Report #0616

DIGHEMV Conductivity Survey

Everglades National Park

Site Characterization

Dighem, A division of CGG Canada Ltd. Douglas L. McConnell, P.Eng.

Mississauga, Ontario Geophysicist

February 1, 1995

A0616FEB.95R

**SUMMARY**

This report describes the logistics and results of a DIGHEMV airborne
geophysical survey carried out for the U.S. Geological Survey over 398
square miles of the Florida Everglades. Total survey coverage amounted
to 1718 miles. The survey was flown from December 9 to December 14,
1994.

The purpose of the survey was to map resistivity variations due to
changes in salt water salinity. The results are being used to map the
fresh-water/salt-water interface in the Everglades National Park, as
well as salinity variations. This is a follow up survey to a an earlier
DIGHEMV survey flown in April 1994. An additional objective is to asses
whether the EM system is capable of mapping resistivity variations due
to changes in fresh-water runoff. The previous survey was flown during
the dry season. It was expected that there should be an increased amount
of fresh-water runoff due to higher rainfall late in the year.

A DIGHEMV multi-coil, multi-frequency electromagnetic system was used.
The information from this system was processed to produce maps which
display the conductive properties of the survey area. A GPS electronic
navigation system, utilizing a UHF link, ensured positioning of the
geophysical data with an accuracy of 3 m. Visual flight path recovery
techniques were used to confirm the location of the helicopter where
visible topographic features could be identified on the ground.

The fresh-water/salt-water interface (FWSWI) is clearly mapped. The
three coplanar frequency resistivities and resistivity sections provide
qualitative information as to the layering and changes in the FWSWI
interface with depth. The maps show sufficient detail to identify near
surface influences on fresh-water and salt-water. Additional conclusions
follow:

\* Highway 9336 (State Highway 27) from Florida City to Flamingo appears
to be acting as a barrier to fresh water. This is particularly evident
on the 56,000 Hz resistivity.

\* An east-west canal, adjacent to a road which connects with Highway
9336, is associated with conductivity which is higher than that of the
surrounding, relatively fresh water.

\* In the western portion of the survey area, salt water encroachment
appears to be associated with drainage. For example, a series of creeks
which converge at Tarpon Bay are associated with higher conductivity
than the surrounding marsh.

\* In the eastern portion of the survey area, Taylor Slough is the main
source of fresh water. A similar feature exists parallel and to the east
of Taylor Slough.

\* Shark Valley Slough, in the western portion of the survey area, does
not appear to contain as thick a layer of fresh-water float as Taylor
Slough, as Shark Valley Slough is more conductive.

\* A similar feature to Taylor Slough may exist near the bend in Highway
9336, centred at 80^0^ 49\' W Lat., 25^0^ 21\' N Long.

A comparison of the results of this survey with the April results shows
that near surface conductivities are very similar except for the
following main differences:

\* The FWSWI appears to have moved to the south by approximately 1/2
mile in the northeast portion of the survey area. This is consistent
with an increase in fresh-water runoff.

\* The 900 Hz resistivities are generally more conductive by about 20%
on this recent survey. As this occurs in both resistive, fresh-water,
and conductive, saline-water, portions of the survey area, the
difference is probably the result of the lack of an absolute calibration
of the EM system. In future surveys, calibration of the system over a
half space of sea water may be useful.

**CONTENTS**

[Section]{.underline}

**INTRODUCTION** 1.1

**SURVEY EQUIPMENT** 2.1

**PRODUCTS AND PROCESSING TECHNIQUES** 3.1

**SURVEY RESULTS** 4.1

**APPENDICES**

A. List of Personnel

**INTRODUCTION**

A DIGHEMV resistivity survey was flown for The U.S. Geological Survey
from December 9 to December 14, 1994 over 398 square miles of the
Florida Everglades. The survey area can be located on map sheet Miami
25080-A1 (see Figure 1).

