Refraction microtremor (ReMi) is a surface-performed

geophysical survey Geophysical survey is the systematic collection of geophysical data for spatial studies. Detection and analysis of the geophysical signals forms the core of Geophysical signal processing. The magnetic and gravitational fields emanating from the E ...

developed by Dr. John Louie (and others) based on previously existing principles of evaluating

surface waves In physics, a surface wave is a mechanical wave that propagates along the interface between differing media. A common example is gravity waves along the surface of liquids, such as ocean waves. Gravity waves can also occur within liquids, at th ...

and in particular

Rayleigh waves Rayleigh waves are a type of surface acoustic wave that travel along the surface of solids. They can be produced in materials in many ways, such as by a localized impact or by piezo-electric transduction, and are frequently used in non-destructive ...

. The refraction microtremor technology was developed at the University of Nevada and is owned by the State of Nevada. Optim of Reno, Nevada has the exclusive license to develop the technology, and SeisOpt® ReMi™ has been available commercially from Optim since 2004. Since Rayleigh waves are dispersive, the propagating waves are measured along a linear seismic array 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 is a geophysical principle governed by Snell's Law of refraction. The seismic refraction method utilizes the refraction of seismic waves by rock or soil layers to characterize the subsurface geologic conditions and geologic str ...

surveys. This equipment consists of a seismograph,

geophone A geophone is a device that converts ground movement (velocity) into voltage, which may be recorded at a recording station. The deviation of this measured voltage from the base line is called the seismic response and is analyzed for structure o ...

s 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.

References

"Faster, Better: Shear-Wave Velocity to 100 Meters Depth from Refraction Microtremor Arrays""Use of refraction microtremor (ReMi) data for shear wave velocity determination at an urban bridge rehabilitation site"
Geophysics {{geophysics-stub