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Geologic Log Form Header Section
The essential elements of a quality field boring or coring log include a header section detailing the location and conditions of the borehole site. The header section should include date, time, investigator names, drilling company name, driller names, GPS coordinates, elevation of ground surface above mean sea level (ASL), site description, weather conditions, surface description and any observations of human-made structures or any evidence of environmental contamination. These data are often recorded by hand in a field notebook or on field boring data log sheets for subsequent computer finalization back at the office or laboratory. The remainder of the log is a columnar map or blueprint of the borehole.
Baseline Elevation and Log Scale
Every well or borehole requires a baseline elevation to use as reference for the creation and scaling of the log. There are two main approaches to this requirement. One is to survey the actual topographic elevation of the ground surface or the top of the outer well casing by tying the borehole or casing to a nearby benchmark. The second approach is to make the ground surface or top of the well casing the baseline and assign each underground feature an elevation with respect to the ground surface or top of the well casing.
Overburden Sampling and Logging
Overburden soils above bedrock are sampled by a split spoon and weighted hammer method in which a two-part split tube spoon is pounded into the ground. The number of strikes it takes to drive the split spoon per inch or per foot is recorded to provide data on compaction and penetrability of the soil in the overburden. When the split spoons are retrieved and opened, the soils are sampled and their descriptions recorded in the field boring log inch by inch from the top of the borehole.
Geologic Refusal: Bedrock
Continuous split spoon sampling is used to sample the overburden soils until pounding no longer drives the split spoons any deeper. Cooperation between team members is essential so that at any given moment, the total depth below baseline is known and agreed upon by all. The depth at which repeated pounding no longer drives the spoons any deeper is known as "refusal" and usually indicates a bedrock surface. This depth or elevation is noted on the borehole log.
Rock Coring and Logging
Rotary drilling with a special coring drill bit yields rock core samples from bedrock, which when returned to the surface are described in detail in the field log. In the absence of a rock core, rock cuttings from the drill bit are returned to the surface with return fluids and are sampled for logging purposes. Throughout boring, coring and drilling, detailed descriptions of the material are recorded inch by inch for the entire borehole. These descriptions and any changes are noted on the field boring log.
Finalizing and Archiving Field Boring Logs
Upon completion of field work, samples and field logs are returned to the office or laboratory for finalization and refinement as well as entry into computerized logging programs. Samples are inspected in greater detail using microscopy and chemical analyses. Final logs are presented to the client or primary investigator and are often considered as proprietary. When no longer needed by geologic and environmental consultants or their clients, archived field boring logs and samples are often given to universities and museums for educational purposes.