USING BOREHOLE DATA TO ESTABLISH LITHOSTRATIGRAPHIC-GEOPHYSICAL UNITS (LGUS) FOR HYDROGEOLOGIC MODELING, FORT IRWIN, CALIFORNIA

Borehole lithostratigraphy and hydrogeologic testing provide important small-to-medium scale links between geologic framework and hydrogeologic modeling in three volcanic and sedimentary basins at Fort Irwin, California. Lithologic and geophysical logs from 12 boreholes were used to establish lithostratigraphic-geophysical units (LGUs) having similar properties. Lithostratigraphic properties such as composition of rocks or clasts, grain size, and porosity influence geophysical properties such as gamma (G), resistivity (R), sonic velocity (V), and spontaneous potential (SP); the combination of these properties influence hydrogeologic properties such as hydraulic conductivity (K). Cuttings collected every 10 ft were described based on grain size (e.g., sandy gravel), and selected cuttings were described based on type of fragments (e.g., granite, basalt, pumice, glass shards, etc.), the amount of original (e.g., rounded) and fractured grain surfaces, and the type of fine-grained matrix (e.g., lithic-rich tuffaceous pebbly sandstone). Cores (1-4 pieces, 3-5 ft long) were described and measured for physical properties including K. Geophysical data (G, R, V, and SP) were collected every 0.1 ft. The LG stratigraphy can be used at scales from “beds” to “sequences” to “cycles” (such as Geologic Framework Model, GFM) that are based on similar characteristic geophysical data. A bed (or bedset) is ~1-10 ft thick where one or more geophysical properties are the same or have a simple increase or decrease in value. A sequence is ~5-40 ft thick with a series of bedsets having similar bed thickness and range in values. A cycle is 20-200 ft thick consisting of two or more sequences. LG contacts are at changes of one or more geophysical properties, and can be sharp, gradational, or defined by a threshold value (e.g., R<15 Ω·m). Use of beds, sequences, cycles, and GFM in a LG system enables description of relative homogeneity or heterogeneity of properties across a range of length scales (thicknesses), including comparisons between boreholes. LGUs are a useful first-order framework for hydrogeologic borehole testing, and are consistent with Airborne Electromagnetic (AEM) data. One difficulty of LG data in relatively small-diameter boreholes is identifying structural features such as fractures and faults.