**APPENDIX I**

#######  Report #03069 {#report-03069 .unnumbered}

**RESOLVE SURVEY**

**FOR**

**U. S. GEOLOGICAL SURVEY**

**NORTHERN BEXAR COUNTY, TEXAS**

![](media/image1.wmf)

######## Fugro Airborne Surveys Corp. Michael J. Cain {#fugro-airborne-surveys-corp.-michael-j.-cain .unnumbered}

######## Mississauga, Ontario Geophysicist {#mississauga-ontario-geophysicist .unnumbered}

> March 2004

##### SUMMARY  {#summary .unnumbered}

This report describes the logistics, data acquisition and processing of
a RESOLVE airborne geophysical survey carried out for the U. S.
Geological Survey, over parts of northern Bexar County, Texas. Total
coverage of the survey blocks amounted to 1281 km. The survey was flown
on December 10^th^ to December 14^th^, 2003.

The purpose of the survey was to map the conductive and magnetic
properties of Northern Bexar County in the area of the Edwards Aquifer
recharge zone. This was accomplished by using a RESOLVE multi-coil,
multi-frequency electromagnetic system, supplemented by a high
sensitivity cesium magnetometer. The information from these sensors was
processed to produce maps that display the magnetic and conductive
properties of the survey area. A GPS electronic navigation system
ensured accurate positioning of the geophysical data with respect to the
base maps.

The survey data were processed and compiled in the Fugro Airborne
Surveys Toronto office. Map products and digital data were provided in
accordance with the scales and formats specified in the Survey
Agreement.

##### CONTENTS {#contents .unnumbered}

[1. INTRODUCTION 1.1](#introduction)

[2. SURVEY OPERATIONS 2.1](#survey-operations)

[3. SURVEY EQUIPMENT 3.1](#survey-equipment)

[Electromagnetic System 3.1](#electromagnetic-system)

[RESOLVE System Calibration 3.2](#resolve-system-calibration)

[Airborne Magnetometer 3.4](#airborne-magnetometer)

[Magnetic Base Station 3.4](#magnetic-base-station)

[Navigation (Global Positioning System)
3.6](#navigation-global-positioning-system)

[Radar Altimeter 3.7](#radar-altimeter)

[Barometric Pressure and Temperature Sensors
3.8](#barometric-pressure-and-temperature-sensors)

[Laser Altimeter 3.8](#laser-altimeter)

[Analog Recorder 3.9](#analog-recorder)

[Digital Data Acquisition System 3.9](#digital-data-acquisition-system)

[Flight Path Video Recording System
3.10](#flight-path-video-recording-system)

[4. QUALITY CONTROL AND IN-FIELD PROCESSING
4.1](#quality-control-and-in-field-processing)

[5. DATA PROCESSING 5.1](#data-processing)

[Flight Path Recovery 5.1](#flight-path-recovery)

[Electromagnetic Data/Apparent Resistivity
5.1](#electromagnetic-dataapparent-resistivity)

[Total Magnetic Field 5.3](#total-magnetic-field)

[Contour, Colour and Shadow Map Displays
5.4](#contour-colour-and-shadow-map-displays)

[Resistivity-depth Sections 5.5](#resistivity-depth-sections)

[6. PRODUCTS 6.1](#products)

[Base Maps 6.1](#base-maps)

[Final Products 6.2](#final-products)

[7. CONCLUSIONS AND RECOMMENDATIONS
7.1](#conclusions-and-recommendations)

#####   {#section .unnumbered}

##### APPENDICES {#appendices .unnumbered}

A.  List of Personnel

B.  Data Archive Description

C.  Background Information

D.  Flight Logs

E.  Tests and Calibrations

F.  Processing Log

G.  Glossary

# INTRODUCTION

A RESOLVE electromagnetic/resistivity/magnetic survey was flown for the
U. S. Geological Survey from December 10^th^ to 14^th^, 2003, over part
of the Edwards Aquifer recharge zone in Northern Bexar County, Texas.

Survey coverage consisted of 1281 line‑km, over 1 block, including a
central detail area. A single line was flown along Salado Creek and
Lewis Creek within the survey area. Flight lines were flown in an
azimuthal direction of 90° with a line separation of 200 metres. The
detail area was flown within the main survey area with an offset of 100
metres, giving an effective line spacing of 100 metres within the detail
area. Several tie lines were flown perpendicular to the survey lines,
but due to flight and schedule restrictions set by the military bases,
tie line coverage was limited and not complete in all areas.

The survey employed the RESOLVE electromagnetic system. Ancillary
equipment consisted of a high sensitivity cesium magnetometer, radar,
laser and barometric altimeters, video camera, analog and digital
recorders, and an electronic navigation system. The instrumentation was
installed in an AS350-B2 turbine helicopter (Registration C-GZTA) that
was provided by Questral Helicopters Ltd. The helicopter flew at an
average airspeed of 135 km/h with an EM sensor height of approximately
35 metres.

![](media/image3.jpeg){width="6.920833333333333in"
height="4.575694444444444in"}

Figure 1: Fugro Airborne Surveys RESOLVE EM bird with AS350-B3

# SURVEY OPERATIONS

The base of operations for the survey was established in north San
Antonio, Texas. The helicopter was based and fueled out of the San
Antonio International airport at Hallmark Aviation. The bird and base
stations were located on Camp Stanley. The survey was flown from
December 10^th^ -- 14^th^, 2003.

**Table 2.1 - Survey Specifications**

+-----------------------------------+-----------------------------------+
| **Parameter**                     | **Specifications**                |
+-----------------------------------+-----------------------------------+
| Traverse line direction           | 90°/270°                          |
|                                   |                                   |
| Traverse line spacing             | 200 m                             |
|                                   |                                   |
| Tie line direction                | approximately 0°/180°             |
|                                   |                                   |
| Tie line spacing                  | variable                          |
|                                   |                                   |
| Sample interval                   | 10 Hz or 3.8 m at 135 km/hr       |
|                                   |                                   |
| Aircraft mean terrain clearance   | 62 m                              |
|                                   |                                   |
| EM sensor mean terrain clearance  | 35 m                              |
|                                   |                                   |
| Mag sensor mean terrain clearance | 35 m                              |
|                                   |                                   |
| Average speed                     | 135 km/hr                         |
|                                   |                                   |
| Navigation (guidance)             | ±5 m, Real-time GPS               |
|                                   |                                   |
| Post-survey flight path           | ±2 m, Differential GPS            |
+-----------------------------------+-----------------------------------+

![](media/image4.jpeg){width="6.679166666666666in"
height="5.799305555555556in"}

Figure 2

Location Map and Sheet Layout

Northern Bexar County, Texas

Job \# 03069

Table 2.2 -- Survey Block Corners

  ------------------ ------------ ----------------- ----------------------
  **Nad27 Utm Zone                                  
  14**                                              

  Block              Corners      X-UTM (E)         Y-UTM (N)

  **03069-1**        1            554854            3291502

  Camp Bullis        2            554775            3290742

  Camp Stanley       3            554225            3289838

                     4            553989            3288514

                     5            548092            3288540

                     6            548092            3288527

                     7            548118            3286340

                     8            546362            3286326

                     9            546152            3286130

                     10           546270            3285422

                     11           546144            3284314

                     12           546193            3283356

                     13           546108            3282944

                     14           546387            3281600

                     15           546302            3280484

                     16           546544            3279853

                     17           546593            3279320

                     18           546447            3278871

                     19           546544            3277333

                     20           545881            3277331

                     21           545881            3276578

                     22           543973            3276578

                     23           543973            3274592

                     24           542480            3274592

                     25           540888            3273797

                     26           539067            3273797

                     27           537932            3277331

                     28           537935            3281908

                     29           537203            3281908

                     30           537199            3282828

                     31           535368            3282828

                     32           532939            3286950

                     33           536213            3286950

                     34           536540            3287382

                     35           536531            3289774

                     36           533853            3289774

                     37           533853            3291523
  ------------------ ------------ ----------------- ----------------------

  ------------------ ------------ ----------------- ----------------------
  **Nad27 Utm Zone                                  
  14**                                              

  Block              Corners      X-UTM (E)         Y-UTM (N)

  **03069-3**        1            544000            3279400

  Camp Bullis        2            537930            3279400

  Infills            3            537935            3281908

                     4            537203            3281908

                     5            537199            3282828

                     6            535368            3282828

                     7            532939            3286950

                     8            544000            3286950

  **03069-5**        1            537199            3287491

  B3 Infills         2            537499            3287491

                     3            537499            3285491

                     4            537199            3285491
  ------------------ ------------ ----------------- ----------------------

# SURVEY EQUIPMENT

This section provides a brief description of the geophysical instruments
used to acquire the survey data and the calibration procedures employed.
The geophysical equipment was installed in an AS350-B2 helicopter. This
aircraft provides a safe and efficient platform for surveys of this
type.

