QUATERNARY FRAMEWORK OF THE LOS ANGELES BASIN: SEDIMENT PHYSICAL PROPERTIES IN THE LONG BEACH AREA

As part of the investigation of the Quaternary stratigraphy, tectonics and hydrogeology of the Los Angeles basin, physical properties measurements and electric log data were acquired from two continuously cored boreholes (LWEB and LBPC) situated in the Long Beach area. The onsite measurements included bulk density (r _b), compressional velocity (V_p), magnetic susceptibility, and thermal conductivity (l). Selected whole-core samples were preserved for laboratory measurements of hydraulic conductivity, bulk density, grain density, porosity (f), and thermal conductivity anisotropy. These core measurements were augmented with surface electrical surveys and a diverse suite of wireline logs, including resistivity, spontaneous potential, electromagnetic soundings, spectral gamma ray, compressional and shear velocity (V_s), nuclear magnetic resonance, borehole televiewer, and formation microimager.

Preliminary analyses indicate that the primary controls on the variability of physical properties in these young sediments are mineralogy and grain size, although the effects of cementation, depth of burial and fluid composition are also significant. In general, fine-grained sediments recovered from both coreholes (primarily clayey silts and silty clays) are characterized by higher f (~40%) and lower r _b (~2.0 g/cm³), V_p (~1600 m/s), V_s (~500 m/s) and l (~1.5 W/m.K) while coarse-grained sediments (primarily sands and gravels) are characterized by lower f (~30-35%) and higher r _b (~2.15 g/cm³), V_p (~2000 m/s), V_s (~700 m/s), and l (~1.7 W/m.K). An exception to this is found in a partially gas-saturated, fine-grained unit encountered below 250 meters depth in LBPC, in which r _b, f and l follow the mineralogy and grain-size trend noted above while both V_p and V_s remain at relatively high values. Ongoing investigations should clarify these relationships and determine if the observed changes are due to the presence of gas in situ or to other factors.