Refraction Microtremor

Refraction Microtremor (ReMi) is a surface-wave seismic method for estimating in-situ Rayleigh-wave (shear-wave) velocities down to depths of 100 meters. Developed by Optim™ of Reno, Nevada, ReMi has a 5 to 15 percent accuracy, with the accuracy decreasing with depth.  Rayleigh waves are surface shear-waves that are also known as “ground roll.”  As the wave passes along the ground, each surface particle moves in a “retrograde elliptical” motion.  This type of motion consists of a roughly circular path in a direction opposite the direction of propagation (see diagram).  With depth, the ellipses become smaller and smaller, until there is no motion.  Lower frequency waves produce elliptical particle motion deeper into the ground.  Surface waves attenuate (decay) in amplitude more slowly than body waves, such as those recorded with seismic refraction, and therefore the ReMi technique can penetrate deeper into the subsurface for a given seismic array.

ReMi Power Spectrum

Testing is performed at the surface using the same seismograph and vertical P-wave geophones as are used to acquire refraction data.  ReMi data are recorded directly before or after the refraction data through the same geophone setup.  The seismic source consists of ambient seismic noise, or microtremors, which are constantly generated by cultural and natural sources.  In addition to the passive source noise, seismic noise can be induced by active sources. The data acquisition procedure consists of obtaining at least ten 30-second seismic noise records at a sample interval of 2 milliseconds.  The result is a 1-D image of the subsurface shear-wave layering below the center of the geophone array.

Applications of Refraction Microtremor

ReMi V_s profiles can be used for:

• Earthquake site response

• IBC site classification based on 100 feet (30m) average shear-wave velocity
• Site amplification maps

• Mapping the subsurface and estimating the strength of subsurface material

• Coupled with P-wave information, one can derive Poisson’s ratio and other engineering parameters

• Complementing seismic refraction analysis in areas characterized by near-surface velocity reversals

• Maps low velocity zones that refraction cannot
• Extends depth of investigation in some cases

• Evaluating compaction quality

Advantages of Refraction Microtremor

ReMi compares well with previously used 1-D shear-wave measurement techniques.

• Economical, accurate, and reliable

• Correlates with SCPT measurements
    ° Detects velocity reversals

• Matches average velocities obtained using an OYO Instruments, LP logger
• Greater depth of investigation compared to borehole and surface methods
• Trends similar to velocity measurements from cross-hole
• Data acquisition and analysis takes about three to four hours

• Determine subsurface properties

• Derive parameters useful for geotechnical engineering
• Determine properties of  buried fill material

• Perform site specific seismic characterization studies efficiently and economically

• Minimizes number of boreholes required
• No permitting required
• Can be carried out in urban settings 
    ° Uses ambient noise as seismic energy source