## Electromagnetic System {#electromagnetic-system .unnumbered}

Model: RESOLVE

Type: Towed bird, symmetric dipole configuration operated at a nominal
survey altitude of 30 metres. Coil separation is 7.9 metres for 400 Hz,
1500 Hz, 6400 Hz, 25,000 Hz and 115,000 Hz coplanar coil-pairs; and 9.0
metres for the 3300 Hz coaxial coil-pair. The EM bird is towed on a
cable measuring 28.7 metres (94 feet). Due to airlift and wind
resistance on the bird and cable during flight, a slightly shorter value
of 27.7 metres (91 feet) is subtracted from the radar altimeter data to
give the approximate bird height. These results agree with the laser
altimeter values at survey height and speed.

Coil orientations/frequencies: [orientation]{.underline}
[nominal]{.underline} [actual]{.underline}

coplanar 400 Hz 389 Hz

coplanar 1500 Hz 1574 Hz

coaxial 3300 Hz 3245 Hz

coplanar 6400 Hz 6075 Hz

coplanar 25,000 Hz 25,300 Hz

coplanar 115,000 Hz 114,940 Hz

Channels recorded: 6 in-phase channels

6 quadrature channels

2 monitor channels

Sensitivity: 0.12 ppm at 400 Hz CP

0.12 ppm at 1500 Hz CP

0.12 ppm at 3300 Hz CX

0.24 ppm at 6400 Hz CP

0.60 ppm at 25,000 Hz CP

0.60 ppm at 115,000 Hz CP

Sample rate: 10 per second, equivalent to 1 sample every 3.8 m, at a
survey speed of 135 km/h.

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 that are maximum coupled to their respective
transmitter coils. The system yields an in-phase and a quadrature
channel from each transmitter‑receiver coil‑pair.

## RESOLVE System Calibration {#resolve-system-calibration .unnumbered}

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 are 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.

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.

## Airborne Magnetometer {#airborne-magnetometer .unnumbered}

Model: Fugro AM102 processor with Scintrex CS2 sensor

Type: Optically pumped cesium vapour

Sensitivity: 0.01 nT

Sample rate: 10 per second

The magnetometer sensor is located inside the EM bird.

## Magnetic Base Station {#magnetic-base-station .unnumbered}

[Primary]{.underline}

Model: Fugro CF1 base station with timing provided by integrated GPS

Sensor type: Geometrics G822

Counter specifications: Accuracy: ±0.1 nT

Resolution: 0.01 nT

Sample rate 1 Hz

GPS specifications: Model: Marconi Allstar

Type: Code and carrier tracking of L1 band,

12-channel, C/A code at 1575.42 MHz

Sensitivity: -90 dBm, 1.0 second update

Accuracy: Manufacturer's stated accuracy for differential

corrected GPS is 2 metres

Environmental

Monitor specifications: Temperature:

-   Accuracy: ±1.5ºC max

-   Resolution: 0.0305ºC

-   Sample rate: 1 Hz

-   Range: -40ºC to +75ºC

Barometric pressure:

-   Model: Motorola MPXA4115A

-   Accuracy: ±3.0º kPa max (-20ºC to 105ºC temp. ranges)

-   Resolution: 0.013 kPa

-   Sample rate: 1 Hz

-   Range: 55 kPa to 108 kPa

[Backup Magnetometer]{.underline}

Model: GEM Systems GSM-19T

Type: Digital recording proton precession

Sensitivity: 0.10 nT

Sample rate: 3 second intervals

A digital recorder is operated in conjunction with the base station
magnetometer to record the diurnal variations of the earth\'s magnetic
field. The clock of the base station is synchronized with that of the
airborne system, using GPS time, to permit subsequent removal of diurnal
drift. The CF1 base station was located at approximately WGS84 LAT
29.7155° and LON 98.6148° at 369.5 metres above the ellipsoid.

## Navigation (Global Positioning System) {#navigation-global-positioning-system .unnumbered}

[Airborne Receiver for Real-time Navigation & Guidance]{.underline}

Model: Ashtech Glonass GG24 with PNAV 2100 interface

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 5 metres

real-time

The antenna for the GPS guidance system is mounted on the tail fin of
the helicopter.

[Airborne Receiver for Flight Path Recovery]{.underline}

Model: Ashtech Dual Frequency Z-Surveyor

Type: Code and carrier tracking of L1 band, 12-channel, dual

frequency C/A code at 1575.2 MHz, and L2 P-code

1227 MHz

Sensitivity: 0.5 second update

Accuracy: Manufacturer's stated accuracy for differential corrected

GPS is better than 1 metre

The antenna for the GPS flight path recovery system is housed on the
rear of the EM bird.

[Primary Base Station for Post-Survey Differential
Correction]{.underline}

Model: Novatel Millennium

Type: Code and carrier tracking of L1-C/A code at 1575.42 MHz

and L2-P code at 1227.0 MHz. Dual frequency, 24-channel

Sample rate: 1.0 second update

Accuracy: Better than 1 metre in differential mode

[Secondary GPS Base Station]{.underline}

Model: Marconi Allstar OEM, CMT-1200

Type: Code and carrier tracking of L1 band, 12-channel, C/A code

at 1575.42 MHz

Sensitivity: -90 dBm, 1.0 second update

Accuracy: Manufacturer's stated accuracy for differential corrected GPS

is 2 metres.

The Ashtech GG24 is a line of sight, satellite navigation system that
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. For flight path processing an Ashtech
Z-surveyor was used as the mobile receiver. A Novatel Millennium duel
frequency system was used as the primary base station receiver. The base
station was located at WGS84 LAT 29° 42' 54.72030" N and LON 98° 36'
52.90654" W at 368.8 metres above the ellipsoid. The mobile and base
station raw XYZ data were recorded, thereby permitting post-survey
differential corrections for theoretical accuracies of better than 2
metres. A Marconi Allstar GPS unit was used as a secondary (back-up)
base station.

## Radar Altimeter {#radar-altimeter .unnumbered}

Manufacturer: Honeywell/Sperry

Model: RT330

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.

## Barometric Pressure and Temperature Sensors {#barometric-pressure-and-temperature-sensors .unnumbered}

Model: DIGHEM D 1300

Type: Motorola MPX4115AP analog pressure sensor

AD592AN high-impedance remote temperature sensors

Sensitivity: Pressure: 150 mV/kPa

Temperature: 100 mV/°C or 10 mV/°C (selectable)

Sample rate: 10 per second

The D1300 circuit is used in conjunction with one barometric sensor and
up to three temperature sensors. Two sensors (baro and temp) are
installed in the EM console in the aircraft, to monitor pressure and
internal operating temperatures.

## Laser Altimeter {#laser-altimeter .unnumbered}

Manufacturer: Optech

Model: G150

Type: Fixed pulse repetition rate of 2 kHz

Sensitivity: ±5 cm from 10ºC to 30ºC

±10 cm from -20ºC to +50ºC

The laser altimeter is housed in the EM bird, and measures the distance
from the EM bird to ground, except in areas of dense tree cover.

