Investigation Methods and Procedures - Geotechnical Engineering Investigation - page 46

normally used for deep exploration and for marine studies for profiling, but does not
directly yield velocity data as do refraction and direct techniques.
Seismic Exploration Techniques
Refraction techniques are used to measure compression (P) wave velocities in each geologic stra-
tum, which are indicative of type of material and location of the groundwater table, to esti-
mate the depths of various substrata, and to indicate the locations of faults and large caverns.
Direct techniques provide information on rock-mass characteristics, such as fracture den-
sity and degree of decomposition, and on dynamic soil and rock properties including Young's
modulus, Poisson's ratio, shear modulus, and bulk modulus (Sections 3.5.3 and 3.5.5).
Reflection techniques have been used primarily in marine investigations. They provide a
pictorial record of the sea-bottom profile showing changes in strata, salt domes, faults, and
marine slides. Since velocities are not directly measured, material types and depths of strata
can only be or cannot be inferred unless inferred when correlations are made with other data.
Energy Sources for Wave Propagation
Impact source (hammer or weight drop), used for shallow explorations on land, tends to
generate disproportionately large Rayleigh surface waves, but also produces large P
waves, helpful for engineering studies. Explosives , used for land and subaqueous studies,
convert a smaller portion of their energy into surface waves, especially when placed at
substantial depths below the surface. High-energy spark is used for subaqueous studies. See
also Griffiths and King (1969) and Mooney (1973).
Seismic Refraction Method
General
Seismic refraction techniques are used to measure material velocities, from which depths of
changes in strata are computed. Material types are judged from correlations with velocities.
Basic equipment includes an energy source (hammer or explosives); elastic-wave detec-
tors (seismometers), which are geophones (electromechanical transducers) for land explo-
ration or hydrophones (pressure-sensitive transducers) for aqueous exploration; and a
recording seismograph that contains a power source, amplifiers, timing devices, and a
recorder. Equipment may provide single or multiple-recording channels.
The recorded elastic waveforms are presented as seismograms.
Operational Procedures
Single-channel seismograph operation employs a single geophone set into the ground, a
short distance from the instrument. A metal plate, located on the ground about 10 ft (3 m)
from the instrument, is struck with a sledge hammer (Figure 2.21). The instant of impact
is recorded through a wire connecting the hammer with the instrument. The shock waves
travel through the soil media and their arrival times are recorded as seismograms or as
digital readouts. The plate is placed alternately at intervals of about 10 ft (3 m) from the
geophone and struck at each location with the hammer. Single-channel units are used for
shallow exploration under simple geologic conditions.
Multi-channel seismographs employ 6 to 24 or more geophones set out in an array to detect
the seismic waves, which are transmitted and recorded simultaneously and continuously.
The older seismographs recorded data on photographic film or magnetic tape; and modern
seismographs record digital data in discrete time units. The energy source is usually some
form of an explosive charge set on the surface or in an auger hole at a shallow depth. The
desired depth of energy penetration is a function of the spread length (distance between the