Seismic Refraction and MASW Shear-Wave Velocity Profiling
Shear-Wave Velocity Profiling for Earthquake Hazards Study
Los Angeles, California
AGI completed non-evasive shear-wave velocity (Vs) profiling at an aircraft manufacturing facility in the Los Angeles Basin. A multi-channel analysis of surface waves (MASW) survey was used to estimate a one-dimensional Vs profile of the upper 100 feet of the subsurface. The Vs layering on this profile was used to calculate the average Vs value of the upper 30 meters.
A portable, 50-pound weight drop was used as the active seismic energy source to generate surface waves that were recorded by a fixed-position, 69-meter long receiver array, consisting of 24 4.5-Hertz geophones spaced 3-meters apart. The geophone spikes were planted in sand bags and securely coupled to the asphalt surface of the survey area. The receiver array was connected to a 24-channel seismic recording system to record the surface waves from source points walked away from each end of the receiver array. The entire field procedure was completed in less than half a day.
The resulting MASW field records were processed with the SurfSeis™ software developed by the Kansas Geological Survey. This software was used to separate frequency components of the higher-amplitude surface waves according to their phase velocity for several field records. This combined phase velocity versus frequency image was used to manually pick the dispersion curve for the fundamental-mode Rayleigh waves recorded by the 24-channel geophone array. The resulting phase velocity-frequency image and dispersion curve are shown below. The dispersion curve underwent an inverse modeling procedure to fit a 1D model of Vs layering to the dispersion curve. Possion's ratio was kept constant at 0.4 during the modeling which was a value consistent with the partially-saturated alluviual conditions at this site. The resulting 1D Vs profile shown below was used to estimate an approximate layer-thickness weighted average Vs for the upper 30 meters.
Subsurface Geophysical Surveys for Feasibility Study of Proposed Sea Water Desalination Systemat Santa Rosa Beach
AGI completed multi-profile seismic surveys to help investigate subsurface conditions beneath a beach area where horizontal wells for a desalination system were proposed. Several survey lines were setup across the area to record higher-resolution reflection, refraction, and multi-channel analysis of surface waves (MASW) data. The seismic reflection profiles and the seismic refraction tomography-generated compressional-wave velocity profiles were used to investigate the topography of the bedrock surface. The MASW shear-wave profiles were used to investigate lithologic conditions of the terrace deposits and younger alluvium overlying the bedrock. These profiles were correlated to Geoprobe sampling data to help locate subsurface bedrock channels where horizontal wells could be positioned below the surf zone in thicker deposits of permeable sands. Our report for this project is available at our client's website: Santa Rosa Beach Report.
A 72-channel seismic recording system was used to record the data for each of these profiles. This system was connected to overlapping arrays of 72 geophones setup at 5-foot intervals along the survey lines. The geophones consisted of 4.5-Hertz vertically-aligned velocity transducers.
The combined MASW/Reflection data set was recorded using an “end-on/roll along” recording configuration. A 20-pound sledge hammer was used as the energy source. The energy source was positioned on the south end of 48 active geophone channels, offset 5 feet from the nearest geophone. As the energy source moved down the line at 10-foot intervals seismic recordings were made into overlapping 48 channel geophone arrays. The resulting MASW field records were processed with the SurfSeis™ software developed by the Kansas Geological Survey. This software was used to separate the frequency components of the recorded surface waves according to their phase velocity and generate dispersion curves for the fundamental-mode Rayleigh waves recorded into each 48 channel geophone array. The resulting dispersion curves underwent inverse modeling to fit 1D Vs models to the dispersion curves. The 2D Vs profile along Line 1 shown below was generated by contouring all of the 1D Vs models from the overlapping geophone arrays positioned along the line.
The refraction data were recorded into overlapping 72 channel geophone arrays as the energy source was moved along the line at 40-foot intervals to record higher-density subsurface coverage for refraction tomography imaging of seismic compressional-wave velocity variations using the RAYFRACT™ software. The field records were used to pick the arrival times for direct and refracted waves at each geophone position and generate a set of travel time curves along the survey line. RAYFRACT first generated an initial velocity-depth model using a one-dimensional, smoothed velocity gradient calculated across the line from all the travel time curves. This initial model was then refined to produce a closer fit to the arrival time data using the Wavepath Eikonal Traveltime tomographic inversion method with 100 iterations and a maximum velocity 4,000 m/sec. This best-fit velocity-depth model was then gridded and color contoured with SURFER to show estimated vertical and lateral variations. The resulting compressional-wave velocity profile for Line 1 is show below.