objects
electrical resistivity
Magnetic
Locates ferrous objects. Used in
Does not locate other metals
combination with self potential and
resistivity surveys
Microgravity
May locate small volumes of low
Use of expensive and sensitive instruments
density materials and larger cavities
in rugged terrain may be impractical
Requires precise levelling.
Ground-
Provides a subsurface profile, locates
Has limited penetration particularly in clay
penetrating
cavities, buried objects, boulders and
materials and laterites
radar
soil-rock interface
-
The geophysical “model” assumes sub-horizontal layering or boundaries. Results may
be misleading where this is not so;
-
The method cannot detect velocity inversions i.e. high velocity material above
low velocity material (unless crosshole or downhole techniques are used, see
Section
5.5.3
);
-
Large energy input (usually explosives) is required in loose material;
-
Accurate ground survey is essential for good results.
A recent development in seismic methods has been Through Dam Tomography (TDT)
where the condition of Bennett Dam in British Columbia was assessed by placing the seis-
mic energy source on the surface of one side of the dam with the geophones on the other
side (Garran, 1999).
5.5.1.2
Self potential
The self potential or SP method is based on the measurement of the natural potential dif-
ference which exists between any two points on the ground. The method has little appli-
cation for new dam construction projects but has been extensively used to locate areas of
leakage from existing dams and reservoirs (Ogilvy et al., 1969; Bogoslovsky and Ogilvy,
1972; Corwin, 1999).
