Comparative Study of the Refraction Microtremor (ReMi) Method: Using Seismic noise and standard P-wave refraction equipment for
deriving 1-D S-wave profiles
6th SEG-J International Symposium, Tokyo, Japan
Satish Pullammanappallil and Bill Honjas
John N. Louie
J. Andrew Siemens
Hidetoshi Miura
University of Nevada Seismological Laboratory, Reno, Nevada, USA
Siemens & Associates, Bend, Oregon, USA
Terra Corporation, Tokyo, Japan
Optim LLC, Reno, Nevada, USA
2003 Optim LLC
Refraction Microtremor TechniqueMicrotremor Technique
• Based on two fundamental ideas– Standard refraction equipment deployed similar to a
shallow P-wave refraction survey to record ambient noise (microtremor)
• 4.5 to 14 Hz (or higher) vertical geophones depending on application. Depth of penetration is inversely proportional to natural frequency of geophones used.
– Slowness-frequency (p-f) transformation of the recorded microtremor
• Separate Rayleigh waves from other seismic arrivals and allow identifying true phase velocity against apparent velocities
SeisOptSeisOpt®® ReMiReMi™™
2003 Optim LLC
• Disadvantages of commonly used methods– Drilling and logging S-wave velocities (such as OYO logger)
• Expensive & time consuming• Permitting required• Physical restrictions
– Surface methods SASW & MASW• Expensive & time consuming• Specialized recording equipment required• Artificial seismic source required
• Advantages of using SeisOpt ReMi– Data acquisition and analysis takes about three hours– No physical restrictions beyond required 100 m to 200 m line deployment space,
minimal permitting• Data can be acquired along roads, in buildings & at active construction sites
– No specialized recording equipment required• Standard refraction seismograph & refraction P-wave geophones
– No artificial seismic source • Uses ambient noise: “Quiet” site not required
– Can be used offshore as effectively as on-shore
Why SeisOpt® ReMi™
2003 Optim LLC
SeisOpt® ReMi™ Method•Acquire 15-20 seconds microtremor data along a linear array. •Array length depends on depth of investigation – recommended minimum length 100m. Crooked line geometry can be handled.
A(p====p0++++ldp, ττττ====kdt) = ΣΣΣΣ A(x====jdx, t====idt ==== τ+τ+τ+τ+px)
Step 1: p-τ transformation or the Slant Stack operation (Thorson and Claerbout, 1985) of the vertical particle velocity
Step 2: Fourier transformation: p-τ to p-f domain (McMechan and Yedlin, 1981)
F A(p,f ==== mdf) ==== ΣΣΣΣ A(p,ττττ====kdt)ei2π π π π m df kdt
Surface Waves Data Courtesy of Terracon, Las Vegas, NV, USA
2003 Optim LLC
SeisOpt® ReMi™ Method
Step 3: Velocity Spectral Analysis (Louie, 2001): Power spectrum
SA(|p|,f ) = [SA(p,f)]p>=0 + [SA(-p,f)]p<0 : Stotal(|p|,f ) = ΣΣΣΣ SAn(p,f)]
•Lower limit of the apparent phase velocities can be recognized as the true phase velocities (Louie, 2001)
Dispersion Picks
SA(p,f ) = FA*(p,f) FA(p,f)
2003 Optim LLC
SeisOpt® ReMi™ Method
Step 4: Interactive Forward Velocity Modeling
• Avoids ambiguities associated with inversion, while providing the user with ability to include constraints while modeling
2003 Optim LLCCase Study: Oregon State University Geotechnical
Engineering Field Research Test Site
•36 10-Hz geophones with 10 ft spacing deployed along a linear array
p-f Image
2003 Optim LLC
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0300 800 1300 1800 2300
Shear-Wave Velocity, ft/s
Dept
h, ft
Vs, ft/s
Case Study: Oregon State University Geotechnical Engineering Field Research Test Site
•36 10-Hz geophones with 10 ft spacing deployed along a linear array
2003 Optim LLCCase Study: Oregon State University Geotechnical
Engineering Field Research Test Site
•Shear-wave profile from ReMi overlain on refraction velocity model from SeisOpt @2D
2003 Optim LLC
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Shear-Wave Velocity, ft/s
Dept
h, ft
Seismic Cone Penetration Test (SCPT)Circa 1997
Case Study: Oregon State University Geotechnical Engineering Field Research Test Site
•Comparison of SeisOpt ReMi with SCPT at OSU
2003 Optim LLC
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Shear-Wave Velocity, ft/s
Dep
th, f
t
ReMi Analysis September
Spectral Analysis of Surface Waves (SASW) Geocon Northwest Circa 1997
Profile from SCPT measurement
Case Study: Oregon State University Geotechnical Engineering Field Research Test Site
•Comparison of SASW with SCPT at OSU
2003 Optim LLC
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Shear-Wave Velocity, ft/s
Dep
th, f
t
Cross-Hole-99-01
Pre-Grout ReMi, V30=673 ft/s
Post-Grout ReMi, V30=827 ft/s
Grout TreatmentZone
Case Study: Wickiup Dam, Deschutes National Forest, Oregon
•Comparison of Cross-hole and SeisOpt ReMi at Wickiup
2003 Optim LLC
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Period, s
Rayl
eigh
Wav
e Pha
se V
eloc
ity, m
/s
Modeled DispersionPicked Dispersion
Case Study: Loire Estuary, western France
Purpose: Soil classification for harbor extensionData collected by SISMOCEAN SAS, France (www.sismocean.com)
•24, standard hydrophones, 5m spacing deployed along a linear array•Source: 40 cu/in air gun. Hydrophone spread and air gun towed along seabed at 2 knots
2003 Optim LLC
The resulting ReMi S-wave velocity profile revealed boundaries that correlated well with logged cores from drill holes, and also provided
the same soil classification standard as the drill holes.
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Shear-Wave Velocity, m/s
Dept
h, m
V30 = 328 m/s (NEHRP D)
Layer 1: Gravel with fine layer of mud
Layer 2: Fine sand and clay
Layer 3: Fine sand and clay
Layer 4: Bedrock (Gneiss)
Layer boundaries from drill hole data
Case Study: Loire Estuary, western France
Contact Jerome Adamy ([email protected]) for data acquisition and project details
2003 Optim LLC
SeisOpt® ReMi™ Market Applicability
• ReMi Vs profiles can be used for:– Earthquake site response– Liquefaction analysis– Mapping the subsurface and estimating the strength
of subsurface material– Complementing seismic refraction analysis in areas
characterized by near-surface velocity reversals– Finding buried cultural features, such as dumps and
fill material in submerged structures– Determining soil classification for offshore projects
2003 Optim LLC
Conclusion: SeisOpt® ReMi™
• Compares well with previously used 1-D shear wave measurement techniques: Economic, accurate and reliable– Correlates with SCPT measurements
• Detects velocity reversals – Matches average velocities obtained using OYO 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 3 to 4 hours
• Determine shear strength of subsurface material– Concrete, water saturation material, buried fill material, etc.
• Save money in performing seismic site characterization studies– Minimizes number of boreholes required– No permitting required– Can be carried out in urban settings
• Uses ambient noise as seismic energy source– Uses standard seismic refraction recording equipment
• Offshore application– Determine seismic soil classification standards for offshore projects
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