Perched Water Identification with Nuclear Logs

Abstract

ABSTRACTThe presence, nature, and extent of perched zones are frequently important during investigation of potential waste disposal sites and contaminated sites. Often such zones are overlooked and cased off during drilling and completion of monitor wells. Typically large differences in water levels over short distances are the first indicator that perched conditions may exist at a site. Installation of piezometers can provide conclusive evidence of the presence, nature, and extent of perched zones, but such activity is costly and time‐consuming. Combined interpretation of the gamma‐gamma (GG) and neutron‐epithermal neutron (NN) geophysical well logs can assist in evaluating perched conditions and guide placement of additional piezometers in an effective and efficient manner. Subsequently, logs from the shallow portion of existing wells that are completed in deeper zones can be used to interpolate and extrapolate knowledge of perched conditions.Perched zones are often indicated on logs by an inverse response of the GG and NN logs at a higher elevation than a direct response of the same logs. Because interpretation of geophysical logs is nonunique, it cannot provide conclusive evidence as to whether a particular zone is saturated or unsaturated. Limitations on conditions that will result in a false response can be determined. When logs are used in conjunction with hydrologic and geologic information, they are a valuable tool for investigating zones of perched ground water.