Survey coverage consisted of approximately 1718 line‑miles, including
tie lines. Flight lines were flown in an azimuthal direction of 0° with
a line separation of 1/4 mile (400 metres).

The survey employed the DIGHEMV electromagnetic system. Ancillary
equipment consisted of a radar altimeter, video camera, analog and
digital recorders and an electronic navigation system. Details on the
survey equipment are given in Section 2. Section 2 also provides details
on the data channels, their respective sensitivities, and the
navigation/flight path recovery procedure.

**SURVEY EQUIPMENT**

The instrumentation was installed in an Aerospatiale AS350B turbine
helicopter (Registration N350LE) which was provided by Bulldog
Helicopters Ltd. The helicopter flew at an average airspeed of 100 km/h
with an EM bird height of approximately 30 m.

**Electromagnetic System**

Model: DIGHEMV

Type: Towed bird, symmetric dipole configuration operated at a nominal
survey altitude of 30 metres. Coil separation is 8 metres for all
coil-pairs except for the 56,000 Hz coil-pair which has a 6.3 metre coil
separation.

Coil orientations/frequencies: coaxial / 900 Hz

coplanar / 900 Hz

coaxial / 5,500 Hz

coplanar / 7,200 Hz

coplanar / 56,000 Hz

Channels recorded: 5 inphase channels

5 quadrature channels

4 monitor channels

Sensitivity: 0.1 ppm at 900 Hz

0.2 ppm at 5,500 Hz

0.2 ppm at 7,200 Hz

0.5 ppm at 56,000 Hz

Sample rate: 10 per second

The electromagnetic system utilizes a multi‑coil coaxial/coplanar
technique to energize conductors in different directions. The coaxial
coils are vertical with their axes in the flight direction. The coplanar
coils are horizontal. The secondary fields are sensed simultaneously by
means of receiver coils which are maximum coupled to their respective
transmitter coils. The system yields an inphase and a quadrature channel
from each transmitter‑receiver coil‑pair.

**Radar Altimeter**

Manufacturer: Honeywell/Sperry

Type: AA 220

Sensitivity: 1 ft

The radar altimeter measures the vertical distance between the
helicopter and the ground. This information is used in the processing
algorithm which determines conductor depth.

The altimeter is calibrated by checking the reading when the helicopter
lifts-off and reaches the end of the 100 ft EM bird cable.

**Analog Recorder**

Manufacturer: RMS Instruments

Type: DGR33 dot‑matrix graphics recorder

Resolution: 4x4 dots/mm

Speed: 1.5 mm/sec

The analog profiles are recorded on chart paper in the aircraft during
the survey. Table 2‑1 lists the geophysical data channels and the
vertical scale of each profile.

