Methods :: Terrestrial Geophysics :: Seismic Methods

Seismic methods are the most commonly conducted geophysical surveys for engineering investigations. They provide engineers and geologists with the most basic of geologic data via simple procedures with common equipment.

Any mechanical vibration is initiated by a source and travels to the location where the vibration is recorded. These vibrations are seismic waves, which include compressional wave and shear wave.

Seismic reflection

The seismic reflection technique maps the subsurface stratigraphy based on density and velocity contrasts between earth materials. The seismic wave, generated at the ground surface, travels through the earth and is reflected an interfaces where a change in density and velocity occurs. The reflected waves are detected by a geophone array and recorded by a seismograph.

Seismic refraction

The refraction method uses seismic waves, introduced into the ground by a weight-drop source, to determine the compressional velocity of earth material. The seismic wave changes direction and speed, or refracted, as it propagates through the earth. When the refracted seismic wave impinges on an interface at a critical incident angle, the energy travels along the interface and sends seismic wavelets back to surface. Geophones placed at selected intervals along the ground surface detect the ground motion and send an electrical signal, via a cable, to the seismograph. The seismograph digitizes, amplifies, filters and records the incoming signals. Analysis of the arrival times of the refracted wave provides a means for calculating the seismic velocity and modeling depths to subsurface layers.

Acoustic pipe tracing

This is most commonly done for broken water mains when the precise location of the leak must be determined. Leak noise detectors enable the user to listen for the noise of a leak through a highly sensitive sensor, referred to as a ground microphone. The sensitive microphone detects the noise of a water leak transmitted along pipes, valves, and hydrants and through the ground. Sound vibrations tend to carry farther and more distinctly along the pipelines themselves. The closer you are to the leak, the louder and clearer the leak noise, and the higher the signal displayed on the device’s meter. Once a reading has peaked and starts to diminish as the microphone continues to travel along the pipe, the leak’s location has been determined with a high degree of accuracy. Typically most leak detectors can detect the sound from most any leak that is pressurized at 10 psi or more.

Electroseismic method

The Seismoelectrical method (also called the Electroseismic method) is based on the generation of electromagnetic fields in soils and rocks by seismic waves. Although the method is not reported to detect groundwater flow, it does measure the hydraulic conductivity, which is related to permeability and, therefore, to the potential for groundwater flow. When a seismic wave encounters an interface, it creates a ge separation at the interface forming an electrical dipole. This dipole radiates an electromagnetic wave that can be detected by antennae on the ground surface.

As the seismic (P or compression) waves stress earth materials, four geophysical phenomenon occur:
  1. The resistivity of the earth materials is modulated by the seismic wave;
  2. Electrokinetic effects analogous to streaming potentials are created by the seismic wave;
  3. Piezoelectric effects are created by the seismic wave; and
  4. High-frequency audio and radio frequency impulsive responses are generated in sulfide minerals (sometimes referred to as RPE).
The dominant application of the electroseismic method is to measure the electrokinetic effect or streaming potential (item 2, above). Electrokinetic effects are initiated by sound waves (typically P-waves) passing through a porous rock inducing relative motion of the rock matrix and fluid. Motion of the ionic fluid through the capillaries in the rock occurs with cations (or less commonly, anions) preferentially adhering to the capillary walls, so that applied pressure and resulting fluid flow relative to the rock matrix produces an electric dipole. In a non-homogeneous formation, the seismic wave generates an oscillating flow of fluid and a corresponding oscillating electrical and EM field. The resulting EM wave can be detected by electrode pairs placed on the ground surface.

Surface seismic sources and measurement electrode pairs are generally used to measure the electrokinetic effect. Borehole systems also have been recently developed. A hammer blow or small explosive (black powder ge) are typically used for the seismic source. Two, short (few meters) electrode pairs are typically located collinear and symmetric to the seismic source. Stacking of repeat seismic "shots" is often used to improve signal to noise.

Surface wave

A wide variety of seismic waves propagate along the surface of the earth. They are called surface waves because their amplitude decreases exponentially with increasing depth. The Rayleigh wave is important in engineering studies because of its simplicity and because of the close relationship of its velocity to the shear-wave velocity for earth materials. As most earth materials have Poisson's ratios in the range of 0.25 to 0.48, the approximation of Rayleigh wave velocities as shear-wave velocities causes less than a 10% error. Rayleigh wave studies for engineering purposes have most often been made in the past by direct observation of the Rayleigh wave velocities. One method consists of excitation of a monochromatic wave train and the direct observation of the travel time of this wave train between two points. As the frequency is known, the wavelength is determined by dividing the velocity by the frequency.

The assumption that the depth of investigation is equal to one-half of the wavelength can be used to generate a velocity profile with depth. This last assumption is somewhat supported by surface wave theory, but more modern and comprehensive methods are available for inversion of Rayleigh-wave observations.

Both spectral analysis of surface waves (SASW) and multi-channel analysis of surface wave (MASW) are active surface wave techniques, which measure surface waves generated by dynamic sources such as hammers, weight drops, electromechanical shakers, vibroseis and bulldozers.

Array microtremor and refraction microtremor (REMI) are passive surface wave techniques, which use ambient noise such as ocean wave activity, traffic, factories, wind, etc.  1.425.890.4321 Monroe, WA, USA
© 2008, 2009 Global Geophysics  ·   Powered by  Weave Media Inc