1710 W MARINE VIEW DR Geotech Report 2023-12-29

GeoEngineers, Inc. -3 - 111-0920.000 3. METHODOLOGY AND INSTRUMENTATION 3.1 MASW and Microtremor Array Measurements (MAM) Methods The Multichannel Analysis of Surface Waves (MASW) method determines variations in surface wave velocities with increasing distances and wavelengths. The data from these measurements are used to model the shear wave velocities of the subsurface. This information can then be used to infer rock/soil types, stratigraphy and soil conditions. The MASW survey requires a seismic source, to generate surface -waves, and at least 24 geophones, to measure the ground response at increasing distances from the source. Surface waves are a special type of seismic wave whose propagation is confined to the near surface medium. The depth of subsurface penetration of a surface -wave is directly proportional to its wavelength. In a non -homogeneous medium, surface -waves are dispersive, i.e. each wavelength has a characteristic velocity stemming from subsurface heterogeneities. The relationship between surface -wave velocity and wavelength is used to calculate the shear -wave velocity of the medium with increasing depth. MAM survey uses the ambient vibrations. A detailed description of the MAM method can be found in Okada, 2003. MAM arrays generally have a greater degree of flexibility with their design and in addition to linear arrays, can be deployed in 2D arrays such as the circular, triangular, T and L arrays. Since this is a passive survey, the ambient vibrations of the surroundings are utilized rather than deliberately generated. These passive sources can come from all directions and include traffic, ocean waves, cultural noise, and construction. MAM arrays should utilize an array size equal to or greater than the depth of investigation (Geometrics, 2009) and record the ambient vibrations for a minimum of 30 seconds and collect a minimum of 10 minutes of data. ReMi (Refraction Microtremor) surveys are linear MAM surveys, commonly collected with wired geophones and can be quickly set up and executed. These are useful for collecting data in locations where space is limited or where wireless geophones are not practical, such as inside buildings where GPS signals can be poor or non-existent. 3.2 Surface Wave Dispersion Curve Modeling Dispersion curves are useful for determining S-Wave velocities of the subsurface and are generated with the help of specialized software. Data files are added to the software and their traces displayed by location versus time, showing the seismic waves that arrive at each geophone over the course of the record. For MAM surveys, the data are transformed with a fast Fourier transform to the frequency domain. Then the coherence (or similarity between traces or waveforms) is calculated. If the coherences are averaged over a long period of time or over many data blocks, the data is considered to be Spatially Auto -Correlated (SPAC) ( (Aki, 1957). From here, the phase velocity can be calculated from each frequency and fundamental and higher modes can be picked. From the fundamental mode, the dispersion curve can be created and edited Global Geophysics