## Analog Recorder {#analog-recorder .unnumbered}

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 3‑1 lists the geophysical data channels and the
vertical scale of each profile.

## Digital Data Acquisition System {#digital-data-acquisition-system .unnumbered}

Manufacturer: RMS Instruments

Model: DGR 33

Recorder: San Disk compact flash card (PCMCIA)

The data are stored on flash cards and are downloaded to the field
workstation PC at the survey base for verification, backup and
preparation of in-field products.

##   {#section-1 .unnumbered}

## Flight Path Video Recording System {#flight-path-video-recording-system .unnumbered}

Recorder: Panasonic AG-720

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.

###  {#section-2 .unnumbered}

###  {#section-3 .unnumbered}

### Table 3-1. The Analog Profiles {#table-3-1.-the-analog-profiles .unnumbered}

+-----------+------------------------------------+---------------------+
| Channel   | Parameter                          | Scale               |
|           |                                    |                     |
| Name      |                                    | 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               |
+-----------+------------------------------------+---------------------+
| > 1X8I    | coaxial in-phase ( 3300 Hz)        | 5 ppm               |
+-----------+------------------------------------+---------------------+
| > 1X8Q    | coaxial quad ( 3300 Hz)            | 5 ppm               |
+-----------+------------------------------------+---------------------+
| > 6K2I    | coplanar in-phase ( 6200 Hz)       | 10 ppm              |
+-----------+------------------------------------+---------------------+
| > 6K2Q    | coplanar quad ( 6200 Hz)           | 10 ppm              |
+-----------+------------------------------------+---------------------+
| > 25KI    | coplanar in-phase ( 25,000 Hz)     | 40 ppm              |
+-----------+------------------------------------+---------------------+
| > 25KQ    | coplanar quad ( 25,000 Hz)         | 40 ppm              |
+-----------+------------------------------------+---------------------+
| > 100I    | coplanar inphase ( 115,000 Hz)     | 40 ppm              |
+-----------+------------------------------------+---------------------+
| > 100Q    | coplanar quad ( 115,000 Hz)        | 40 ppm              |
+-----------+------------------------------------+---------------------+
| > ALTL    | altimeter (laser)                  | 3 m                 |
+-----------+------------------------------------+---------------------+
| > ALTR    | altimeter (radar)                  | 3 m                 |
+-----------+------------------------------------+---------------------+
| > MAG1    | magnetics, coarse                  | 20 nT               |
+-----------+------------------------------------+---------------------+
| > 1SP     | coaxial spherics monitor           |                     |
+-----------+------------------------------------+---------------------+
| > 2SP     | coplanar spherics monitor          |                     |
+-----------+------------------------------------+---------------------+
| > 2PL     | coplanar powerline monitor         |                     |
+-----------+------------------------------------+---------------------+
| > 1KPA    | altimeter (barometric)             | 30 m                |
+-----------+------------------------------------+---------------------+
| > 2TDC    | internal (console) temperature     | 1º C                |
+-----------+------------------------------------+---------------------+
| > 3TDC    | external temperature               | 1º C                |
+-----------+------------------------------------+---------------------+

# QUALITY CONTROL AND IN-FIELD PROCESSING

Digital data for each flight were transferred to the field workstation,
in order to verify data quality and completeness. A database was created
and updated using Geosoft Oasis Montaj and proprietary Fugro Atlas
software. This allowed the field personnel to calculate, display and
verify both the positional (flight path) and geophysical data on a
screen or printer. Analog records were examined as a preliminary
assessment of the data acquired for each flight.

In-field processing of Fugro survey data consists of differential
corrections to the airborne GPS data, verification of EM calibrations,
drift correction of the raw airborne EM data, spike rejection and
filtering of all geophysical and ancillary data, verification of flight
videos, calculation of preliminary resistivity data, diurnal correction,
and preliminary leveling of magnetic data.

All data, including base station records, were checked on a daily basis,
to ensure compliance with the survey contract specifications. Reflights
were required if any of the standard specifications were not met.

# DATA PROCESSING

## Flight Path Recovery {#flight-path-recovery .unnumbered}

The raw range data from at least four satellites are simultaneously
recorded by both the base and mobile GPS units. The geographic positions
of both units, relative to the model ellipsoid, are calculated from this
information. Differential corrections, which are obtained from the base
station, are applied to the mobile unit data to provide a post-flight
track of the aircraft, accurate to within 2 m. Speed checks of the
flight path are also carried out to determine if there are any spikes or
gaps in the data.

The corrected WGS84 latitude/longitude coordinates are transformed to
the coordinate system used on the final maps. Images or plots are then
created to provide a visual check of the flight path.

## Electromagnetic Data/Apparent Resistivity {#electromagnetic-dataapparent-resistivity .unnumbered}

EM data are processed at the recorded sample rate of 10 samples/second.
Spheric rejection median and Hanning filters were applied to reduce
noise to acceptable levels.

The apparent resistivity in ohm‑m were generated from the in-phase and
quadrature EM components for all of the coplanar frequencies, using a
pseudo‑layer half-space model. The inputs to the resistivity algorithm
are the inphase and quadrature amplitudes of the secondary field. The
algorithm calculates the apparent resistivity in ohm-m, and the apparent
height of the bird above the conductive source. Any difference between
the apparent height and the true height, as measured by the radar
altimeter, is called the pseudo-layer and reflects the difference
between the real geology and a homogeneous halfspace. This difference is
often attributed to the presence of a highly resistive upper layer. Any
errors in the altimeter reading, caused by heavy tree cover, are
included in the pseudo-layer and do not affect the resistivity
calculation. The apparent depth estimates, however, will reflect the
altimeter errors. Apparent resistivity calculated in this manner may
behave quite differently from those calculated using other models.

In areas of high magnetic permeability or dielectric permittivity, the
calculated resistivities will be erroneously high. Various algorithms
and inversion techniques can be used to partially correct for this
effect.

The preliminary apparent resistivity maps and images were carefully
inspected to identify any lines or line segments that might require base
level adjustments. Subtle changes between in-flight calibrations of the
system can result in line-to-line differences that are more recognizable
in resistive (low signal amplitude) areas. Manual leveling was carried
out to eliminate or minimize resistivity differences that can be
attributed, in part, to changes in operating temperatures. These
leveling adjustments were usually very subtle, and do not result in the
degradation of discrete anomalies.

After the manual leveling process is complete, the data were subjected
to a microleveling technique in order to remove any remaining
line-to-line differences within the calculated resistivities.

Apparent resistivity grids, which display the conductive properties of
the survey areas, were produced from the 400 Hz, 1500 Hz, 6400 Hz,
25,000 Hz and 115,000 Hz coplanar data. The calculated resistivities for
the five coplanar frequencies are included in the XYZ and grid archives.
Values are in ohm-metres on all final products.

## Total Magnetic Field {#total-magnetic-field .unnumbered}

The aeromagnetic data were inspected in grid and profile format. Spikes
were removed manually with the aid of a fourth difference calculation. A
Geometrics G822 cesium vapour magnetometer was operated at the survey
base to record diurnal variations of the earth's magnetic field. The
clock of the base station was synchronized with that of the airborne
system to permit subsequent removal of diurnal drift. The data were
inspected for spikes and filtered. The filtered diurnal data were
subtracted from the total field magnetic data. Grids of the diurnally
corrected aeromagnetic data were created and contoured. A lag correction
was applied to the magnetic data. The results were then leveled using
tie and traverse line intercepts. Manual adjustments were applied to any
lines that required leveling, as indicated by shadowed images of both
the total field magnetic data and the calculated vertical gradient data.
A microleveling algorithm was used to make any remaining subtle leveling
adjustments.

