Time Series Electromagnetic Induction-Log Datasets, Including Logs Collected through the 2015 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 repeatability specifications of the probe (Mount Sopris Instrument Co., Inc., 2002). 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. To create TSEMIL datasets, 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 to 3 mS/m of completely irregular variation between successive logs that cannot be removed. Even if complete numerical alignment is achieved at one or two depths, the 2 to 3 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 allowed, in some instances, vertical water flow within the well bore that has biased water conductivity profiles (Prinos and Valderrama, 2015). Although a small number of logs from a well can be combined into a TSEMIL dataset to evaluate changes in salinity through time, many logs may be needed to estimate bulk conductivity to within 2 mS/m. As new logs are collected, the TSEMIL dataset may be adjusted to the median bulk conductivity of the entire dataset at the alignment depth. For this reason, the most recent version of a TSEMIL dataset supersedes previous versions.

The U.S. Geological Survey produced TSEMIL datasets or updated existing TSEMIL datasets in cooperation with Broward County, the City of Hallandale Beach, the City of Hollywood, the Florida Power & Light Company, Miami-Dade County, the South Florida Water Management District, and the U.S. Army Corps of Engineers. These TSEMIL datasets are published in USGS data releases on an ongoing basis.

Mount Sopris Instrument Co., Inc., 2002, 2PIA-1000 Poly induction probe, 2EMA-1000 conductivity probe (2 EMB-1000 and 2ECM-1000) EMP-2493 and EMP-4493: Golden CO, Mount Sopris, 19 p.

Prinos, S. T., and Valderrama, Robert, 2015, Changes in the saltwater interface resulting from installation of a seepage barrier near Lake Okeechobee, Florida: U.S. Geological Survey, Open-File Report 2015-1256, 24 p.

Prinos, S.T., Wacker, M.A., Cunningham, K.J., and Fitterman, D.V., 2014, Origins and delineation of saltwater intrusion in the Biscayne aquifer and changes in the distribution of saltwater in Miami-Dade County, Florida: U.S. Geological Survey Scientific Investigations Report 20145025, 101 p., Also available at http://dx.doi.org/10.3133/sir20145025.