USGS_GeophysicalLoggingInSLV_Report

driller?s method of describing the lithologies. Thus, to perforations may have been obscured due to the extent simplify use of the driller?s logs, the many lithologic of corrosion or colonies of iron bacteria. descriptions were generalized to four different litho- Caliper Log. The caliper log is a continuous logic types based on the expected permeabilities record of the inside diameter of a well casing or bore- (fig. 2): (1) permeable?composed of sand, gravel, hole (the uncased part of a well). Many wells in the cobbles, and so forth, or a combination of these types San Luis Valley are constructed with several diameters of deposits; (2) nonpermeable?composed of clays; of casing pipes. The uppermost pipe typically is (3) mostly permeable with some nonpermeable char- referred to as the ?surface casing,? and the lower acter?mostly sand or gravel with a nondominant clay (normally smaller diameter) pipes are referred to as component; and (4) mostly nonpermeable with some ?liners.? The caliper tool is used to measure the inside permeable character?mostly clay with a nondominant diameter of well casing, to detect changes in casing sand or gravel component. diameter such as at the top of a liner, and to locate the bottom of the casing in wells that are not cased the entire length of the borehole. Several wells measured Permeable? Sand, gravel, or cobbles in varying proportions for this study had at least some of the borehole left uncased. The inside diameter of some wells recorded Mostly permeable with some non- on the caliper logs was affected by thick accumula- permeable character? Mostly sand or gravel with a nondominant clay tions of corrosion or iron bacteria. These thick accu- component mulations cause the trace of the caliper log to be Mostly nonpermeable with some irregular rather than smooth, as would be expected for permeable character? Mostly clay a clean well casing. with a nondominant sand or gravel A caliper log was not recorded in several wells component due to difficulties in getting past the tops of liners. In Nonpermeable? Clay these cases, the trace of the caliper log shown in the figures was constructed from the reported well diame- ters and the video observations. Water Specific-Conductance Log. The Figure 2. Explanation of colors repre- specific conductance of water is a measure of the senting generalized lithologic types ability of water to conduct an electrical current. Typi- (figs. 6, 7, 10, 12, 15, 19, 20, 22?25, 30?32, 34, and 35). cally, an inflection will be observed in the specific- conductance log where water enters the well. The Other layers may exist within each of the gener- specific conductance in the water in a well is related to alized layers, such as a description on a driller?s log of the concentration of dissolved solids in the water. As ?sand and clay and gravel.? This description would be the dissolved-solids concentration increases in the generalized to being sand or gravel with some clay. water, the specific conductance of the water increases. Neither the driller?s log nor this generalization might Changes in dissolved solids and, thus, specific conduc- delineate layers that are relatively thin but which are tance can occur as water from different zones with hydrogeologically significant in affecting ground- different dissolved-solids concentrations enters the water flow. well. Specific-conductance logs were recorded in Video Log. Videotapes of each well were flowing wells under free-flowing conditions and in recorded by using a well video camera designed to be nonflowing wells under pumped and static conditions. used in wells and submerged in water. The well video Water-Temperature Log. The trace of the camera was useful for determining casing integrity, water-temperature log is similar to the trace of the locating torch-cut slots or well screens, verifying well water specific-conductance log. An inflection is seen construction, and detecting the presence of corrosion in the temperature logs as water with different temper- or colonies of iron bacteria on the casing walls. Only atures from different zones in the aquifer mixes in the the video observations of torch-cut, saw-cut, or stain- well. Because of the geothermal gradient of the San less-steel screened intervals are plotted in the figures Luis Valley, water from different depths in the for each well. In some wells, torch-cut slots or casing confined-aquifer system can have different tempera- tures, usually increasing temperature with greater DESCRIPTION OF WELL LOGS 5