**Table 2‑1. The Analog Profiles**

+---------+-----------------------------+----------+------------------+
| Channel | Parameter                   | Scale    | Designation on   |
|         |                             |          |                  |
| Name    |                             | units/mm | digital profile  |
+---------+-----------------------------+----------+------------------+
| 1XI     | coaxial inphase (900 Hz)    | 2.5 ppm  | CXI (900 Hz)     |
+---------+-----------------------------+----------+------------------+
| 1XQ     | coaxial quad (900 Hz)       | 2.5 ppm  | CXQ (900 Hz)     |
+---------+-----------------------------+----------+------------------+
| 3PI     | coplanar inphase (900 Hz)   | 2.5 ppm  | CPI (900 Hz)     |
+---------+-----------------------------+----------+------------------+
| 3PQ     | coplanar quad (900 Hz)      | 2.5 ppm  | CPQ (900 Hz)     |
+---------+-----------------------------+----------+------------------+
| 2PI     | coplanar inphase (7200 Hz)  | 5 ppm    | CPI (7200 Hz)    |
+---------+-----------------------------+----------+------------------+
| 2PQ     | coplanar quad (7200 Hz)     | 5 ppm    | CPQ (7200 Hz)    |
+---------+-----------------------------+----------+------------------+
| 4XI     | coaxial inphase (5500 Hz)   | 5 ppm    | CXI (5500 Hz)    |
+---------+-----------------------------+----------+------------------+
| 4XQ     | coaxial quad (5500 Hz)      | 5 ppm    | CXQ (5500 Hz)    |
+---------+-----------------------------+----------+------------------+
| 5PI     | coplanar inphase (56000 Hz) | 10 ppm   | CPI (56 kHz)     |
+---------+-----------------------------+----------+------------------+
| 5PQ     | coplanar quad (56000 Hz)    | 10 ppm   | CPQ (56 kHz)     |
+---------+-----------------------------+----------+------------------+
| ALTR    | altimeter                   | 3 m      | ALT              |
+---------+-----------------------------+----------+------------------+
| CXSP    | coaxial sferics monitor     |          | CXS              |
+---------+-----------------------------+----------+------------------+
| 4XSP    | coaxial sferics monitor     |          |                  |
+---------+-----------------------------+----------+------------------+
| CPSP    | coplanar sferics monitor    |          | CPS              |
+---------+-----------------------------+----------+------------------+
| 3PSP    | coplanar sferics monitor    |          |                  |
+---------+-----------------------------+----------+------------------+
| CXPL    | coaxial powerline monitor   |          | CXP              |
+---------+-----------------------------+----------+------------------+
| CPPL    | coplanar powerline monitor  |          | CPP              |
+---------+-----------------------------+----------+------------------+
| 3PPL    | coplanar powerline monitor  |          |                  |
+---------+-----------------------------+----------+------------------+

**Digital Data Acquisition System**

Manufacturer: Picodas

Type: PDAS 1000

The PDAS 1000 has a built-in hard drive for digital data storage, and
two internal magnetometer counters. The data are downloaded from the
hard drive to a DC2120 cassette at the end of each flight.

The digital data are used to generate several computed parameters. Both
measured and computed parameters are plotted as \"multi-channel stacked
profiles\" during data processing. These parameters are shown in Table
2‑2. In Table 2‑2, the log resistivity scale of 0.06 decade/mm means
that the resistivity changes by an order of magnitude in 16.6 mm. The
resistivities at 0, 33 and 67 mm up from the bottom of the digital
profile are respectively 1, 100 and 10,000 ohm‑m.

**Table 2‑2. The Digital Profiles**

+---------------+-----------------------------------------+-----------+
| Channel       | Observed parameters                     | Scale     |
|               |                                         |           |
| Name (Freq)   |                                         | units/mm  |
+---------------+-----------------------------------------+-----------+
| ALT           | bird height                             | 6 m       |
+---------------+-----------------------------------------+-----------+
| CXI (900 Hz)  | vertical coaxial coil-pair inphase      | 8 ppm     |
+---------------+-----------------------------------------+-----------+
| CXQ (900 Hz)  | vertical coaxial coil-pair quadrature   | 8 ppm     |
+---------------+-----------------------------------------+-----------+
| CPI (900 Hz)  | horizontal coplanar coil-pair inphase   | 8 ppm     |
+---------------+-----------------------------------------+-----------+
| CPQ (900 Hz)  | horiztonal coplanar coil-pair           | 8 ppm     |
|               | quadrature                              |           |
+---------------+-----------------------------------------+-----------+
| CXI (5500 Hz) | vertical coaxial coil-pair inphase      | 16 ppm    |
+---------------+-----------------------------------------+-----------+
| CXQ (5500 Hz) | vertical coaxial coil-pair quadrature   | 16 ppm    |
+---------------+-----------------------------------------+-----------+
| CPI (7200 Hz) | horizontal coplanar coil-pair inphase   | 16 ppm    |
+---------------+-----------------------------------------+-----------+
| CPQ (7200 Hz) | horiztonal coplanar coil-pair           | 16 ppm    |
|               | quadrature                              |           |
+---------------+-----------------------------------------+-----------+
| CPI (56 kHz)  | horizontal coplanar coil-pair inphase   | 40 ppm    |
+---------------+-----------------------------------------+-----------+
| CPQ (56 kHz)  | horiztonal coplanar coil-pair           | 40 ppm    |
|               | quadrature                              |           |
+---------------+-----------------------------------------+-----------+
| CXS           | coaxial sferics monitor                 |           |
+---------------+-----------------------------------------+-----------+
| CXP           | coaxial powerline monitor               |           |
+---------------+-----------------------------------------+-----------+
|               |                                         |           |
+---------------+-----------------------------------------+-----------+
|               | Computed Parameters                     |           |
+---------------+-----------------------------------------+-----------+
| RES (900 Hz)  | log resistivity                         | .06       |
|               |                                         | decade    |
+---------------+-----------------------------------------+-----------+
| RES (7200 Hz) | log resistivity                         | .06       |
|               |                                         | decade    |
+---------------+-----------------------------------------+-----------+
| DP (900 Hz)   | apparent depth                          | 6 m       |
+---------------+-----------------------------------------+-----------+
| DP (7200 Hz)  | apparent depth                          | 6 m       |
+---------------+-----------------------------------------+-----------+
| DP (56 kHz)   | apparent depth                          | 6m        |
+---------------+-----------------------------------------+-----------+
|               |                                         |           |
+---------------+-----------------------------------------+-----------+