#####  {#section-4 .unnumbered}

## Contour, Colour and Shadow Map Displays {#contour-colour-and-shadow-map-displays .unnumbered}

The geophysical data are interpolated onto a regular grid using a
modified Akima spline technique. The resulting grid is suitable for
image processing and generation of contour maps. The grid cell size was
20% of the line interval; 40 metres for the 200 metre spaced portion of
the survey area, 20 metres for the 100 metre spaced detail area.

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.

Monochromatic shadow maps or images can be generated by employing an
artificial sun to cast shadows on a surface defined by the geophysical
grid. There are many variations in the shadowing technique. These
techniques can be applied to total field or enhanced magnetic data,
magnetic derivatives, resistivity, etc. The shadowing technique is also
used as a quality control method to detect subtle changes between lines.

## Resistivity-depth Sections {#resistivity-depth-sections .unnumbered}

The apparent resistivities for all frequencies can be displayed
simultaneously as coloured resistivity-depth sections. Usually, only the
coplanar data are displayed as the close frequency separation between
the coplanar and adjacent coaxial data tends to distort the section. The
sections can be plotted using the topographic elevation profile as the
surface. The digital terrain values, in metres a.m.s.l., can be
calculated from the GPS Z-value or barometric altimeter, minus the
aircraft radar altimeter.

Resistivity-depth sections can be generated in three formats:

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

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

\(3\) Occam[^3] or Multi-layer[^4] inversion.

Both the Sengpiel and differential methods are derived from the
pseudo-layer half-space model. Both yield a coloured resistivity-depth
section that attempts to portray a smoothed approximation of the true
resistivity distribution with depth. Resistivity-depth sections are 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 in-phase responses have been suppressed by the effects of
magnetite, or adversely affected by cultural features, the computed
resistivities shown on the sections may be unreliable.

Both the Occam and multi-layer inversions compute the layered earth
resistivity model that would best match the measured EM data. The Occam
inversion uses a series of thin, fixed layers (usually 20 x 5m and 10 x
10m layers) and computes resistivities to fit the EM data. The
multi-layer inversion computes the resistivity and thickness for each of
a defined number of layers (typically 3-5 layers) to best fit the data.

# PRODUCTS

This section lists the final maps and products that have been provided
under the terms of the survey agreement. Other products can be prepared
from the existing dataset, if requested. These include magnetic
enhancements or derivatives, percent magnetite, resistivities corrected
for magnetic permeability and/or dielectric permittivity, digital
terrain, resistivity-depth sections, inversions, and overburden
thickness.

## Base Maps {#base-maps .unnumbered}

Base maps of the survey area were produced by scanning published
topographic maps to a TIF format. This process provides a relatively
accurate, distortion free base that facilitates correlation of the
navigation data to the map coordinate system. The topographic files were
combined with geophysical data for plotting the final maps. All maps
were created using the following parameters:

[Projection Description]{.underline}:

Datum: NAD 27

Ellipsoid: Clarke 1866

DX,DY,ZY shift 8 -159 -175

Projection: UTM (Zone: 14)

Central Meridian: 99° West

False Northing: 0

False Easting: 500000

Scale Factor: 0.9996

The following parameters are presented on 1 map sheet for each target,
at a scale of 1:24,000. Preliminary products are not listed.

## Final Products {#final-products .unnumbered}

######### Colour Maps (2 copies) at 1:24000 {#colour-maps-2-copies-at-124000 .unnumbered}

Apparent Resistivity 400 Hz

Apparent Resistivity 1500 Hz

Apparent Resistivity 6200 Hz

Apparent Resistivity 25,000 Hz

Apparent Resistivity 115,000 Hz

[Black & White Maps at 1:24000 (Mylar)]{.underline}

Total Magnetic Field maps

[Additional Products]{.underline}

Digital Archive on CD-ROM 2 copies

Survey Report 2 copies

Analog Chart Records All flights

Flight Path Video (VHS) All flights

Flight Path Video (DVD) All flights

# CONCLUSIONS AND RECOMMENDATIONS

This report provides a description of the equipment, data processing
procedures and logistics of the survey.

The various maps included with this report display the magnetic and
conductive properties of the survey area. It is recommended that a
complete assessment and detailed evaluation of the survey results be
carried out, in conjunction with all available geophysical, geological
and geochemical information.

It is also recommended that image processing of existing geophysical
data be considered, in order to extract the maximum amount of
information from the survey results. Current software and imaging
techniques often provide valuable information on structure and
lithology, which may not be clearly evident on the contour and colour
maps. These techniques can yield images that define subtle, but
significant, structural details.

Respectfully submitted,

**FUGRO AIRBORNE SURVEYS CORP.**

Michael Cain, P. Eng.

Geophysicist

### APPENDIX A {#appendix-a .unnumbered}

### LIST OF PERSONNEL {#list-of-personnel .unnumbered}

###  {#section-5 .unnumbered}

The following personnel were involved in the acquisition, processing,
interpretation and presentation of data, relating to a RESOLVE airborne
geophysical survey carried out for the U. S. Geological Survey, over
northern Bexar County, Texas

> David Miles Manager, Helicopter Operations
>
> Emily Farquhar Manager, Data Processing and Interpretation
>
> Bill Brown Sales and Marketing
>
> Michael Cain Project Geophysicist
>
> Elizabeth Bowslaugh Data processor
>
> Darcy Blouin Geophysical Operator
>
> Terry Thomson Pilot (Questral Helicopters Ltd.)
>
> Lyn Vanderstarren Drafting Supervisor
>
> Albina Tonello Secretary/Expeditor

The survey consisted of 1281 km of coverage, flown from December 10^th^
to 14^th^, 2003.

All personnel are employees of Fugro Airborne Surveys, except for the
pilot who is an employee of Questral Helicopters Ltd.

### APPENDIX B {#appendix-b .unnumbered}

### ARCHIVE DESCRIPTION {#archive-description .unnumbered}

This CD-ROM contains final data archives of an airborne survey conducted
by Fugro Airborne Surveys on behalf of the U. S. Geological Survey. The
survey was flown from December 10^th^ to 14^th^, 2003.

Fugro Job #03069

CD Archive number: CCD02102

**Fugro Airborne Surveys Job #03069**

The archives contain three directories.

1.  Line Data: Geosoft GDB database with archive description.

2.  Grids: Grids in Geosoft GRD format for the following parameters:

```{=html}
<!-- -->
```
1.  Magnetic total field (IGRF corrected)

2.  5 coplanar resistivities

3\. Report: A digital copy of the operations report in PDF format

[Projection Description]{.underline}:

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

EM PARAMETERS

FREQUENCY

orientation nominal actual COIL SPACING

\-\-\-\-\-\-\-\-\-\-- \-\-\-\-\-\-- \-\-\-\-\-- \-\-\-\-\-\-\-\-\-\-\--

coplanar 400 Hz 389 Hz 7.9 m

coplanar 1 500 Hz 1 574 Hz 7.9 m

coaxial 3 300 Hz 3 245 Hz 9.0 m

coplanar 6 200 Hz 6 075 Hz 7.9 m

coplanar 25 000 Hz 25 300 Hz 7.9 m

coplanar 115 000 Hz 114 940 Hz 7.9 m

DATABASE : 03069_BEXAR.GDB

====================================================

NUMBER OF DATABASE CHANNELS : 99

====================================================

1 - ALTBIRDM BIRD RADAR ALTIMETER (METRES)

2 - ALTBIRDR BIRD RADAR ALTIMETER RAW (METRES)

3 - ALTLASM BIRD LASER ALTIMETER PROCESSED (METRES)

4 - ALTLASR BIRD LASER ALTIMETER RAW (METRES)

5 - BALT BAROMETRIC ALTIMETER RAW (METRES)

