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Geophysical Studies of the Southwest Florida Coast

Project Proposal for 1998

Project Title: Geophysical Mapping of Freshwater/Saltwater Interface
Location of Study Area: Initial scope of the project was a 362-km2 area in the Everglades where the first HEM survey was flown in April 1994. Based upon the effectiveness of this survey and evaluation of alternative methods, the study area has been expanded to the west to include a 1036-km2 region of the Everglades. The third survey was flown in November 1996. This 1010-km2 survey covered a core region over Taylor Slough and moved our coverage further north to investigate the influence of the main Park road, as well as the upper portions of Taylor Slough. The accompanying map shows the survey boundaries, TEM sounding locations, and observation wells.
Project Start Date: October 1994
Project End Date: September 1999
Project Number: N/A
Project Chief: David V. Fitterman
Region/Division/Team/Section: N/A
Phone: 303-236-1382
Fax: 303-236-1425
Mailing Address:
U.S. Geological Survey
Box 25046 MS 964
Denver, CO 80225
Program Element(s)/Task(s) Ecosystems: Element 1, Task 1.5; Element 2, Task 2.3; Element 4, Task 4.2
Collaborators, Clients:


The following agencies are interested in this project: National Park Service (NPS), South Florida Water Management District (SFWMD), and the U.S. Army Corps of Engineers (USACE). Our maps form a basis for the ground-water modeling efforts of the these agencies. The NPS is using our maps for studies of the influence of roadbeds on surface water flow in ENP. These agencies will use our data as one means of assessing the effectiveness of restoration efforts. Metropolitan Dade County Environmental Resource Management (DERM) is interested in having HEM data collected in the area to the east of our study area. There may be other potential clients such as agencies in Dade, Monroe, and Collier Counties and Big Cypress National Preserve that are interested in this work.

Collaboration and Partnerships

Within USGS there has been collaboration with Roy Sonenshein of WRD-Miami on mapping the FWSWI in South Dade County. Roy has provided equipment and field help on an informal basis, and I have provided geophysical expertise. I have worked with Gene Shinn, who drilled several monitoring wells, and with Bob Halley to discuss how this project will provide information for his studies of Florida Bay salinity.

There has been considerable extramural collaboration. I have worked closely with Robert Fennema, the ENP hydrologist responsible for developing a hydrologic model for the ENP. He has provided field support in the form of vehicles (truck, airboat, and helicopter) and personnel for logging operations. We have co-authored a paper which was published, and a report on the influence of roadways on surface-water flow in ENP is in preparation. He has provided information on and access to wells inside the ENP. In addition his staff have provided GIS information for incorporation into map products.

Aaron Higer has provided many valuable contacts with the SFWMD. Of greatest importance was the commitment of Keith Smith and Milt Switanek to drill seven monitoring wells for this project. Five of these wells are inside the ENP and two are on SFWMD right of ways. Robert Fennema obtained the necessary permits to drill inside ENP, and Switanek did the same for the other wells. The SFWMD has done some geophysical logging to complement the work that I am doing.

Richard Green and Ken Campbell (Florida Geological Survey) have provided early versions of geologic maps from the Homestead area for use by this project. I have provided them with HEM apparent resistivity maps which they have used to help locate push-core sites for their mapping in the western part of ENP.

Gwen Burzycki of DERM and Bob Kesler of Florida Power and Light have expressed interest in having an HEM survey flown in the area between U.S. 1 and Card Sound Road (immediately adjacent to the eastern border of our study area). We had numerous discussions about this, but were not able to come up with a satisfactory funding procedure.

There has been discussions with employees of the U.S. Army Corps of Engineers involved in formulating a hydrologic flow model for South Florida. This include Jerry Linn of Vicksburg and Jim May from Jacksonville. As our interpreted products become available I anticipate that this collaboration will increase.


Project Summary: This project focuses on the use of airborne, ground, and borehole geophysical measurements to map the freshwater/saltwater interface in Everglades National Park (ENP) and to monitor changes in this interface over time. In addition, these data provide information to hydrologic modelers studying possible ground-water seepage into Florida Bay and the influence of the Buttonwood Embankment on these flows.