**Tracking Camera**

Type: Panasonic Video

Model: AG 2400/WVCD132

Fiducial numbers are recorded continuously and are displayed on the
margin of each image. This procedure ensures accurate correlation of
analog and digital data with respect to visible features on the ground.

**Navigation System (RT-DGPS)**

Model: Sercel NR106, Real-time differential positioning

Type: SPS (L1 band), 10-channel, C/A code, 1575.42 MHz.

Sensitivity: -132 dBm, 0.5 second update

Accuracy: \< 5 metres in differential mode,

± 50 metres in S/A (non differential) mode

The Global Positioning System (GPS) is a line of sight, satellite
navigation system which utilizes time-coded signals from at least four
of the twenty-four NAVSTAR satellites. In the differential mode, two GPS
receivers are used. The base station unit is used as a reference which
transmits real-time corrections to the mobile unit in the aircraft, via
a UHF radio datalink. The on-board system calculates the flight path of
the helicopter while providing real-time guidance. The raw XYZ data are
recorded for both receivers, thereby permitting post-survey processing
for accuracies of approximately 2 metres.

Although the base station receiver is able to calculate its own latitude
and longitude, a higher degree of accuracy can be obtained if the
reference unit is established on a known benchmark or triangulation
point. The GPS records data relative to the WGS84 ellipsoid, which is
the basis of the revised North American Datum (NAD83).

Conversion software is used to transform the WGS84 coordinates to the
system displayed on the base maps.

**Field Workstation**

Manufacturer: Dighem

Model: FWS: V2.41

Type: 80386 based P.C.

A portable PC-based field workstation is used at the survey base to
verify data quality and completeness. Flight tapes are dumped to a hard
drive to permit the creation of a database. This process allows the
field operators to display both the positional (flight path) and
geophysical data on a screen or printer.

**PRODUCTS AND PROCESSING TECHNIQUES**

The following products are available from the survey data. Those which
are not part of the survey contract may be acquired later. Refer to
Table 3‑1 for a summary of the maps which accompany this report, some of
which may be sent under separate cover. Most parameters can be displayed
as contours, profiles, or in colour.

**Base Maps**

Base maps of the survey area have been produced from the 1:100,000 scale
Cape Sable, Everglades City, Miami and Homestead topographic map sheets.
These were joined and photographically reproduced to a scale of
1:48,000. The topographic base maps were hand digitized to produce the
skeletal topography for the colour maps.

