Report # 6025
Logistics Report
Helicopter-borne
RESOLVE Electromagnetic and Magnetic
Geophysical Survey
Seco Creek, Texas
Prepared by:
Fugro Airborne Surveys
2270 Argentia Road
Mississauga (Toronto), Ontario, Canada L5N 6A6
For:
the United States Geological Survey
October 2002
By: Michael J. Cain, P.Eng.
Fugro Airborne Surveys
Toronto, Canada
SUMMARY
This report describes the logistics and results of a RESOLVE EM and magnetic airborne geophysical survey carried out for the United States Geological Survey over the Seco Creek area, Texas. Total coverage of the survey blocks amounted to 953 miles (1534 kilometres) over 2 survey blocks and several lines following the creeks through the survey area. The survey was flown from May 21 to 27, 2002.
The purpose of the survey was to map the conductive and magnetic properties of Seco Creek and the surrounding area. This was accomplished by using a RESOLVE multi-coil, multi-frequency electromagnetic system supplemented by a high sensitivity cesium magnetometer. A GPS electronic navigation system ensured accurate positioning of the geophysical data.
The RESOLVE system comprises of five coplanar coil pairs and one coaxial coil pair with a frequency spread of 386 Hz to 106 400 Hz. This system provided optimum definition of the conductive properties of the Earth’s layering in the top 100 m.
TABLE OF CONTENTS
1.0 INTRODUCTION 1.1
2.0 SURVEY EQUIPMENT AND PROCEDURES 2.1
2.1 Electromagnetic System 2.1
2.2 Magnetometer 2.4
2.3 Magnetic Base Station 2.4
2.4 Radar Altimeter 2.5
2.5 Analog Recorder 2.5
2.6 Digital Data Acquisition System 2.6
2.7 Video Flight Path Recording System 2.7
2.8 Navigation (Global Positioning System) 2.7
2.9 Field Workstation and Data Verification 2.9
3.0 PRODUCTS AND PROCESSING TECHNIQUES 3.1
3.1 Base Maps (optional) 3.1
3.2 Apparent Resistivity 3.2
3.3 EM Magnetite (optional) 3.3
3.4 Total Magnetic Field 3.4
3.5 Calculated Vertical Magnetic Gradient (optional) 3.4
3.6 Magnetic Derivatives (optional) 3.4
3.7 Multi-channel Stacked Profiles (optional) 3.5
3.8 Contour, Colour and Shadow Map Displays (optional) 3.6
3.9 Resistivity-depth Sections (optional) 3.6
4.0 CONCLUSIONS AND RECOMMENDATIONS 4.1
APPENDIX A LIST OF PERSONNEL
APPENDIX B ARCHIVE DESCRIPTION
APPENDIX C PROCESSING LOG
APPENDIX D BACKGROUND INFORMATION
D.1 Resistivity Mapping
D.2 Reduction of Geologic Noise
D.3 Magnetics
APPENDIX E FLIGHT LOGS
APPENDIX F TESTS AND CALIBRATIONS
FIGURES
Figure 1.1 Seco Creek area, Texas. 1.3
TABLES
Table 1.1 Breakdown for the Seco Creek Survey 1.1
Table 2.1 The Analog Profiles, RESOLVE System 2.6
Table 3.1 Survey Products 3.2
1.3
1.0 INTRODUCTION
A RESOLVE electromagnetic and magnetic survey was flown for the United States Geological Survey over the Seco Creek area, Texas. Details of the survey areas are shown in Table 1.1
Block/Line / Line Spacing / Line Direction / Total distanceBlock 1 / 200 metres
655 feet / 0° / 1370 km
851 miles
Block 2
(north extension) / 200 metres
655 feet / 0° / 54 km
33.6 miles
River lines / 200- 250 metres
655-820 feet / variable / 110 km
68.4 miles
TOTAL / 1534 km
953 miles
Table 1.1 Breakdown for the Seco Creek Survey
Total coverage of the survey blocks amounted to 953 miles over 2 survey blocks and 2 river sections. The survey was flown from May 21 to 27, 2002. The blocks were flown in an azimuthal direction of 0°. Three tie lines were flown over block 1 perpendicular to the flight lines. Figure 1.1 displays the layout and location of the survey blocks.
Ancillary equipment consisted of a radar altimeter, a video camera, analog and digital recorders, and an electronic navigation system. The instrumentation was installed in an Ecureuil AS350-B2 turbine helicopter (registration CF-ZTA) which was provided by Questral Helicopters Ltd. The helicopter flew at an average airspeed of 70 mph (113 km/h) with an average EM sensor height of approximately 110 feet (33.5 metres).
Section 2 provides details on the survey equipment, the data channels, their respective sensitivities, and the navigation/flight path recovery procedure.
Figure 1.1 Seco Creek area, Texas.
2.7
2.0 SURVEY EQUIPMENT AND PROCEDURES
This section provides a brief description of the geophysical instruments, quality control and calibration procedures used to acquire the survey data.