Project Justification:

Management Issues

Water quality in coastal areas of South Florida such as the Everglades National Park (ENP) and the discharge of fresh water into Florida Bay are closely tied to water use and water management policies. Determination and monitoring of water quality is essential to restoration of the South Florida ecosystem (SFE). The flow of fresh water through the Everglades into Florida Bay is critical to the well being of the SFE. Increased domestic use of water, drainage of land to allow farming, increased farming and subsequent nutrient loading of runoff, and changes in water management practices over the years have had a profound effect on the SFE. Monitoring of these effects is made difficult by the inaccessibility of much of this area. Airborne geophysical methods provide a means of rapidly and economically monitoring large areas where access is difficult.

Project Objectives:

Proposed FY-1998 Activity

The emphasis of this project during FY-98 will be completion of reports, publication of maps, and continued data interpretation. Having resolved problems associated with HEM inversion procedures during FY- 97, we should be able to devote more time to releasing products of use by our clients. Proposed activities for FY-98 include:

1. Publication of interpreted HEM maps showing models. interpreted resistivity-depth

2. Continued collection of borehole geophysical data including induction and water conductivity data.

3. Processing and interpretation of existing borehole geophysical data

4. Investigation of constraints imposed by geophysical data on the hypothesized "river of sand" and its transport of fresh ground water to Florida Bay.

Overall Strategy, Study Design, and Planned Major Products:

Study Design

This project relies upon the fact that changes in water salinity produce changes in specific conductance (SC) or water resistivity. (SC is measured in units of (S/cm. Resistivity ( has units of ohm-m. These two properties are related by SC[(S/cm]=10000/( [ohm-m]. Geophysicists often refer to conductivity ([S/m] and its reciprocal resistivity (.) Changes in SC produce changes in the bulk resistivity of geologic materials. The relationship between the rock resistivity and the pore fluid resistivity is called Archie's Law and is given by (rock=a (fluid(-m where a is a constant approximately equal to 1, ( is the fluid accessible porosity, and m is a constant ranging from 1.8-2.0.

We are utilizing several electromagnetic geophysical techniques to map ground resistivity, each having different depths of exploration and preferred targets. Our primary tool is helicopter electromagnetic surveying which allows rapid mapping of large tracts of otherwise inaccessible terrain. HEM surveying is accomplished by flying an instrument package, which is slung below the helicopter, along parallel flight lines spaced 400 meters apart which cover the survey area. These data provide information on how the resistivity of the ground varies with position and depth. We have developed procedures to convert these data into interpreted resistivity-depth models. Time-domain electromagnetic soundings are being used in this process and to provide information about resistivity variations at greater depth (as much as 100 meters). Finally, we are using borehole geophysical measurements to obtain information on formation and fluid conductivity. By sampling from various locations and depths, we are developing a correlation between formation-resistivity and water-quality. This will allow the interpreted HEM maps to be converted into estimated water quality maps.


Overall: N/A


 Geophysical Mapping of Fresh/Saltwater Interface	

    Activity                              FY-94    FY-95     FY-96   FY-97    FY-98    FY-99  
                                           1994     1995     1996     1997    1998     1999
                                         1 2 3 4  1 2 3 4  1 2 3 4  1 2 3 4  1 2 3 4  1 2 3 4

Fly HEM Survey #1                         -V
Delivery of HEM Survey #1 data               *
Processing and interpretation of
   HEM Survey #1                                 -------V

Fly HEM Survey #2                              V
Delivery of HEM Survey #2 data                      *
Processing and interpretation of 
  HEM Survey #2                                       -----V
Publication of interpreted map of 
  HEM Survey #2                                                         ---V   

Fly HEM Survey #3                                                V
Delivery of HEM Survey #3 data                                        *
Processing and interpretation of 
  HEM Survey #3                                                        ---V
Publication of interpreted map of HEM Survey #3                                 ---V

Geophysical logging, existing holes            --V
Installation of new wells                           ----------V
Geophysical logging, new wells                       ----------V
Analysis of well log data                                          --------V--------V
Repeat logging studies                                            ------------------V

Surface geophysical measurements                        V    V
Processing and interpretation of data                     V   V