**Resistivity**

The apparent resistivity in ohm‑m was generated from the inphase and
quadrature EM components for the 900 Hz, 7,200 Hz and 56,000 Hz
frequencies, using a pseudo‑layer halfspace model. A resistivity map
portrays all the EM information for that frequency over the entire
survey area. The large dynamic range makes the resistivity parameter an
excellent mapping tool.

**Table 3‑1 Survey Products**

[Final Transparencies @ 1:48,000]{.underline}

900 Hz resistivity contours

7,200 Hz resistivity contours

56,000 Hz resistivity contours

The final prints display the geophysical parameter combined with the
flight path on a screened topographic base. Two sets of black and white
prints are supplied.

[Final Colour maps @ 1:48,000]{.underline}

900 Hz resistivity colours

7,200 Hz resistivity colours

56,000 Hz resistivity colours

The colour maps display superimposed contours, flight path, skeletal
topography, Lat/Long\'s and UTM coordinates. Two sets of colour maps are
supplied.

[Other Products]{.underline}

Digital XYZ archives with documentation

Digital grid archives

Multi-channel \'stacked\' geophysical profiles

Sengpiel and differential resistivity sections for four lines

[Note]{.underline}: The XYZ archives are supplied on a DOS compatible
CD-ROM in Geopak ASCII format.

**Multi-channel Stacked Profiles**

Distance‑based profiles of the digitally recorded geophysical data are
generated and plotted by computer. These profiles also contain the
calculated parameters which are used in the interpretation process.
These are produced in the final corrected form after interpretation.

**Contour and Colour Displays**

The geophysical data are interpolated onto a regular grid using a
modified Akima spline technique. The resulting grid is suitable for
generating contour maps of excellent quality.

Colour maps are produced by interpolating the grid down to the pixel
size. The parameter is then incremented with respect to specific
amplitude ranges to provide colour \"contour\" maps.

**Conductivity-depth Sections**

The apparent resistivities for all frequencies can be displayed
simultaneously as coloured conductivity-depth sections. Usually, only
the coplanar data are displayed as the quality tends to be higher than
that of the coaxial data.

Conductivity-depth sections can be generated in two formats:

\(1\) Sengpiel resistivity sections, where the apparent resistivity for
each frequency is plotted at the depth of the centroid of the inphase
current flow[^1]; and,

\(2\) Differential resistivity sections, where the differential
resistivity is plotted at the differential depth[^2].

Both the Sengpiel and differential methods are derived from the
pseudo-layer halfspace model. Both yield a coloured conductivity-depth
section which attempts to portray a smoothed approximation of the true
resistivity distribution with depth. The Sengpiel method is most useful
in conductive layered situations, but may be unreliable in areas of
moderate to high resistivity where signal amplitudes are weak. In areas
where inphase responses have been suppressed by the effects of
magnetite, the computed resistivities shown on the sections may be
unreliable. The differential technique was developed by Dighem to
overcome problems in the Sengpiel technique. The differential
resistivity section is more sensitive than the Sengpiel section to
changes in the earth\'s resistivity and it reaches deeper.

**SURVEY RESULTS**

Resistivity maps, which display the conductive properties of the survey
area, were produced from the 900 Hz, 7,200 Hz and 56,000 Hz coplanar
data. The conductivity units, in mS/m, are calculated from the
resistivity units in ohm-m as: mS/m = 1000/ohm-m.

The maximum resistivity value, which is calculated for each frequency,
is approximately 1.15 times the numerical value of the frequency. This
cutoff eliminates the meaningless higher resistivities which would
result from very small EM amplitudes. The minimum resistivity value is
.0000054 times the frequency. This cutoff eliminates errors due to the
lack of an absolute phase control for the EM data. This cutoff only
affected the 56,000 Hz data.