2.1 Electromagnetic System
Model: RESOLVE
Type: Towed bird, symmetric dipole configuration operated at a nominal survey altitude of 98 feet (30 metres). Coil separation is 7.9 metres for 400 Hz, 1500 Hz, 6200 Hz, 25,000 Hz and 100,000 Hz and 9.0 metres for the 3300 Hz coil-pair.
Coil orientations/frequencies: orientation nominal actual
coplanar 400 Hz 386 Hz
coplanar 1 500 Hz 1 514 Hz
coaxial 3 300 Hz 3 315 Hz
coplanar 6 200 Hz 6 122 Hz
coplanar 25 000 Hz 25 960 Hz
coplanar 100 000 Hz 106 400 Hz
Channels recorded: 6 in-phase channels
6 quadrature channels
2 monitor channels
Sensitivity: 0.13 ppm at 400 Hz CP
0.12 ppm at 1 500 Hz CP
0.06 ppm at 3 300 Hz CX
0.24 ppm at 6 200 Hz CP
0.44 ppm at 25 000 Hz CP
0.44 ppm at 100 000 Hz CP
Sample rate: 10 per second, equivalent to 1 sample every 3.5 m, at a survey speed of 125 km/h.
The electromagnetic system utilizes a multicoil coaxial/coplanar technique to energize conductors in different directions. The coaxial coil is vertical with its axis in the flight direction. The coplanar coils are horizontal. The secondary fields are sensed simultaneously by means of receiver coils which are maximally coupled to their respective transmitter coils. The system yields an in-phase and a quadrature channel from each transmitterreceiver coilpair.
Calibration of the system during the survey uses the Fugro AutoCal automatic, internal calibration process. At the beginning and end of each flight, and at intervals during the flight, the system is flown up to high altitude to remove it from any “ground effect” (response from the earth). Any remaining signal from the receiver coils (base level) is measured as the zero level, and removed from the data collected until the time of the next calibration. Following the zero level setting, internal calibration coils, for which the response phase and amplitude have been determined at the factory, are automatically triggered – one for each frequency. The on-time of the coils is sufficient to determine an accurate response through any ambient noise. The receiver response to each calibration coil “event” is compared to the expected response (from the factory calibration) for both phase angle and amplitude, and the applied phase and gain corrections adjusted to bring the data to the correct value.
In addition, the output of the transmitter coils are continuously monitored during the survey, and the applied gains adjusted to correct for any change in transmitter output (due to heating, etc.)
Because the internal calibration coils are calibrated at the factory (on a resistive halfspace) ground calibrations using external calibration coils on-site are not necessary for system calibration. A check calibration may be carried out on-site to ensure all systems are working correctly. All system calibrations will be carried out in the air, at sufficient altitude that there will be no measurable response from the ground.
The internal calibration coils are rigidly positioned and mounted in the system relative to the transmitter and receiver coils. In addition, when the internal calibration coils are calibrated at the factory, a rigid jig is employed to ensure accurate response from the external coils.
Using real time Fast Fourier Transforms and the calibration procedures outlined above, the data will be processed in real time from measured total field at a high sampling rate to in-phase and quadrature values at 10 samples per second.
2.2 Magnetometer
Model: Fugro AM102 processor with Geometrics G822 sensor
Type: Optically pumped cesium vapour
Sensitivity: 0.01 nT
Sample rate: 10 per second
The magnetometer sensor is housed in the EM bird, 29 m below the helicopter.
2.3 Magnetic Base Station
Model: Fugro CF1
Type: Digital recording proton precession
Sensitivity: 0.10 nT
Sample rate: 1 second intervals
The base station was located in a magnetically quiet area away from cultural interference. The clock on the base station was synchronized with the airborne magnetometer to UTC time, permitting subsequent removal of the diurnal variation. The magnetic base station was located at approximately WGS84 latitude 29°21.87’N and longitude 99° 9.92’W.
2.4 Radar Altimeter
Manufacturer: Honeywell/Sperry
Model: AA 330
Type: Short pulse modulation, 4.3 GHz
Sensitivity: 0.3 m
The radar altimeter measures the vertical distance between the helicopter and the ground. This information is used in the processing algorithm that determines conductor depth.
2.5 Analog Recorder
Manufacturer: RMS Instruments
Type: DGR33 dotmatrix 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.
