Timothy J. Walsh

Geophysics and Geologic Hazards

Sinkholes in Florida pose significant geotechnical, engineering, and hydrogeological challenges for using the land in constructive ways. In some instances, the sinkholes may prove unstable, thus limiting the overburden stress that can be applied. Additionally, the sinkholes may provide a conduit for accelerated contaminant transport from surface activities. In this case study, we use electrical resistivity tomography (ERT) to understand the scope of sinkhole activity under a planned landfill. As part of their application, the landfill permit applicant submitted a dense network of parallel, twodimensional electrical resistivity profiles as described in the following. We provided an alternative, three dimensional analysis of this data set to enhance detection of subsurface sinkhole targets. Eighty five parallel resistivity lines spaced 6m (20ft) apart were coalesced into a large three-dimensional resistivity model to map the 14 hectare (35 acre) site. The results revealed that resistive sand-filled sinkholes could extend at least 30m (100ft) below ground surface with a diameter that ranged from 30 to 100m (100-300ft). The host conductive limestone was shown to have a complex undulating topography with eroded pinnacles. Using cone penetrometer technology (CPT), the edge of the limestone pinnacles were also shown to have significant raveling, which coincided with a narrow range of resistivity values. The implications of the correlation between direct characterization using CPT and indirect characterization with ERT suggest that raveling could cover as much as 17% of the site. Based on these findings, the site was determined to be ill suited for landfill construction.

PASSIVE SEISMIC ANALYSES IN THE LAKE CHAPLAIN 7.5-MINUTE QUADRANGLE, KING AND SNOHOMISH COUNTIES, WASHINGTON

We performed active and passive surface wave methods at four sites in the Cherry Creek fault zone (CCFZ) in Lake Chaplain, Washington to detect the depth to bedrock below Quaternary deposits. The surface wave method was performed at four sites (PS-3, PS-1, PS-4 a nd PS-5) from east to west across the CCFZ with approximately one kilometer separations. We used a multichannel analysis of surface waves (MASW), a passive surface wave measurement using 4.5Hz geophones on a linear array (Linear-MAM) and a passive surface wave measurement using broadband accelerometers (SPAC) to obtain dispersion curves. Dispersion curves obtained by the three different methods are in excellent agreement in ranges of overlap. Maximum wavelengths obtained using the 2ST-SPAC, Linear-MAM and MASW were about 850, 200 and 50 m, respectively. As a rule of thumb, 1/2 to 1/3 of the maximum Rayleigh wave wavelength is indicative of the penetration depth of the surface wave method. The extremely deep penetration capability of the SPAC is obvious when compared to conventional surface wave methods, such as MASW or ReMi™. Phase velocities obtained from three methods were combined to produce a single dispersion curve for each site. The inversion scheme based on Genetic Algorithm was applied to the observed dispersion curves, resulting in the S-wave velocity site profiles. Results from the 2ST-SPAC survey near Sultan, Washington, near the Skykomish River (Figure 1a) provide estimates for the depth to bedrock below Quaternary sedimentary deposits as a companion study to the “Geologic map of the Lake Chaplin 7.5-minute quadrangle, King and Snohomish County, Washington” (Dragovich and others, 2014a). Four seismic survey sites marked PS-1, PS-3, PS-4 and PS-5 (Figure 1a) straddle the main strand of the Cherry Creek fault zone (CCFZ) as shown on Plate 1 of Dragovich and others (2013). Earlier study sites of Sultan B and Sultan C (Hayashi and others, 2014) are also shown for comparison (Figure 1a).

Seismic and Geotechnical Site Characterizations at Four Earthquake Strong Motion Sites in Washington State

As part of on going program for generating maps addressing geologic site effects in Washington, the Washington State Department of Natural Resources (DNR), Division of Geology and Earth Resources (DGER) drilled 30-meter-deep geotechnical boreholes at four strong-motion sites operated by the Pacific Northwest Seismic Network (PNSN). Invasive (soil sampling, Standard Penetration Test (SPT) and downhole seismic) and noninvasive (Multichannel Analysis of Surface Waves, (MASW)) methods were used to determine shear-wave velocity profiles, specifically for the top 100 feet of soil column, at each of these strong motion sites. The boreholes were logged for visual soil classification and SPTs, and selected soil samples were tested in the laboratory to obtain plasticity and gradation values. In addition, S-and P-waves generated at the surface were received by a 3-component geophone placed at 1-m intervals in each borehole. Then the borehole sites were later surveyed by using MASW method to estimate the Vs profiles at each borehole site. The MASW survey shows a very good correlation with the downhole S-velocity profiles. SPT blow counts are consistent with soil conditions in the Puget Sound area. Site classes E, D-E, D, C-D and C, as described in the International Building Codes (IBC 2006), were determined by using average shear-wave velocities and SPT blow count values to 30 meters (~100 feet) of soil column of the four boreholes. An updateable geospatial database incorporating shear-wave data and velocities, borehole geotechnical information (soil samples and their laboratory tests, SPT blow counts, etc.) will be generated and periodically updated. This database will directly be available through the DGER‘s interactive mapping service for end users such as federal and local government agencies, urban planning and emergency response groups and seismic networks, such as PNSN.