6 - CEN115k CENTROID DEPTH 115K HZ

7 - CEN1500 CENTROID DEPTH 1500 HZ

8 - CEN25k CENTROID DEPTH 25K HZ

9 - CEN400 CENTROID DEPTH 400 HZ

10 - CEN6200 CENTROID DEPTH 6200 HZ

11 - CPI115K COPLANAR INPHASE FULLY PROCESSED 115K HZ

12 - CPI1500 COPLANAR INPHASE FULLY PROCESSED 1500 HZ

13 - CPI25K COPLANAR INPHASE FULLY PROCESSED 25K HZ

14 - CPI400 COPLANAR INPHASE FULLY PROCESSED 400 HZ

15 - CPI6200 COPLANAR INPHASE FULLY PROCESSED 6200 HZ

16 - CPQ115K COPLANAR QUADRATURE FULLY PROCESSED 115K HZ

17 - CPQ1500 COPLANAR QUADRATURE FULLY PROCESSED 1500 HZ

18 - CPQ25K COPLANAR QUADRATURE FULLY PROCESSED 25K HZ

19 - CPQ400 COPLANAR QUADRATURE FULLY PROCESSED 400 HZ

20 - CPQ6200 COPLANAR QUADRATURE FULLY PROCESSED 6200 Hz

21 - CXI3300 COAXIAL INPHASE FULLY PROCESSED 3300 HZ

22 - CXQ3300 COAXIAL QUADRATURE FULLY PROCESSED 3300 HZ

23 - CPPL COPLANAR POWERLINE MONITOR

24 - CXPL COAXIAL POWERLINE MONITOR

25 - Date FLIGHT DATE

26 - DDEP115K DIFFERENTIAL DEPTH 115K HZ FILTERED

27 - DDEP1500 DIFFERENTIAL DEPTH 1500HZ FILTERED

28 - DDEP25K DIFFERENTIAL DEPTH 25K HZ FILTERED

29 - DDEP400 DIFFERENTIAL DEPTH 400 HZ FILTERED

30 - DDEP6200 DIFFERENTIAL DEPTH 6200 HZ FILTERED

31 - DEP115K APPARENT DEPTH 115K HZ FILTERED

32 - DEP1500 APPARENT DEPTH 1500HZ FILTERED

33 - DEP25K APPARENT DEPTH 25K HZ FILTERED

34 - DEP3300 APPARENT DEPTH 3300 HZ FILTERED

35 - DEP400 APPARENT DEPTH 400 HZ FILTERED

36 - DEP6200 APPARENT DEPTH 6200 HZ FILTERED

37 - DIURNAL DIURNAL CORRECTION

38 - DRES115K DIFFERENTIAL RESISTIVITY 115K HZ FILTERED

39 - DRES1500 DIFFERENTIAL RESISTIVITY 1500 HZ FILTERED

40 - DRES25K DIFFERENTIAL RESISTIVITY 25K HZ FILTERED

41 - DRES400 DIFFERENTIAL RESISTIVITY 400 HZ FILTERED

42 - DRES6200 DIFFERENTIAL RESISTIVITY 6200 HZ FILTERED

43 - DTM DIGITAL TERRAIN MODEL

44 - FID FIDUCIAL COUNTER

45 - Flight FLIGHT NUMBER

46 - GPSZ4 RAW GPS ELEVATION

47 - L100I BASE LEVELED INPHASE 115K HZ

48 - L100Q BASE LEVELED QUADRATURE 115K HZ

49 - L1K5I BASE LEVELED INPHASE 1500 HZ

50 - L1K5Q BASE LEVELED QUADRATURE 1500 HZ

51 - L1K7I BASE LEVELED INPHASE 3300 HZ

52 - L1K7Q BASE LEVELED QUADRATURE 3300 HZ

53 - L25KI BASE LEVELED INPHASE 25K HZ

54 - L25KQ BASE LEVELED QUADRATURE 25K HZ

55 - L400I BASE LEVELED INPHASE 400 HZ

56 - L400Q BASE LEVELED QUADRATURE 400 HZ

57 - L6K2I BASE LEVELED INPHASE 6200 HZ

58 - L6K2Q BASE LEVELED QUADRATURE 6200 HZ

59 - LAT WGS84 LATITUDE

60 - LON WGS84 LONGITUDE

61 - LINE LINE NUMBER

62 - MAGD DIURNAL CORRECTED MAG

63 - MAGRAW RAW MAG

64 - MAG_GL MAG MICROLEVELED

65 - MAG_IGRFZ MAG IGRF REMOVED USING Z ALTITUDE

66 - MAG_LEVEL TIE LINE LEVELED MAG

67 -- TMI MAG Final

68 - RES115K RESISTIVITY 115K HZ

69 - RES1500 RESISTIVITY 1500 HZ

70 - RES25K RESISTIVITY 25K HZ

71 - RES3300 RESISTIVITY 3300 HZ

72 - RES400 RESISTIVITY 400 HZ F

73 - RES6200_filt RESISTIVITY 6200 HZ

74 - RES115K_filt RESISTIVITY 115K HZ FILTERED

75 - RES1500_filt RESISTIVITY 1500 HZ FILTERED

76 - RES25K_filt RESISTIVITY 25K HZ FILTERED

77 - RES400_filt RESISTIVITY 400 HZ FILTERED

78 - RES6200_filt RESISTIVITY 6200 HZ FILTERED

79 - UTC UTC TIME

80 - X FINAL BIRD UTM X CLARK 1866 (NAD27)

81 - X4 RAW BIRD UTM X CLARK 1866 (NAD27)

82 - Y FINAL UTM Y CLARK 1866 (NAD27)

83 - Y4 RAW UTM Y CLARK 1866 (NAD27)

84 - X_HELI UTM X FROM HELI GPS CLARK 1866 (NAD27)

85 - Y_HELI UTM X FROM HELI GPS CLARK 1866 (NAD27)

86 - Z FINAL GPS ELEVATION

87 - \_100I RAW INPHASE 115K HZ

88 - \_100Q RAW QUADRATURE 115K HZ

89 - \_1K5I RAW INPHASE 1500 HZ

90 - \_1K5Q RAW QUADRATURE 1500 HZ

91 - \_1K7I RAW INPHASE 3300 HZ

92 - \_1K7Q RAW QUADRATURE 3300 HZ

93 - \_25KI RAW INPHASE 25K HZ

94 - \_25KQ RAW QUADRATURE 25K HZ

95 - \_400I RAW INPHASE 400 HZ

96 - \_400Q RAW QUADRATURE 400 HZ

97 - \_6K2I RAW INPHASE 6200 HZ

98 - \_6K2Q RAW QUADRATURE 6200 HZ

====================================================

### APPENDIX C {#appendix-c .unnumbered}

### BACKGROUND INFORMATION {#background-information .unnumbered}

### Electromagnetics {#electromagnetics .unnumbered}

Fugro electromagnetic responses fall into two general classes, discrete
and broad. The discrete class consists of sharp, well-defined anomalies
from discrete conductors such as sulphide lenses and steeply dipping
sheets of graphite and sulphides. The broad class consists of wide
anomalies from conductors having a large horizontal surface such as
flatly dipping graphite or sulphide sheets, saline water-saturated
sedimentary formations, conductive overburden and rock, kimberlite pipes
and geothermal zones. A vertical conductive slab with a width of 200 m
would straddle these two classes.

The vertical sheet (half plane) is the most common model used for the
analysis of discrete conductors. All anomalies plotted on the
geophysical maps are analyzed according to this model. The following
section entitled **Discrete Conductor Analysis** describes this model in
detail, including the effect of using it on anomalies caused by broad
conductors such as conductive overburden.

The conductive earth (half-space) model is suitable for broad
conductors. Resistivity contour maps result from the use of this model.
A later section entitled **Resistivity Mapping** describes the method
further, including the effect of using it on anomalies caused by
discrete conductors such as sulphide bodies.

##### Geometric Interpretation {#geometric-interpretation .unnumbered}

The geophysical interpreter attempts to determine the geometric shape
and dip of the conductor. Figure C-1 shows typical HEM anomaly shapes
which are used to guide the geometric interpretation.

