Completing Accurate Well Logs
An accurate, complete log of a water well provides a written record about the well that will long survive the memory of the driller or well owner. The log may ultimately be used to make important decisions about the well. The record describes the subsurface penetrated, observations of the well while it was being constructed, and method of construction.
The water well log should indicate the finished depth of the well, whether the well was constructed with a well screen, the length of the screen, the total number of feet of casing that was used, and should describe in as much detail as possible the different rocks that were drilled through. The well log also should identify the depth, thickness, and yield of water-bearing zones.
A simple guide for logging wells is provided by the Pennsylvania Geological Survey. In addition, Hanna (2006) wrote a useful, concise guide for drillers on the methodology of logging water wells. It includes a field guide for selecting color, grain size, and other characteristics that the driller can use in rock descriptions.
Uses of Water Well Logs
Water wells can have or develop problems. A good well log can help to understand and correct the problems. Some examples are:
- Cloudy water/caving. The log may show soft shale or other rock that disintegrated.
- Hard water. The log may show limestone.
- Sulfur. The log may show black shale.
- Iron/manganese. The log may show rock that produces iron/manganese.
- Voids: The log may show caves or old coal mines.
In some wells, a zone producing undesirable water can be cased off. If this is considered to be an option, it is important to know the depths and yields of water-bearing zones, so that the well will continue to produce an adequate volume of water. In other cases, a new well may be the answer.
Well logs can be useful in determining the local and regional depth to bedrock and depth to water. Well casing lengths can be a reasonable indicator of depth to bedrock. Geologists can use well logs to construct contour maps for water depths and geologic structures.
Knowledge of the possible rock types in the area of drilling can be helpful for the driller in planning and drilling at the site. Nevertheless, drill cuttings should be described as they come up from the hole, not as an expected rock type based on a regional map. County geologic maps developed based on the Survey’s 1984 Map 63, Rock Types of Pennsylvania, provide an overview of rock types. Click on our “County rock type maps of Pennsylvania” web page for county maps showing up to 19 different rock types.
Berg, T. M., Edmunds, W. E., Geyer, A. R., and others, 1980, Geologic map of Pennsylvania, 2nd. Ed., Pennsylvania Geological Survey, 4th ed., Map 1, scale 1:250,000, 3 sheets.
Berg, T. M., Sevon, W. D., and Abel, Robin, compilers, 1984, Rock types of Pennsylvania: Pennsylvania Geological Survey, 4th ser., Map 63, scale 1:500,000, web edition.
Hanna, T. M., 2006, Guide for using the hydrologic classification system for logging water well boreholes, NGWA Press, 16p.
Sevon, W. D., 2000, Physiographic provinces of Pennsylvania, 4th edition, Pennsylvania Geological Survey, 4th series, Map 13, scale 1:2,000,000.