Several points of interest, which appear on the coplanar resistivity
maps, in addition to those identified in the report of the April 1994
survey results, are as follows:

\* A red-orange to green-blue resistivity contrast is obvious on all
three resistivity colour maps. It undoubtedly reflects a major salinity
contrast, and indicates relatively saline water dominates the western
and southern portions of the survey area. This contrast, which is
characterized by resistivities below 10 ohm-m (100 mS/m) to the south
and west and resistivities over 10 ohm-m to the north and east, is

> almost identical in its location on the 7,200 and 900 Hz maps, but
> differs in location on the 56,000 Hz maps, east of 80^0^ 33\' W, in
> that for the most part the 10 ohm-m contour does not go north of 25^0^
> 18\' N. This contrast on the 56,000 Hz maps, which is less obvious on
> the other frequencies, may reflect a secondary salinity interface due
> to some controlling source that is not indicated on the topographic
> maps.

\* A comparison of the results from the April survey with this recent
survey shows that the main interface mentioned above has moved about 1/2
mile to the south, north of 25^0^ 20\' and east of 80^0^ 31\'. This may
reflect an influx of fresh-water.

\* The 900 Hz is consistently more conductive in this recent survey than
in the April survey. This occurs in both fresh-water, high resistivity
areas, and salt-water low resistivity areas. The difference between the
resistivity values in any one location for the two surveys is about 20%.
This is probably due to the lack of an absolute calibration of the EM
system. A calibration test over a salt-water half space would allow
post-survey phase rotation to ensure that all EM parameters were giving
the same value for salt-water. Assuming that the resistivity of the
salt-water did not change over time, this calibration would allow season
to season comparisons.

\* Highway 9336 (State Highway 27) from Florida City to Flamingo appears
to be acting as a barrier to fresh water. This is particularly evident
on the 56,000 Hz resistivity.

\* A relatively resistive feature, which is similar to Taylor Slough,
may exist near the bend in Highway 9336, centred at 80^0^ 49\' W Lat.,
25^0^ 21\' N Long. The shape of this resistive area may be affected by
the road.

\* An east-west canal, adjacent to a road which connects with Highway
9336, is associated with conductivity which is higher than that of the
surrounding, relatively fresh water.

\* In the western portion of the survey area, the area hosting a series
of creeks that converge at Tarpon Bay is associated with higher
conductivity than the surrounding marsh. This indicates that saline
water encroaches more where there is existing drainage channels.

\* Shark Valley Slough, in the western portion of the survey area, does
not appear to contain as thick a layer of fresh-water float as Taylor
Slough, as Shark Valley Slough is more conductive.

Respectfully submitted,

**DIGHEM**

Douglas L. McConnell, P.Eng.

Geophysicist

**APPENDIX A**

**LIST OF PERSONNEL**

The following personnel were involved in the acquisition, processing,
interpretation and presentation of data, relating to a DIGHEM^V^
airborne geophysical survey carried out for the U.S. Geological Survey
of a selected area of the Florida Everglades.

Don Ellis Senior Geophysical Operator

Jordan Cronkwright Second Geophysical Operator

Steve Alexander Pilot (Bulldog Helicopters Ltd.)

Gordon Smith Data Processing Supervisor

Dak Darbha Computer Processor

Doug McConnell Interpretation Geophysicist

Lyn Vanderstarren Drafting Supervisor

Mike Armstrong Draftsperson (CAD)

Susan Pothiah Word Processing Operator

Albina Tonello Secretary/Expeditor

All personnel are employees of Dighem, A Division of CGG Canada Ltd.,
except for the pilot who is an employee of Bulldog Helicopters Ltd.

**DIGHEM**

Douglas L. McConnell, P.Eng.

Geophysicist

[^1]: ^?^ Approximate Inversion of Airborne EM Data from Multilayered
    Ground: Sengpiel, K.P., Geophysical Prospecting 36, 446-459, 1988.

[^2]: ^?^ The Differential Resistivity Method for Multi-frequency
    Airborne EM Sounding: Huang, H. and Fraser, D.C., presented at
    Intern. Airb. EM Workshop, Tucson, Ariz., 1993.