COMPARISON OF LITHOLOGIC LOGS WITH GEOPHYSICAL LOGS FROM A SAMPLE OF GROUNDWATER WELLS IN CALIFORNIA

In California, groundwater well drillers are required to complete a well completion report that includes a lithologic log that provides a short description of continuous subsurface intervals. These logs vary in the length of the each description (“sand” versus “medium sand with streaks of gray clay”) and in the vertical spacing of descriptions (a description every 10 to 20 feet versus one description every 100 feet). The ability of a log to provide an accurate description of the subsurface also depends on the skills and care of the logger. While these logs have been widely used in construction of geologic frameworks, it is difficult to decide which are the “best” available logs. While a criteria based on the length or vertical spacing of descriptions can be used, these only evaluate data density, and do not directly assess the ability of the logger. Comparison of logs with those from nearby wells can be useful but does not work in stratigraphic units with steep lateral facies changes.

In this presentation, we use borehole geophysical logs, which are occasionally included with well completion reports, to make a direct comparison between the geophysical logs and lithologic logs. Geophysical logs, which contain spontaneous potential, resistivity, and gamma ray readings, can be interpreted in terms of lithology providing an independent description of the same intervals covered by lithologic logs. We present cases from a sample of groundwater wells from the Central Valley where the lithologic logs confuse coarse- and fine-grained intervals, or where key intervals, such as clays, are offset from their true depth. While more accurate, the use of geophysical logs to create stratigraphic frameworks may be limited by the small number that are available, even when supplemented by those recorded in deeper oil and gas wells.