Refraction microtremor
Encyclopedia
Refraction microtremor is a surface-performed geophysical survey
developed by Dr. John Louie (and others) based on previously existing principles of evaluating surface waves and in particular Rayleigh waves. Since Rayleigh waves are dispersive
, the propagating waves are measured along the seismic traverse and evaluated relative to wave frequency and slowness (or the inverse of the velocity). Due to the dispersive characteristics of higher frequency waves travelling through the more shallow conditions and lower frequency waves passing through deeper materials, a 1-D subsurface profile can be generated based on the velocity with depth.
Equipment and Field Procedures
The method utilizes equipment typically employed in seismic refraction
surveys. This equipment consists of a seismograph, geophones
placed in an array, and a seismic source. An array consists of 6 to 48+ geophones placed at some interval along the ground surface to measure the propagating waves and are connected to a seismograph that records the data. Rayleigh waves are generated from seismic sources described to be active, passive, or a combination of both. For example, active sources can be generated sources from a sledge hammer striking a plate or other sources that are created near the traverse. Passive sources can be nearby highway traffic, construction equipment working in the distance, etc. Several recordings (typically 15 to 60+ seconds long) are captured and stored for later analysis. As multiple traverses are performed along the ground surface, the 1-D profiles can be compiled to generate a 2-D profile of the subsurface conditions. It is worth noting that since seismic traverses typically "average" conditions along the traverse length to some extent, the expectations and limitations of geophysical methods should be realized.
Geophysical survey
Geophysical survey is the systematic collection of geophysical data for spatial studies. Geophysical surveys may use a great variety of sensing instruments, and data may be collected from above or below the Earth's surface or from aerial or marine platforms. Geophysical surveys have many...
developed by Dr. John Louie (and others) based on previously existing principles of evaluating surface waves and in particular Rayleigh waves. Since Rayleigh waves are dispersive
Dispersive
*Dispersive partial differential equation*Dispersive phase from Biological dispersal*Dispersive medium*dispersive line*dispersive mass transfer*dispersive power*dispersive fading...
, the propagating waves are measured along the seismic traverse and evaluated relative to wave frequency and slowness (or the inverse of the velocity). Due to the dispersive characteristics of higher frequency waves travelling through the more shallow conditions and lower frequency waves passing through deeper materials, a 1-D subsurface profile can be generated based on the velocity with depth.
Equipment and Field Procedures
The method utilizes equipment typically employed in seismic refraction
Seismic refraction
Seismic refraction is a geophysical principle governed by Snell's Law. Used in the fields of engineering geology, geotechnical engineering and exploration geophysics, seismic refraction traverses are performed using a seismograph and/or geophone, in an array and an energy source...
surveys. This equipment consists of a seismograph, geophones
Geophone
The term geophone derives from the Greek word "geo" meaning "earth" and "phone" meaning "sound".A geophone is a device which converts ground movement into voltage, which may be recorded at a recording station...
placed in an array, and a seismic source. An array consists of 6 to 48+ geophones placed at some interval along the ground surface to measure the propagating waves and are connected to a seismograph that records the data. Rayleigh waves are generated from seismic sources described to be active, passive, or a combination of both. For example, active sources can be generated sources from a sledge hammer striking a plate or other sources that are created near the traverse. Passive sources can be nearby highway traffic, construction equipment working in the distance, etc. Several recordings (typically 15 to 60+ seconds long) are captured and stored for later analysis. As multiple traverses are performed along the ground surface, the 1-D profiles can be compiled to generate a 2-D profile of the subsurface conditions. It is worth noting that since seismic traverses typically "average" conditions along the traverse length to some extent, the expectations and limitations of geophysical methods should be realized.