Refraction Microtremor (ReMi) is a method Geotechnology uses to determine shear wave velocity profiles to a depth of approximately 100 feet (per International Building Code 2003). ReMi surveys utilize the dispersive property of surface waves and are conducted by passively recording background surface wave "noise" such as the vibrations generated by passing vehicles, airplanes and trains. Sometimes additional noise is added by initiating impacts (via sledgehammer) at the ground surface. The surface waves are recorded using a seismic system comprised of geophones, cables and a seismograph. Shear wave velocity profiles are constructed from this data.

Geophones (above) are set up at intervals to receive surface waves (or microtremors) produced by random traffic and sledgehammer impacts (second photo). The surface wave energy is recorded using a seismograph (last photo) and later processed and modeled to determine shear wave velocity profiles.

ReMi Benefits

This data can provide valuable information to structural engineers for design of new and expanded structures in all seismic zones such as St. Louis and the New Madrid Fault. The building code provides classifications based on seismic risk. These classifications can require expensive design and construction requirements. The shear wave assessment is considered more representative of seismic response. It can provide information that allows an owner to receive a less conservative seismic classification than an assessment from boring data alone, saving potentially hundreds of thousands of dollars in design and construction costs.

Because ReMi is non-invasive and nondestructive, and uses ambient noise as a seismic source, surveys can be conducted in highly urbanized areas and no permits are required. ReMi seismic lines can be deployed within road medians, at active construction sites or along highways, without disturbing work or traffic flow.

ReMi Results - Shear Wave Velocity and IBC Classification

Structural designers following the International Building Code (IBC 2003) must take into consideration the site classification for structure protection. The IBC site classification can be determined from site specific shear wave velocity measurements using methods such as ReMi, or estimated from a subsurface exploration using standard penetration tests (N) or soil undrained shear strength (su).

The graph below demonstrates how direct shear wave measurements often result in an improved IBC site classification when compared with a subsurface exploration. Thus ReMi surveying results can save the owner significant costs by requiring less conservative seismic designs and construction requirements.