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Lanbo Liu

Publications and source records attributed to Lanbo Liu.

5 recordsLinked to original sources
USGS2018

Seismic hazard classifications and Vs30 in Connecticut using MASW and HVSR methods

Five new seismic hazard classifications for Hartford County, Connecticut (CT), were proposed by New England State Geologists (NESG) in an effort to improve the current USGS Seismic Hazard Map. These classes were derived from mapped surficial materials, but in situ information is required to verify this approach. Therefore, active and passive surface wave techniques were performed at thirty field sites to determine V S30 and compare the results to the NESG map. Passive data were processed using the Horizontal-to-Vertical Spectral Ratio (HVSR); active data were processed with the multi-channel analysis of surface waves (MASW) technique. The field investigation demonstrated that the surficial material-based system was not sufficient for 66% of the field sites and in-situ velocity information from at least two methods should be considered for improved classification. The geophysical work discussed here represents the first field de-rived V S30 values for Hartford County, CT.

Conference Paper↗
USGS2014

Borehole radar interferometry revisited

Single-hole, multi-offset borehole-radar reflection (SHMOR) is an effective technique for fracture detection. However, commercial radar system limitations hinder the acquisition of multi-offset reflection data in a single borehole. Transforming cross-hole transmission mode radar data to virtual single-hole, multi-offset reflection data using a wave interferometric virtual source (WIVS) approach has been proposed but not fully demonstrated. In this study, we compare WIVS-derived virtual single-hole, multi-offset reflection data to real SHMOR radar reflection profiles using cross-hole and single-hole radar data acquired in two boreholes located at the University of Connecticut (Storrs, CT USA). The field data results are similar to full-waveform numerical simulations developed for a two-borehole model. The reflection from the adjacent borehole is clearly imaged by both the real and WIVS-derived virtual reflection profiles. Reflector travel-time changes induced by deviation of the two boreholes from the vertical can also be observed on the real and virtual reflection profiles. The results of this study demonstrate the potential of the WIVS approach to improve bedrock fracture imaging for hydrogeological and petroleum reservoir development applications.

Conference Paper↗
USGS2009

Marine electrical resistivity imaging of submarine groundwater discharge: Sensitivity analysis and application in Waquoit Bay, Massachusetts, USA

Electrical resistivity imaging has been used in coastal settings to characterize fresh submarine groundwater discharge and the position of the freshwater/salt-water interface because of the relation of bulk electrical conductivity to pore-fluid conductivity, which in turn is a function of salinity. Interpretation of tomograms for hydrologic processes is complicated by inversion artifacts, uncertainty associated with survey geometry limitations, measurement errors, and choice of regularization method. Variation of seawater over tidal cycles poses unique challenges for inversion. The capabilities and limitations of resistivity imaging are presented for characterizing the distribution of freshwater and saltwater beneath a beach. The experimental results provide new insight into fresh submarine groundwater discharge at Waquoit Bay National Estuarine Research Reserve, East Falmouth, Massachusetts (USA). Tomograms from the experimental data indicate that fresh submarine groundwater discharge may shut down at high tide, whereas temperature data indicate that the discharge continues throughout the tidal cycle. Sensitivity analysis and synthetic modeling provide insight into resolving power in the presence of a time-varying saline water layer. In general, vertical electrodes and cross-hole measurements improve the inversion results regardless of the tidal level, whereas the resolution of surface arrays is more sensitive to time-varying saline water layer.

Massachusetts↗
USGS1997

Cross‐hole radar attenuation tomography using a frequency centroid down‐shift method: Consideration of non‐linear frequency dependence of EM wave attenuation

This paper presents a cross-hole radar attenuation tomography method based on analysis of the down-shift in the spectrum centroid frequency, and spectral broadening of the received radar signals. The method uses a parameter that combines centroid frequency down shift and variance increase for the projection function to construct the tomography algorithm. In comparison with other methods for estimating attenuation, the frequency down shift method is relatively insensitive to the effects of geometric spreading, antenna coupling, antenna radiation pattern,and instrument response, but the method requires the data to be broad-band so the frequency shift is easily measured. This method is well suited for difference tomography when electrically conductive tracers are used. The method was tested using cross-hole radar data acquired before and during a saline tracer injection experiment at the U.S.Geological Survey’s Fractured Rock Research Site at Mirror Lake, in Grafton County, New Hampshire. The attenuation-difference tomogram clearly outlines the location of the saline tracer within the tomography plane.

New Hampshire↗