ChannelName / Parameter / Scale
units/mm
400I / coaxial in-phase (400 Hz) / 5 ppm
400Q / coaxial quad (400 Hz) / 5 ppm
1K5I / coplanar in-phase (1500 Hz) / 5 ppm
1K5Q / coplanar quad (1500 Hz) / 5 ppm
6K2I / coplanar in-phase (6200 Hz) / 10 ppm
6K2Q / coplanar quad (6200 Hz) / 10 ppm
1X8I / coaxial in-phase (3300 Hz) / 10 ppm
1X8Q / coaxial quad (3300 Hz) / 10 ppm
25KI / coplanar in-phase (25000 Hz) / 20 ppm
25KQ / coplanar quad (25000 Hz) / 20 ppm
100I / coplanar in-phase (100000 Hz) / 20 ppm
100Q / coplanar quad (100000 Hz) / 20 ppm
ALTL / altimeter (laser) / 3 m
ALTR / altimeter (radar) / 3 m
MAGC / magnetics, coarse / 20 nT
MAGF / magnetics, fine / 2.0 nT
2SP / coplanar spherics monitor
2PL / coplanar power line monitor
1KPA / altimeter (barometric) / 30 m
2TDC / internal (console) temperature / 1º C
3TDC / external temperature / 1º C
Table 2.1 The Analog Profiles, RESOLVE System
2.6 Digital Data Acquisition System
Manufacturer: RMS Instruments
Model: DGR 33
Recorder: SanDisk 48-64 Mb flash cards
The data are stored on PCMCIA flash cards and are transferred to the field workstation PC at the survey base for verification, backup and preparation of in-field products.
2.7 Video Flight Path Recording System
Type: VHS Colour Video Camera (NTSC)
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.
2.8 Navigation (Global Positioning System)
Airborne Receiver for Navigation
Model: Ashtech Glonass GG24
Type: SPS (L1 band), 24-channel, C/A code at 1575.42 MHz,
S code at 0.5625 MHz, Real-time differential.
Sensitivity: -132 dBm, 0.5 second update
Accuracy: Manufacturer’s stated accuracy is better than 10 metres
real-time
Airborne Receiver for Processing
Model: Ashtech Z-Surveyor
Type: Code and carrier tracking of L1 band, 12-channel,
dual-frequency C/A code at 1575.42 MHz, and L2 P-code
at 1227 MHz
Sensitivity: 0.5 second update
Accuracy: Manufacturer’s stated accuracy for differential corrected
GPS is better than 1 metre
Base Station
Model: Ashtech Z-Surveyor
Type: Code and carrier tracking of L1 band, 12-channel,
dual-frequency C/A code at 1575.42 MHz, and L2 P-code
at 1227 MHz
Sensitivity: 0.5 second update
Accuracy: Manufacturer’s stated accuracy for differential corrected
GPS is better than 1 metre
The Ashtech GG24 is a line of sight, satellite navigation system which utilizes time-coded signals from at least four of forty-eight available satellites. Both Russian GLONASS and American NAVSTAR satellite constellations are used to calculate the position and to provide real time guidance to the helicopter. The Ashtech system can be combined with a RACAL or similar GPS receiver which further improves the accuracy of the flying and subsequent flight path recovery to better than 5 metres. The differential corrections, which are obtained from a network of virtual reference stations, are transmitted to the helicopter via a spot-beam satellite. For flight path processing an Ashtech Z-surveyor was used as the mobile receiver and base station receiver. The mobile and base station raw XYZ data are recorded, thereby permitting post-survey processing for theoretical accuracies of better than 5 metres. The final navigation channels were produced from the Ashtech Z-Surveyor GPS unit which was located on the EM sensor.
The GPS base station was run for a period of at least 24 hours to obtained a master position for post processing. For the Seco Creek survey the GPS station was located at latitude 29º 21' 53.5493”N, longitude 99º 09' 55.2231”W at an elevation of 277.5 metres a.m.s.l. The GPS records data relative to the WGS84 ellipsoid, which is the basis of the revised North American Datum (NAD83).
2.9 Field Workstation and Data Verification
A laptop computer is used at the survey base to verify data quality and completeness. Flight data are transferred to the PC hard drive to permit the creation of a Geosoft database. This process allows the geophysicist to display both the positional and geophysical data on a screen or printer.
3.8
3.0 PRODUCTS AND PROCESSING TECHNIQUES
3.1 Base Maps
Base maps of the survey areas are produced from published topographic maps. These provide a relatively accurate, distortion-free base which facilitates correlation of the navigation data to the UTM grid. The original topographic maps are scanned to a digital format and combined with geophysical data for plotting the final maps. All maps are created using the following parameters:
Projection Description:
Datum: NAD27
Ellipsoid: Clarke 1866
Projection: UTM (Zone: 14)
Central Meridian: 99º West
False Northing: 0
False Easting: 500000
Scale Factor: 0.9996
WGS84 to Local Conversion: Molodensky
X,Y,Z Datum Shifts: 8 -159 -175
Table 3.1 Survey Products
Digital Geosoft GDB archive format on CD-ROM (2 copies)
Digital grid archives in Geosoft GRD format on CD-ROM (2 copies)
Survey report (2 copies)
Analog chart records
Flight path videocassettes (VHS format)
Note: Other products can be produced from existing survey data, if requested.
3.2 Apparent Resistivity
The apparent resistivity in ohm-metres (ohm-m) can be generated from the in-phase and quadrature EM components for any of the frequencies, using a pseudolayer half-space model. Resistivity maps portray all the EM information for that frequency over the entire survey area. This contrasts with discrete electromagnetic anomaly maps which provide information only over interpreted conductors. The large dynamic range makes the resistivity parameter an excellent mapping tool.