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nents contribute to the total head in an aquifer: pres- Geophysical logs were recorded by the USGS by
<br />sure head, elevation head, and velocity head. In a static lowering various tools attached to the end of a cable
<br />water column within a well, the total head at a depth of
<br />into each of the wells. These tools included a well
<br />800 ft equals the total head at a depth of 1,500 ft video camera, a caliper (to measure the inside diam-
<br />because although the pressure head is greater at 1,500 eter of a well), a water specific-conductance meter, a
<br />ft, the elevation head is greater at 800 ft. When the water temperature meter, and a flowmeter. The logs
<br />component of velocity head is introduced, as in a were recorded as a digital signal at the surface and
<br />flowing well, the total head can be greater at 800 ft were later printed on strip charts and plotted on graphs
<br />when the lithologic layers at that depth are more for analysis.
<br />permeable than layers at 1,500 ft, allowing water to Natural gamma logging typically is used to aid
<br />flow at a higher velocity out of the layers at 800 ft in delineating permeable and nonpermeable lithologic
<br />(Fetter, 1988). Thus, under flowing conditions, a head units (Keys, 1990). Gamma logs were recorded in
<br />difference caused by the addition of the velocity head most of the wells in the CAWN, but the traces of these
<br />can occur. When flow from the well is turned off and logs did not indicate responses typical of the various
<br />the water column again becomes static (shut-in), the lithologies such as clay or sand. This likely is due to
<br />total head at 1,500 ft is likely to be higher than the several factors: (1) most of the boreholes for these
<br />total head at 800 ft because of the greater loss of water wells are larger than 8 inches in diameter, which might
<br />and, thus, pressure in the layers at 800 ft. Water can cause attenuation of the natural gamma signal, (2) the
<br />then flow within the shut-in well from the layers at lithologies present in the San Luis Valley are not high
<br />1,500 ft and into the layers at 800 ft (interzonal flow) in natural gamma radiation, or (3) the lithologies that
<br />to equalize pressures among the layers. After a suffi- typically emit higher natural gamma (for example,
<br />cient amount of time, the heads in the permeable clays) are interspersed within lithologies that do not
<br />layers may equalize and flow will stop or the heads emit high natural gamma (for example, sand and
<br />may remain different and interzonal flow from one gravel) thus dampening the gamma response. The
<br />layer to the others will continue through the well. large diameters of most wells in the CAWN likely
<br />cause the natural gamma signal to be difficult to detect
<br />The water levels in the confined-aquifer system
<br />with the logging tools used.
<br />can vary with depth depending on factors, such as
<br />Logging with each of the tools was attempted in
<br />pumpage or wells left open and flowing, that can cause
<br />most wells. One well was not logged with the flow-
<br />head differences between different zones in the
<br />meter because there was a possibility of getting the
<br />aquifer. The water level in wells open to multiple
<br />flowmeter stuck in the well. Logs were recorded in
<br />aquifer layers or a long interval of the confined-aquifer
<br />some wells but not used in the analysis because the
<br />system is a composite head measurement (Domenico
<br />logs did not provide information useful for this study.
<br />and Schwartz, 1990). If the water levels, or heads,
<br />Thus, the same logs are not shown for all wells in the
<br />differ in each of the aquifer layers open to a well, then
<br />figures that show all logs for each well (figs. 3, 4,
<br />the measured water level will represent a composite
<br />6?26, 28, 30?36).
<br />head and equal some intermediate value between the
<br />Driller?s Log (generalized lithology). The
<br />highest and lowest heads in the aquifer layers. If heads
<br />driller?s log shows the lithologic layers that were pene-
<br />do not differ in aquifer layers open to a well, then the
<br />trated during drilling. The depths to layers and the
<br />composite head is equal throughout the open intervals.
<br />compositions of the layers are not as exact a represen-
<br />tation of lithologies as a continuous core sample would
<br />provide. This is because the determination of litho-
<br />DESCRIPTION OF WELL LOGS
<br />logic character is made (1) by the response of the
<br />Several types of well logs were used to deter-
<br />drilling apparatus to the differing lithologies pene-
<br />mine the construction of wells, the lithologic units trated during drilling, and (2) by viewing cuttings that
<br />penetrated by wells, and the zones contributing water reach the surface fairly pulverized and delayed in time
<br />to wells in the CAWN. Driller?s logs, from which a from when they were reached by the drilling appa-
<br />generalized description of the lithologic layers pene- ratus. There were many different lithologic descrip-
<br />trated by a well was determined, were obtained from tions among the logs obtained for the CAWN wells.
<br />the Colorado Division of Water Resources in Denver. Each description differed according to the particular
<br />4Geophysical Logging to Determine Construction, Contributing Zones, and Appropriate Use of Water Levels Measured in
<br />Confined-Aquifer Network Wells, San Luis Valley, Colorado, 1998?2000
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