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Transient Electromagnetic Sounding Data

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Frequently-anticipated questions:


What does this data set describe?

Title: Transient Electromagnetic Sounding Data
Abstract:
This data set contains time-domain electromagnetic (TEM) soundings collected at 63 sites in and near Everglades National Park. The data set includes the sounding name, date measured, location information in the form of a descriptive location, zone 17 UTM coordinates, and latitude and longitude, transmitter loop size [meters], and location of receiver coil with respect to the center of the transmitter loop, layered-earth model best fitting the data. This includes the layer resistivity [ohm-meters] and layer thickness [meters]. Also included is the percentage rms misfit error between the observed and calculated apparent resistivity of the model.
  1. How should this data set be cited?

    Fitterman, Dave, 2005, Transient Electromagnetic Sounding Data.

    Online Links:

  2. What geographic area does the data set cover?

    West_Bounding_Coordinate: -81.1
    East_Bounding_Coordinate: -80.4
    North_Bounding_Coordinate: 25.5
    South_Bounding_Coordinate: 25.15

  3. What does it look like?

    <https://sofia.usgs.gov/exchange/fitterman/locationTEM.html> (GIF)
    sounding locations for TEM data

  4. Does the data set describe conditions during a particular time period?

    Beginning_Date: 17-Aug-1995
    Ending_Date: 10-Dec-1996
    Currentness_Reference: ground condition

  5. What is the general form of this data set?

    Geospatial_Data_Presentation_Form: text files

  6. How does the data set represent geographic features?

    1. How are geographic features stored in the data set?

      Indirect_Spatial_Reference: southwest Florida coast
      This is a Point data set. It contains the following vector data types (SDTS terminology):
      • Entity point (63)

    2. What coordinate system is used to represent geographic features?

      Horizontal positions are specified in geographic coordinates, that is, latitude and longitude. Latitudes are given to the nearest 1. Longitudes are given to the nearest 0.1. Latitude and longitude values are specified in Decimal degrees.

      The horizontal datum used is North American Datum of 1983.
      The ellipsoid used is Geodetic Reference System 80.
      The semi-major axis of the ellipsoid used is 6378137.
      The flattening of the ellipsoid used is 1/298.257.

  7. How does the data set describe geographic features?

    Entity_and_Attribute_Overview:
    The time-domain electromagnetic (TEM) data were collected in and near Everglades National Park. The TEM data sets contain all necessary information required for data interpretation. The data were recorded using a Geonics PROTEM receiver and EM-47 transmitter.

    The data set includes the following information:

    1. Sounding name

    2. Date measured

    3. Location information in the form of: a descriptive location, zone 17 UTM coordinates, and lattitude and longitude.

    4. Transmitter loop size [meters] and location of receiver coil with respect to the center of the transmitter loop.

    5. Layered-earth model best fitting the data. This includes the layer resistivity [ohm-meters] and layer thickness [meters]. Also included is the percentage rms misfit error between the observed and calculated apparent resistivity of the model.

    6. Information on the measurement system including: the TEM system used, the repetition frequency [hertz] of the transmitted waveform, the data set code (Used by interpretation software to represent the repetition frequency.), the transmitter current [amperes], the length of the transmitter turnoff ramp [microseconds], the receiver coil area-turns product [turns-meters2], and the receiver gain setting. The actual receiver gain is 52.1x2G, where G is the gain setting.

    7. Apparent resistivity-time data are provided for two base frequencies: the higher base-frequency data are on the left hand side and the lower base-frequency data on the right hand side. For each frequency the following information is given:

    * the time of the measurement point after transmitter turn off [milliseconds]; * the measured apparent resistivity [ohm-meters]; * the estimate measurement error [percent]; * a mask code which indicates if the data were used (u) in the least-squares parameter estimation, masked (m) from the parameter estimation but retained for possible inclusion in modeling, or deleted (d) from the parameter estimation because the observation error was too large; * and the apparent resistivity calculated for the best-fit model.

    Entity_and_Attribute_Detail_Citation: USGS personnel


Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)

    • Dave Fitterman

  2. Who also contributed to the data set?

  3. To whom should users address questions about the data?

    David Fitterman
    U.S. Geological Survey
    Project Chief
    P. O. Box 25046

    Denver Federal Center MS 964
    Denver, CO 80225
    USA

    303 236-1382 (voice)
    303 236-1425 (FAX)
    fitter@usgs.gov


Why was the data set created?

The purpose of the work was to locate the freshwater-saltwater interface (FWSWI), and to provide data used to remove calibration errors in helicopter electromagnetic data. These data can be used to estimate formation resistivity as a function of depth to depths of up to 100 meters below the surface. This information is a value in determining if the aquifer is freshwater or saltwater saturated.


How was the data set created?

  1. From what previous works were the data drawn?

  2. How were the data generated, processed, and modified?

    Date: Unknown (process 1 of 1)
    The techniques used in this study rely upon time-varying magnetic fields from a transmitter to induce electrical currents into the ground. The flow of these currents is controlled by the electrical conductivity of the ground. More conductive zones tend to let the induced currents flow unimpeded, while less conductive zones impede the current flow. The induced currents in the ground produce a secondary magnetic field which is recorded by a receiver coil. Analysis of the received signals determines how conductivity (or its reciprocal, resistivity) varies with depth and position.

    The transient electromagnetic (TEM) sounding method uses the transition from a steady to zero transmitter current to induce current in the ground. The ground response is measured during the transmitter off-time. We employed a 40-m by 40-m transmitter loop with the receiver coil located at the center of the transmitter loop. The data are converted to apparent resistivity before modeling. Layered-earth model parameters are determined using commercially available nonlinear least-squares inversion software. Because of the large number of data points (typically 25-35) compared to the 10 for each HEM measurement, model parameter estimates are more reliable for the TEM data than the HEM data. The TEM method also has the ability to probe to greater depths than the HEM method. From these data we were able to locate the FWSWI, as well as the depth to the base of the Biscayne aquifer.

