Vertical seismic profiles are the most cost-effective and easiest borehole seismic technique. An external seismic energy source, such as a hammer striking a plate, is applied at a fixed distance from the borehole. An array of special geophones is inserted into the borehole and the one-way travel time of the initial arrival of the generated seismic energy is measured.
Shear-wave surveys may be conducted using a tri-axial geophone that is coupled to the borehole wall at various depths within the borehole. Shear wave energy is generated by striking the ends of a large wooden block that is held in place by a vehicle or heavy weight. By striking both ends of the wooden block, both positive and negative shear waves are generated, which enables the seismic interpreter to distinguish the shear wave arrivals from compressional wave and extraneous signal noise.
Cross-hole seismic methods measure the one-way travel time of seismic energy transmitted between boreholes to determine the bulk material properties of the intervening materials. An external seismic energy source is used within one borehole, and arrays of receiving geophones are lowered into near-by boreholes. The one-way seismic wave travel times are based on the initial arrival of energy at each geophone. Knowing the separations of each borehole and the depths of the external seismic source and each receiver, the apparent compressional seismic velocities may be calculated for each depth interval. These data are then used to derive a profile of the intervening geological boundaries, as well as generate detailed information on bulk rock properties such as rippability.
In a similar manner to standard cross-hole seismic surveys, cross-hole tomography measures the one-way travel times of seismic waves emanating from an external source placed with a borehole to an array of geophone receivers placed in a secondary hole or holes. For each seismic shot position, acoustic energy travel times are collected along the entire length of the receiver borehole(s). This gives more coverage than is obtained through crosshole seismic profiling. By varying the depth of the external seismic source, a dense network of overlapping raypaths is constructed which is then used to construct a highly accurate velocity profile between the boreholes.
The results of a cross-hole tomography survey enable the identification of anomalous velocity zones that may exist between boreholes, as well as imaging of individual velocity layers. The primary application of cross-hole seismic tomography is for the civil and environmental engineering fields in the detection of fault zones and voids. Seismic tomography in concert with shear-wave seismic surveying can also be used to provide information on material stiffness properties.
The Parallel Seismic (PS) method is a borehole test method primarily used to determine the depths of foundations. However, it can also be used to measure the thickness of the scour zone when it has been filled with mud or soft sand after the flood surge has passed.
Parallel Seismic tests can be performed on concrete, wood, masonry, and steel foundations. Some portion of the structure that is connected to the foundation must be exposed for the hammer impacts. A borehole is also required.
The PS method involves striking any part of the exposed structure that is connected to the foundation (or hitting the foundation itself, if accessible) and using a hydrophone or a three-component geophone to record compressional and/or shear waves traveling down the foundation. Analysis of the PS data is performed in the time domain. In PS tests, reliance is placed on identifying direct arrival times of compressional and shear waves at the receiver locations, as well as the wave amplitudes