##### Discrete Conductor Analysis {#discrete-conductor-analysis .unnumbered}

The EM anomalies appearing on the electromagnetic map are analyzed by
computer to give the conductance (i.e., conductivity-thickness product)
in siemens (mhos) of a vertical sheet model. This is done regardless of
the interpreted geometric shape of the conductor. This is not an
unreasonable procedure, because the computed conductance increases as
the electrical quality of the conductor increases, regardless of its
true shape. DIGHEM anomalies are divided into seven grades of
conductance, as shown in Table C-1. The conductance in siemens (mhos) is
the reciprocal of resistance in ohms.

![](media/image5.png){width="8.247916666666667in"
height="6.189583333333333in"}

### Figure C-1 {#figure-c-1 .unnumbered}

The conductance value is a geological parameter because it is a
characteristic of the conductor alone. It generally is independent of
frequency, flying height or depth of burial, apart from the averaging
over a greater portion of the conductor as height increases. Small
anomalies from deeply buried strong conductors are not confused with
small anomalies from shallow weak conductors because the former will
have larger conductance values.

### Table C-1. EM Anomaly Grades {#table-c-1.-em-anomaly-grades .unnumbered}

  ------------------------------- ---------------------------------------
  Anomaly Grade                   Siemens

  7                               \> 100

  6                               50 - 100

  5                               20 - 50

  4                               10 - 20

  3                               5 - 10

  2                               1 - 5

  1                               \< 1
  ------------------------------- ---------------------------------------

Conductive overburden generally produces broad EM responses which may
not be shown as anomalies on the geophysical maps. However, patchy
conductive overburden in otherwise resistive areas can yield discrete
anomalies with a conductance grade (cf. Table C-1) of 1, 2 or even 3 for
conducting clays which have resistivities as low as 50 ohm-m. In areas
where ground resistivities are below 10 ohm-m, anomalies caused by
weathering variations and similar causes can have any conductance grade.
The anomaly shapes from the multiple coils often allow such conductors
to be recognized, and these are indicated by the letters S, H, and
sometimes E on the geophysical maps (see EM legend on maps).

For bedrock conductors, the higher anomaly grades indicate increasingly
higher conductances. Examples: the New Insco copper discovery (Noranda,
Canada) yielded a grade 5 anomaly, as did the neighbouring copper-zinc
Magusi River ore body; Mattabi (copper-zinc, Sturgeon Lake, Canada) and
Whistle (nickel, Sudbury, Canada) gave grade 6; and the Montcalm
nickel-copper discovery (Timmins, Canada) yielded a grade 7 anomaly.
Graphite and sulphides can span all grades but, in any particular survey
area, field work may show that the different grades indicate different
types of conductors.

Strong conductors (i.e., grades 6 and 7) are characteristic of massive
sulphides or graphite. Moderate conductors (grades 4 and 5) typically
reflect graphite or sulphides of a less massive character, while weak
bedrock conductors (grades 1 to 3) can signify poorly connected graphite
or heavily disseminated sulphides. Grades 1 and 2 conductors may not
respond to ground EM equipment using frequencies less than 2000 Hz.

The presence of sphalerite or gangue can result in ore deposits having
weak to moderate conductances. As an example, the three million ton
lead-zinc deposit of Restigouche Mining Corporation near Bathurst,
Canada, yielded a well-defined grade 2 conductor. The 10 percent by
volume of sphalerite occurs as a coating around the fine grained massive
pyrite, thereby inhibiting electrical conduction. Faults, fractures and
shear zones may produce anomalies that typically have low conductances
(e.g., grades 1 to 3). Conductive rock formations can yield anomalies of
any conductance grade. The conductive materials in such rock formations
can be salt water, weathered products such as clays, original
depositional clays, and carbonaceous material.

For each interpreted electromagnetic anomaly on the geophysical maps, a
letter identifier and an interpretive symbol are plotted beside the EM
grade symbol. The horizontal rows of dots, under the interpretive
symbol, indicate the anomaly amplitude on the flight record. The
vertical column of dots, under the anomaly letter, gives the estimated
depth. In areas where anomalies are crowded, the letter identifiers,
interpretive symbols and dots may be obliterated. The EM grade symbols,
however, will always be discernible, and the obliterated information can
be obtained from the anomaly listing appended to this report.

The purpose of indicating the anomaly amplitude by dots is to provide an
estimate of the reliability of the conductance calculation. Thus, a
conductance value obtained from a large ppm anomaly (3 or 4 dots) will
tend to be accurate whereas one obtained from a small ppm anomaly (no
dots) could be quite inaccurate. The absence of amplitude dots indicates
that the anomaly from the coaxial coil-pair is 5 ppm or less on both the
in-phase and quadrature channels. Such small anomalies could reflect a
weak conductor at the surface or a stronger conductor at depth. The
conductance grade and depth estimate illustrates which of these
possibilities fits the recorded data best.

The conductance measurement is considered more reliable than the depth
estimate. There are a number of factors that can produce an error in the
depth estimate, including the averaging of topographic variations by the
altimeter, overlying conductive overburden, and the location and
attitude of the conductor relative to the flight line. Conductor
location and attitude can provide an erroneous depth estimate because
the stronger part of the conductor may be deeper or to one side of the
flight line, or because it has a shallow dip. A heavy tree cover can
also produce errors in depth estimates. This is because the depth
estimate is computed as the distance of bird from conductor, minus the
altimeter reading. The altimeter can lock onto the top of a dense forest
canopy. This situation yields an erroneously large depth estimate but
does not affect the conductance estimate.

Dip symbols are used to indicate the direction of dip of conductors.
These symbols are used only when the anomaly shapes are unambiguous,
which usually requires a fairly resistive environment.

A further interpretation is presented on the EM map by means of the
line-to-line correlation of bedrock anomalies, which is based on a
comparison of anomaly shapes on adjacent lines. This provides conductor
axes that may define the geological structure over portions of the
survey area. The absence of conductor axes in an area implies that
anomalies could not be correlated from line to line with reasonable
confidence.

The electromagnetic anomalies are designed to provide a correct
impression of conductor quality by means of the conductance grade
symbols. The symbols can stand alone with geology when planning a
follow-up program. The actual conductance values are printed in the
attached anomaly list for those who wish quantitative data. The anomaly
ppm and depth are indicated by inconspicuous dots which should not
distract from the conductor patterns, while being helpful to those who
wish this information. The map provides an interpretation of conductors
in terms of length, strike and dip, geometric shape, conductance, depth,
and thickness. The accuracy is comparable to an interpretation from a
high quality ground EM survey having the same line spacing.

The appended EM anomaly list provides a tabulation of anomalies in ppm,
conductance, and depth for the vertical sheet model. No conductance or
depth estimates are shown for weak anomalous responses that are not of
sufficient amplitude to yield reliable calculations.

Since discrete bodies normally are the targets of EM surveys, local base
(or zero) levels are used to compute local anomaly amplitudes. This
contrasts with the use of true zero levels which are used to compute
true EM amplitudes. Local anomaly amplitudes are shown in the EM anomaly
list and these are used to compute the vertical sheet parameters of
conductance and depth.

##### Questionable Anomalies {#questionable-anomalies .unnumbered}

The EM maps may contain anomalous responses that are displayed as
asterisks (\*). These responses denote weak anomalies of indeterminate
conductance, which may reflect one of the following: a weak conductor
near the surface, a strong conductor at depth (e.g., 100 to 120 m below
surface) or to one side of the flight line, or aerodynamic noise. Those
responses that have the appearance of valid bedrock anomalies on the
flight profiles are indicated by appropriate interpretive symbols (see
EM legend on maps). The others probably do not warrant further
investigation unless their locations are of considerable geological
interest.

