Several seismic methods have
been developed for the determination of in-situ soil dynamic elastic
moduli. These include seismic cross-hole, uphole, and downhole surveys.
While the cross-hole method is the most exacting of these, for fairly
uniform site conditions (thick, fairly homogeneous soils), the downhole
method offers a fast, acceptable alternative method at significant
savings in terms of the number of required boreholes or specialized
equipment.
The method requires one
borehole per test location. The borehole is PVC-cased and grouted to
ensure the hole remains open and that the casing is in firm contact with
the soil or rock mass. The test consists of lowering a geophone (motion
transducer) to a specified depth in the borehole and clamping it to the
casing. An energy source is placed at the surface, near the top of
casing. Generally, the source is a sledgehammer which is struck
vertically onto a steel plate (compressional or P wave source) or,
alternatively, horizontally onto a steel plate (shear or S wave source).
The source is struck for P wave transmission as well as for S wave
transmission, and travel time from the moment of source initiation until
reception at the geophone is recorded.
The downhole
seismic refraction equipment EMC uses for this type of survey are
a Geometrics Seismograph, Model SmartSeis S-24 and a Geostuff BHG-2
borehole geophone. The SmartSeis is a 24-channel digital recording
seismograph system specially designed for collecting high-resolution
seismic information. (Three channels are used on the seismograph for
recording the three responses from the geophones.)
