Time Series Electromagnetic Induction-Log Datasets, Including Logs Collected through the 2014 Water Year in South Florida

Time series electromagnetic-induction log (TSEMIL) datasets are collected from PVC cased or uncased monitoring wells to evaluate changes in water conductivity over time. TSEMIL datasets consist of a series of individual electromagnetic-induction logs collected at a frequency of months or years that have been compiled into a dataset by eliminating small uniform offsets in bulk conductivity between logs likely caused by minor variations in calibration. At depths where water conductivity is not changing through time, each log is typically within about ±15 mS/m of the median of the dataset at any given depth, which is within the stated resolution, repeatability, and accuracy specifications of the probe. Though the offsets between logs are small (±15 mS/m), they obscure the ability to identify small but real changes in bulk conductivity resulting from changes in aquifer salinity. These offsets are removed by selecting a depth at which no changes are apparent from year to year, and by adjusting individual logs to the median of all logs at the selected depth. Generally, these depths are within the freshwater saturated part of the aquifer, well below the water table. Once the offsets have been removed there is generally only about ±2 mS/m of completely irregular variation between successive logs that cannot be removed. Even if perfect numerical alignment is achieved at one or two depths, the ±2 mS/m of random variation remains at other depths. Given these corrections, however, changes from year to year caused by saltwater intrusion are easier to identify. Detailed descriptions of how these corrections are applied are described in Prinos and others (2014) and Prinos and Valderrama (2015). TSEMIL datasets can be used to monitor changes in water conductivity throughout the full thickness of an aquifer, without the need for long open-interval wells, which have, in some instances, allowed vertical water flow within the well bore that has biased water conductivity profiles (Prinos and Valderrama, 2015). Although TSEMIL datasets are most commonly used to evaluate saltwater intrusion some other observed changes evident in TSEMIL datasets are: (1) variations in bulk conductivity near the water table where water saturation of pore spaces may vary, and water temperature may be more variable, (2) dissipation of conductive water in high porosity rock layers, which may have entered these layers during drilling.

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