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Geophysics - Vibration & Noise - Rock Mechanics

Providing Quality Geophysical Consulting Services since 1984

Downhole Seismic Velocity Measurement Methods

This method is commonly used to determine compressional and shear wave velocity versus depth.  These velocity data are used to help asses the geology and seismic response of a particular site.  (See the ReMi method for site-specific NEHRP/IBC soil classification without the need for a specially cased boring.)

In a downhole seismic survey, a seismic source is placed on the surface near a borehole, and two tri-axial geophones are placed at selected depths in the borehole. The raw data obtained from a downhole survey are the travel times for compressional and shear waves from the source to the geophones and the distance between the source and geophones.

Downhole Shear Wave Data Acquisition
Downhole shear wave data acquisition
Compressional waves are generated by striking a steel plate with a sledge hammer. The steel plate is located two to five feet from the boring. Shear waves travel slower than compressional waves. Therefore, compressional waves often interfere with shear waves. This interference sometimes makes identification of the first shear wave arrival difficult. To improve the resolution of the shear wave arrival, the seismic source is designed to produce a signal which contains a large shear wave component and a signal enhancement seismograph is used to process the received signals from the geophones. The shear wave source consists of sledge hammer impacts on alternate ends of a 8"x8"x8' wooden beam with steel end plates. The beam is coupled to the ground by weighing it down with the front tires of the field recording truck. The beam is offset a distance of five to ten feet from the borehole to minimize direct coupling of the seismic energy to the casing.

    The downhole sensors consist of two triaxial geophone assemblies. Each assembly contains three sensing elements: one vertical and two orthogonal horizontal elements. The geophone assemblies are separated by a distance of five or ten feet. Two geophone assemblies at a fixed separation are used so that interval velocities can be determined from the same set of impulses. This method reduces timing errors caused by differences in seismic triggering and variations in source impulse characteristics.

    The data are analyzed by determining the interval velocity for each geophone placement. Interval velocity is determined by first computing the distance from the source to each geophone and the difference in arrival times between the upper and lower geophones. The interval velocity is computed by dividing the difference in distance between the geophones by the difference in arrival times. The interval velocity is then plotted as a function of depth. Typical travel time plot and velocity profiles are shown in the figure below:

Downhole Seismic Data