##### The Thickness Parameter {#the-thickness-parameter .unnumbered}

A comparison of coaxial and coplanar shapes can provide an indication of
the thickness of a steeply dipping conductor. The amplitude of the
coplanar anomaly (e.g., CPI channel) increases relative to the coaxial
anomaly (e.g., CXI) as the apparent thickness increases, i.e., the
thickness in the horizontal plane. (The thickness is equal to the
conductor width if the conductor dips at 90 degrees and strikes at right
angles to the flight line.) This report refers to a conductor as
[thin]{.underline} when the thickness is likely to be less than 3 m, and
[thick]{.underline} when in excess of 10 m. Thick conductors are
indicated on the EM map by parentheses \"( )\". For base metal
exploration in steeply dipping geology, thick conductors can be high
priority targets because many massive sulphide ore bodies are thick. The
system cannot sense the thickness when the strike of the conductor is
subparallel to the flight line, when the conductor has a shallow dip,
when the anomaly amplitudes are small, or when the resistivity of the
environment is below 100 ohm-m.

##### Resistivity Mapping {#resistivity-mapping .unnumbered}

Resistivity mapping is useful in areas where broad or flat lying
conductive units are of interest. One example of this is the clay
alteration which is associated with Carlin-type deposits in the south
west United States. The resistivity parameter was able to identify the
clay alteration zone over the Cove deposit. The alteration zone appeared
as a strong resistivity low on the 900 Hz resistivity parameter. The
7,200 Hz and 56,000 Hz resistivities showed more detail in the covering
sediments, and delineated a range front fault. This is typical in many
areas of the south west United States, where conductive near surface
sediments, which may sometimes be alkalic, attenuate the higher
frequencies.

Resistivity mapping has proven successful for locating diatremes in
diamond exploration. Weathering products from relatively soft kimberlite
pipes produce a resistivity contrast with the unaltered host rock. In
many cases weathered kimberlite pipes were associated with thick
conductive layers that contrasted with overlying or adjacent relatively
thin layers of lake bottom sediments or overburden.

Areas of widespread conductivity are commonly encountered during
surveys. These conductive zones may reflect alteration zones,
shallow-dipping sulphide or graphite-rich units, saline ground water, or
conductive overburden. In such areas, EM amplitude changes can be
generated by decreases of only 5 m in survey altitude, as well as by
increases in conductivity. The typical flight record in conductive areas
is characterized by in-phase and quadrature channels that are
continuously active. Local EM peaks reflect either increases in
conductivity of the earth or decreases in survey altitude. For such
conductive areas, apparent resistivity profiles and contour maps are
necessary for the correct interpretation of the airborne data. The
advantage of the resistivity parameter is that anomalies caused by
altitude changes are virtually eliminated, so the resistivity data
reflect only those anomalies caused by conductivity changes. The
resistivity analysis also helps the interpreter to differentiate between
conductive bedrock and conductive overburden. For example, discrete
conductors will generally appear as narrow lows on the contour map and
broad conductors (e.g., overburden) will appear as wide lows.

The apparent resistivity is calculated using the pseudo-layer (or
buried) half-space model defined by Fraser (1978)[^5]. This model
consists of a resistive layer overlying a conductive half-space. The
depth channels give the apparent depth below surface of the conductive
material. The apparent depth is simply the apparent thickness of the
overlying resistive layer. The apparent depth (or thickness) parameter
will be positive when the upper layer is more resistive than the
underlying material, in which case the apparent depth may be quite close
to the true depth.

The apparent depth will be negative when the upper layer is more
conductive than the underlying material, and will be zero when a
homogeneous half-space exists. The apparent depth parameter must be
interpreted cautiously because it will contain any errors that might
exist in the measured altitude of the EM bird (e.g., as caused by a
dense tree cover). The inputs to the resistivity algorithm are the
in-phase and quadrature components of the coplanar coil-pair. The
outputs are the apparent resistivity of the conductive half-space (the
source) and the sensor-source distance. The flying height is not an
input variable, and the output resistivity and sensor-source distance
are independent of the flying height when the conductivity of the
measured material is sufficient to yield significant in-phase as well as
quadrature responses. The apparent depth, discussed above, is simply the
sensor-source distance minus the measured altitude or flying height.
Consequently, errors in the measured altitude will affect the apparent
depth parameter but not the apparent resistivity parameter.

The apparent depth parameter is a useful indicator of simple layering in
areas lacking a heavy tree cover. Depth information has been used for
permafrost mapping, where positive apparent depths were used as a
measure of permafrost thickness. However, little quantitative use has
been made of negative apparent depths because the absolute value of the
negative depth is not a measure of the thickness of the conductive upper
layer and, therefore, is not meaningful physically. Qualitatively, a
negative apparent depth estimate usually shows that the EM anomaly is
caused by conductive overburden. Consequently, the apparent depth
channel can be of significant help in distinguishing between overburden
and bedrock conductors.

##### Interpretation in Conductive Environments {#interpretation-in-conductive-environments .unnumbered}

Environments having low background resistivities (e.g., below 30 ohm-m
for a 900 Hz system) yield very large responses from the conductive
ground. This usually prohibits the recognition of discrete bedrock
conductors. However, Fugro data processing techniques produce three
parameters that contribute significantly to the recognition of bedrock
conductors in conductive environments. These are the in-phase and
quadrature difference channels (DIFI and DIFQ, which are available only
on systems with "common" frequencies on orthogonal coil pairs), and the
resistivity and depth channels (RES and DEP) for each coplanar
frequency.

The EM difference channels (DIFI and DIFQ) eliminate most of the
responses from conductive ground, leaving responses from bedrock
conductors, cultural features (e.g., telephone lines, fences, etc.) and
edge effects. Edge effects often occur near the perimeter of broad
conductive zones. This can be a source of geologic noise. While edge
effects yield anomalies on the EM difference channels, they do not
produce resistivity anomalies. Consequently, the resistivity channel
aids in eliminating anomalies due to edge effects. On the other hand,
resistivity anomalies will coincide with the most highly conductive
sections of conductive ground, and this is another source of geologic
noise. The recognition of a bedrock conductor in a conductive
environment therefore is based on the anomalous responses of the two
difference channels (DIFI and DIFQ) and the resistivity channels (RES).
The most favourable situation is where anomalies coincide on all
channels.

The DEP channels, which give the apparent depth to the conductive
material, also help to determine whether a conductive response arises
from surficial material or from a conductive zone in the bedrock. When
these channels ride above the zero level on the depth profiles (i.e.,
depth is negative), it implies that the EM and resistivity profiles are
responding primarily to a conductive upper layer, i.e., conductive
overburden. If the DEP channels are below the zero level, it indicates
that a resistive upper layer exists, and this usually implies the
existence of a bedrock conductor. If the low frequency DEP channel is
below the zero level and the high frequency DEP is above, this suggests
that a bedrock conductor occurs beneath conductive cover.

##### Reduction of Geologic Noise {#reduction-of-geologic-noise .unnumbered}

Geologic noise refers to unwanted geophysical responses. For purposes of
airborne EM surveying, geologic noise refers to EM responses caused by
conductive overburden and magnetic permeability. It was mentioned
previously that the EM difference channels (i.e., channel DIFI for
in-phase and DIFQ for quadrature) tend to eliminate the response of
conductive overburden.

Magnetite produces a form of geological noise on the in-phase channels.
Rocks containing less than 1% magnetite can yield negative in-phase
anomalies caused by magnetic permeability. When magnetite is widely
distributed throughout a survey area, the in-phase EM channels may
continuously rise and fall, reflecting variations in the magnetite
percentage, flying height, and overburden thickness. This can lead to
difficulties in recognizing deeply buried bedrock conductors,
particularly if conductive overburden also exists. However, the response
of broadly distributed magnetite generally vanishes on the in-phase
difference channel DIFI. This feature can be a significant aid in the
recognition of conductors that occur in rocks containing accessory
magnetite.

