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This paper presents the time history maps of horizontal-to-vertical spectral ratio (HVSR) of 12 long-term microtremor recordings (LTRs) in Northern Mississippi and the coastal area of Gulf of Mexico. These maps show that a stable predominant frequency ( f 0 ) value can be estimated across most of Northern Mississippi where the unconsolidated sediment thickness (UST) is larger than 300 m. On the other hand, estimates of amplification factor, taken as HVSR values at f 0 (HVSR@ f 0 ), display a strong time-dependency, potentially caused by variations of source location and energy level of microseism, i.e. ocean wave activities in Atlantic Ocean and Gulf of Mexico. Validity of this observation is examined by calculating transfer functions between HVSR@ f 0 and ocean data (ocean wave height, ocean wind speed and atmospheric pressure above ocean). Additionally, 3D microtremor spectra of each LTR and those of all STRs within each 100 m-UST group are converted into spatial spectral vectors and projected on stereographic nets. Patterns of the clusters formed by these projections show that the HVSR@ f 0 values are related to both UST and vibration source location and energy level. Finally, a modified HVSR method based on stereographic projection is proposed to determine the average spectral vector of each cluster, which enables a more accurate estimation of amplification factor. [ABSTRACT FROM AUTHOR]

The US Gulf of Mexico coastal region has repeatedly been subjected to major flood events. Local geotechnical site characteristics and geomorphology can change due to sediment transport processes during such events. However, field measurements during extreme conditions are challenging. This paper discusses initial attempts at a combined geotechnical and geophysical site investigation of the uppermost layers of riverbeds following severe flooding events at three different rivers in Texas: the Guadalupe, Brazos, and Colorado Rivers in terms of sediment strength derived from a portable free fall penetrometer, backscatter intensity recorded by a chirp sonar, and soil sample characterization. Results show low strength sediments (<40 kPa) along the investigated sections of the Guadalupe and Brazos riverbanks. Although sediments in the center of the Brazos River were characterized with higher strength (>50 kPa) and larger grain sizes (d 50 ∼ 0.3 mm), sediment strength of the Guadalupe and Colorado Rivers displayed more variations around bridge piers. The spatial variations likely resulted from sediment remobilization processes and local scour under severe hydrodynamic conditions. Both, geotechnical and geophysical results, reflected the observed variations in the riverbed sediments; nonetheless, a quantitative correlation among the rivers was impeded by challenges primarily related to limitations of spatial accuracy and the significant riverbed heterogeneity, as well as shallow water limitations of the chirp sonar. • Combined geotechnical and geophysical site characterization at three Texas river sections • Measurements from small vessels and in areas of unknown riverbed conditions • Sediment dynamics were reflected in geotechnical and geophysical data • Qualitative relationship between portable free fall penetrometer and chirp sonar • Three river sections differed significantly in local sediment properties [ABSTRACT FROM AUTHOR]