Formulation of long-term monitoring surveys                               ---------V
HEM surveys in other South Florida locations                                           -------

     Activity                              FY-94    FY-95     FY-96   FY-97    FY-98    FY-99  
                                            1994     1995     1996     1997    1998     1999
                                          1 2 3 4  1 2 3 4  1 2 3 4  1 2 3 4  1 2 3 4  1 2 3 4

ENP HEM poster completion                      *
Revised ENP HEM poster completed                         *
SAGEEP paper completion                            *                                        
Everglades HEM fact sheet completion                          *
TEM survey OFR report                                                     o
HEM calibration error report                                              o
Map of intepreted HEM Survey #2 for GW modelers                           o
Borehole geophysical data OFR report                                               o
Geophysical models for GW modelers OFR report                             o
Map of intepreted HEM Survey #3 for GW modelers                                    o

HEM Survey #1 resistivity map                   *
HEM Survey #2 resistivity map                          *
HEM Survey #3 resistivity map                                          *

Research paper on HEM methods                                            o
Research paper                                                                    o        o

- = start activity
V = finish activity
o = target product completion
* = product complete

Planned Deliverables/Products:

The following products are planned for this project:

1. Open-file report describing the interpretation of the TEM data (publication 9/97). These data will also form the basis of a journal article.
2. Open-file report on analysis of HEM calibration errors (publication 9/97). This information will also be published in a journal.
3. Interpreted airborne resistivity map from December 1994 survey (review submittal 6/97, publication 9/97).
4. Interpreted airborne resistivity map from November 1996 survey (review submittal 6/98, publication 9/98).
5. Open-file Report describing the borehole geophysical data and their analysis (publication 10/98).
6. Outside journal paper on new airborne processing and interpretation methods developed by this project (publication 10/98)

Planned Outreach Activities: N/A

Prior Accomplishments in Proposed Area of Work: There were no new formal publications completed during FY-96. However, work continues on two open-file reports: 1) interpretation of time-domain electromagnetic soundings from Everglades National Park, and 2) analysis of errors in HEM calibration procedures.

During the fiscal year the following non-publication products have been completed:

1. Interpreted resistivity-depth sections from the December 1994 HEM survey. These have been furnished to people as they are requested.

2. Ecosystem web site document describing the project and some of our results.

New Directions, Expansion of Continuing Project (if applicable): If all goes as planned, this project should be completed in FY-98, however, there may be work required for completion of publications in FY-99. The question of long-term monitoring surveys is still open. After analysis of the November 1996 survey and comparison with the previous surveys, we will be in a position to make recommendations for future surveys. These surveys might take place every three to five years over selected portions of the study area to look for long-term changes.

The success of the HEM surveys in mapping the FWSWI in great detail makes it an ideal tool to use in other parts of Florida. In particular, additional work westward from the study area towards Big Cypress National Preserve and the Naples area could be persued if funding is available. Likewise the HEM technique could be of value in the water conservation and storm-water treatment areas north of Miami to get constraints on the geology and hydrology. Work in undeveloped portions of Palm Beach, St. Martin, and St. Lucie counties could be done to determine background saltwater intrusion levels for planning of ground-water resource development.


Accomplishments and Outcomes, Including Outreach:


During the current fiscal year we have made a number of technical breakthroughs with regard to helicopter electromagnetic (HEM) data interpretation which allow us to realize the full value of our data sets. Progress to date includes the following:

1. An analysis of the effect of calibration errors on HEM data was completed resulting in new procedures which will be used for subsequent surveys.

2. In conjunction with the HEM error analysis we perfected a procedure for removing calibration errors from the HEM data which allows stable inversion models to be computed. As a result, we are now able to produce interpreted resistivity at various depths. These data have shown the results of man-made structures-primarily roads and canals-on the ground-water regime.

3. Data from a third HEM survey were collected in November 1996. We have recently received the resistivity maps from the contractor.

4. Preliminary processing of the November 1996 survey has begun using the newly developed techniques mentioned above.

5. Geophysical logging of wells in the study area continues.

Deliverables, Products Completed: N/A


Required Expertise: N/A

Names of Key Project Staff: N/A

Major Equipment/Facility Needs: N/A

Approximate FTE:

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