    Using the TEM method in the Everglades required slight modification of standard methods as most of the soundings were made in water-covered areas. Equipment had to be floated in plastic tubs, and the transmitter wire was strung over saw grass, while the receiver coil was stood on long legs to keep it above the water.

    At the few sites where we had observation wells, induction logs were measured. The induction tool uses a frequency-domain electromagnetic system to determine the formation resistivity outside the borehole. The borehole must be cased with non-conducting material such as PVC. Induction logs provide very detailed resistivity-depth information within the vicinity of the borehole about 1 m radius from the well. This information is useful in determining the relationship between formation resistivity and pore water quality.

    Time-domain electromagnetic (TEM) soundings were made using a Geonics PROTEM system. A square transmitter loop measuring approximately 40 meters on a side was used. The loop was connected to a Geonics EM-47 transmitter which produced a 50-percent duty-cycle square wave. The current in the transmitter loop creates a primary magnetic field. When the transmitter current is interrupted, the decaying primary magnetic field induces a circulating current flow in the ground below the transmitter loop. The decay of this induced current system is controlled by the electrical resistivity of the ground below the transmitter loop.

    The decaying current system produces a secondary magnetic field, which is sensed by a receiver coil located at the center of the transmitter loop and recorded by the receiver. The recorded signal is called a transient. Many transients are recorded and averaged to reduce noise in the data and to estimate measurement error. The averaged data are converted to apparent resistivity to allow comparison of one sounding to another.

    The data are modeled as one-dimensional, layered-earth models. The electrical resistivity and thickness of model layers is determined by least-squares parameter estimation.

    Person who carried out this activity:

    David Fitterman
    U.S. Geological Survey
    Project Chief
    P. O. Box 25046

    Denver Federal Center MS 964
    Denver, CO 80225
    USA

    303 236-1382 (voice)
    303 236-1425 (FAX)
    fitter@usgs.gov

  3. What similar or related data should the user be aware of?

    Fitterman. David Deszcz-Pan, Maria, 199907, Geophysical Mapping of Saltwater Intrusion in Everglades National Park: Proceedings 3rd International Symposium on Ecohydraulics, International Association for Hydraulic Research (IAHR), Salt Lake City, UT.

    Online Links:

    Stewart, M. A. Bhatt, T. N., Fennema, R. J, 2002, The Road to Flamingo: an Evaluation of Flow Pattern Alterations and Salinity Intrusion in the Lower Glades, Everglades National Park: USGS Open-File Report OFR 02-59, U.S. Geological Survey, Reston, VA.

    Online Links:

    Fitterman, David V. Deszcz-Pan, Maria, 2002, Helicopter Electromagnetic Data from Everglades National Park and Surrounding Areas, Florida: Collected 9-14 December 1994: USGS Open-File Report 02-101, U.S. Geological Survey, Reston, VA.

    Online Links:

    Fitterman, David V. Deszcz-Pan, Maria; Stoddard, 1999, Results of Time-Domain Electromagnetic Soundings in Everglades National Park, Florida: USGS Open-File Report 99-426, U.S. Geological Survey, Reston, VA.

    Online Links:

    Fitterman, David V. Deszcz-Pan,, Maria, 2004, Characterization of Saltwater Intrusion in South Florida Using Electromagnetic Geophysical Methods: Proceedings 18th Salt Water Intrusion Meeting, unknown, Cartagena, Spain.

    Online Links:

    Fitterman, D. V. Deszcz-Pan M., 2001, Saltwater intrusion in Everglades National Park, Florida measured by airborne electromagnetic surveys: proceedings First Internationl Conference on Saltwater Intrusion and Coastal Aquifers-Monitoring, Modeling, and Management (SWICA-M3, Labratoire d'Analyse des Systemes Hydrauliques (LASH), Essaouira, Morocco.

    Online Links:

    Fitterman, D.V. Deszcz-Pan, M., 2001, Using airborne and ground electromagnetic data to map hydrologic features in Everglades National Pak: Proceedings Symposium on the Application of Geophysics to Engineering and Environmental Problems SAGEEP, Environmental and Engineering Geophysical Society, Denver, Co.

    Online Links:


How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?

  2. How accurate are the geographic locations?

  3. How accurate are the heights or depths?

  4. Where are the gaps in the data? What is missing?

    not applicable

  5. How consistent are the relationships among the observations, including topology?

    not applicable


How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?

Access_Constraints: none
Use_Constraints: none

  1. Who distributes the data set? (Distributor 1 of 1)

    Heather S.Henkel
    U.S. Geological Survey
    600 Fourth St. South
    St. Petersburg, FL 33701
    USA

    727 803-8747 ext 3028 (voice)
    727 803-2030 (FAX)
    hhenkel@usgs.gov

  2. What's the catalog number I need to order this data set?

    Transient Electromagnetic Data

  3. What legal disclaimers am I supposed to read?

    The data have no implied or explicit guarantees

  4. How can I download or order the data?


Who wrote the metadata?

Dates:
Last modified: 07-May-2007
Metadata author:
Heather Henkel
U.S. Geological Survey
600 Fourth Street South
St. Petersburg, FL 33701
USA

727 803-8747 ext 3028 (voice)
727 803-2030 (FAX)
sofia-metadata@usgs.gov

Metadata standard:
Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)


This page is <https://sofia.usgs.gov/metadata/sflwww/fitter.faq.html>

U.S. Department of the Interior, U.S. Geological Survey, Center for Coastal Geology
Comments and suggestions? Contact: Heather Henkel - Webmaster
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