|Home||Archived October 29, 2018||(i)|
Denver Federal Center MS 964
Bhatt, T. N., Fennema, R. J., Fitterman, D. V.
Deszcz-Pan, Maria; Stoddard, Carl E.
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.
Denver Federal Center MS 964
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.
U.S. Department of the Interior, U.S. Geological Survey, Center for
Comments and suggestions? Contact: Heather Henkel - Webmaster
Generated by mp version 2.8.18 on Wed May 23 20:03:54 2007
|Home||Archived October 29, 2018|