##### EM Magnetite Mapping {#em-magnetite-mapping .unnumbered}

The information content of HEM data consists of a combination of
conductive eddy current responses and magnetic permeability responses.
The secondary field resulting from conductive eddy current flow is
frequency-dependent and consists of both in-phase and quadrature
components, which are positive in sign. On the other hand, the secondary
field resulting from magnetic permeability is independent of frequency
and consists of only an in-phase component that is negative in sign.
When magnetic permeability manifests itself by decreasing the measured
amount of positive in-phase, its presence may be difficult to recognize.
However, when it manifests itself by yielding a negative in-phase
anomaly (e.g., in the absence of eddy current flow), its presence is
assured. In this latter case, the negative component can be used to
estimate the percent magnetite content.

A magnetite mapping technique, based on the low frequency coplanar data,
can be complementary to magnetometer mapping in certain cases. Compared
to magnetometry, it is far less sensitive but is more able to resolve
closely spaced magnetite zones, as well as providing an estimate of the
amount of magnetite in the rock. The method is sensitive to 1/4%
magnetite by weight when the EM sensor is at a height of 30 m above a
magnetitic half-space. It can individually resolve steep dipping narrow
magnetite-rich bands which are separated by 60 m. Unlike magnetometry,
the EM magnetite method is unaffected by remanent magnetism or magnetic
latitude.

The EM magnetite mapping technique provides estimates of magnetite
content which are usually correct within a factor of 2 when the
magnetite is fairly uniformly distributed. EM magnetite maps can be
generated when magnetic permeability is evident as negative in-phase
responses on the data profiles.

Like magnetometry, the EM magnetite method maps only bedrock features,
provided that the overburden is characterized by a general lack of
magnetite. This contrasts with resistivity mapping which portrays the
combined effect of bedrock and overburden.

##### The Susceptibility Effect {#the-susceptibility-effect .unnumbered}

When the host rock is conductive, the positive conductivity response
will usually dominate the secondary field, and the susceptibility
effect[^6] will appear as a reduction in the in-phase, rather than as a
negative value. The in-phase response will be lower than would be
predicted by a model using zero susceptibility. At higher frequencies
the in-phase conductivity response also gets larger, so a negative
magnetite effect observed on the low frequency might not be observable
on the higher frequencies, over the same body. The susceptibility effect
is most obvious over discrete magnetite-rich zones, but also occurs over
uniform geology such as a homogeneous half-space.

High magnetic susceptibility will affect the calculated apparent
resistivity, if only conductivity is considered. Standard apparent
resistivity algorithms use a homogeneous half-space model, with zero
susceptibility. For these algorithms, the reduced in-phase response
will, in most cases, make the apparent resistivity higher than it should
be. It is important to note that there is nothing wrong with the data,
nor is there anything wrong with the processing algorithms. The apparent
difference results from the fact that the simple geological model used
in processing does not match the complex geology.

##### Measuring and Correcting the Magnetite Effect {#measuring-and-correcting-the-magnetite-effect .unnumbered}

Theoretically, it is possible to calculate (forward model) the combined
effect of electrical conductivity and magnetic susceptibility on an EM
response in all environments. The difficulty lies, however, in
separating out the susceptibility effect from other geological effects
when deriving resistivity and susceptibility from EM data.

Over a homogeneous half-space, there is a precise relationship between
in-phase, quadrature, and altitude. These are often resolved as phase
angle, amplitude, and altitude. Within a reasonable range, any two of
these three parameters can be used to calculate the half space
resistivity. If the rock has a positive magnetic susceptibility, the
in-phase component will be reduced and this departure can be recognized
by comparison to the other parameters.

The algorithm used to calculate apparent susceptibility and apparent
resistivity from HEM data, uses a homogeneous half-space geological
model. Non half-space geology, such as horizontal layers or dipping
sources, can also distort the perfect half-space relationship of the
three data parameters. While it may be possible to use more complex
models to calculate both rock parameters, this procedure becomes very
complex and time-consuming. For basic HEM data processing, it is most
practical to stick to the simplest geological model.

Magnetite reversals (reversed in-phase anomalies) have been used for
many years to calculate an "FeO" or magnetite response from HEM data
(Fraser, 1981). However, this technique could only be applied to data
where the in-phase was observed to be negative, which happens when
susceptibility is high and conductivity is low.

##### Applying Susceptibility Corrections {#applying-susceptibility-corrections .unnumbered}

Resistivity calculations done with susceptibility correction may change
the apparent resistivity. High-susceptibility conductors, that were
previously masked by the susceptibility effect in standard resistivity
algorithms, may become evident. In this case the susceptibility
corrected apparent resistivity is a better measure of the actual
resistivity of the earth. However, other geological variations, such as
a deep resistive layer, can also reduce the in-phase by the same amount.
In this case, susceptibility correction would not be the best method.
Different geological models can apply in different areas of the same
data set. The effects of susceptibility, and other effects that can
create a similar response, must be considered when selecting the
resistivity algorithm.

##### Susceptibility from EM vs Magnetic Field Data {#susceptibility-from-em-vs-magnetic-field-data .unnumbered}

The response of the EM system to magnetite may not match that from a
magnetometer survey. First, HEM-derived susceptibility is a rock
property measurement, like resistivity. Magnetic data show the total
magnetic field, a measure of the potential field, not the rock property.
Secondly, the shape of an anomaly depends on the shape and direction of
the source magnetic field. The electromagnetic field of HEM is much
different in shape from the earth's magnetic field. Total field magnetic
anomalies are different at different magnetic latitudes; HEM
susceptibility anomalies have the same shape regardless of their
location on the earth.

In far northern latitudes, where the magnetic field is nearly vertical,
the total magnetic field measurement over a thin vertical dike is very
similar in shape to the anomaly from the HEM-derived susceptibility (a
sharp peak over the body). The same vertical dike at the magnetic
equator would yield a negative magnetic anomaly, but the HEM
susceptibility anomaly would show a positive susceptibility peak.

##### Effects of Permeability and Dielectric Permittivity {#effects-of-permeability-and-dielectric-permittivity .unnumbered}

Resistivity algorithms that assume free-space magnetic permeability and
dielectric permittivity, do not yield reliable values in highly magnetic
or highly resistive areas. Both magnetic polarization and displacement
currents cause a decrease in the in-phase component, often resulting in
negative values that yield erroneously high apparent resistivities. The
effects of magnetite occur at all frequencies, but are most evident at
the lowest frequency. Conversely, the negative effects of dielectric
permittivity are most evident at the higher frequencies, in resistive
areas.

The table below shows the effects of varying permittivity over a
resistive (10,000 ohm-m) half space, at frequencies of 56,000 Hz
(DIGHEM^V^) and 102,000 Hz (RESOLVE).

### Apparent Resistivity Calculations {#apparent-resistivity-calculations .unnumbered}

### Effects of Permittivity on In-phase/Quadrature/Resistivity {#effects-of-permittivity-on-in-phasequadratureresistivity .unnumbered}

<table>
<colgroup>
<col style="width: 10%" />
<col style="width: 8%" />
<col style="width: 7%" />
<col style="width: 8%" />
<col style="width: 8%" />
<col style="width: 9%" />
<col style="width: 10%" />
<col style="width: 9%" />
<col style="width: 13%" />
<col style="width: 14%" />
</colgroup>
<tbody>
<tr class="odd">
<td><p>Freq</p>
<p>(Hz)</p></td>
<td>Coil</td>
<td><p>Sep</p>
<p>(m)</p></td>
<td><p>Thres</p>
<p>(ppm)</p></td>
<td><p>Alt</p>
<p>(m)</p></td>
<td><p>In</p>
<p>Phase</p></td>
<td><p>Quad</p>
<p>Phase</p></td>
<td><p>App</p>
<p>Res</p></td>
<td><p>App Depth</p>
<p>(m)</p></td>
<td>Permittivity</td>
</tr>
<tr class="even">
<td>56,000</td>
<td>CP</td>
<td>6.3</td>
<td>0.1</td>
<td>30</td>
<td>7.3</td>
<td>35.3</td>
<td>10118</td>
<td>-1.0</td>
<td>1 Air</td>
</tr>
<tr class="odd">
<td>56,000</td>
<td>CP</td>
<td>6.3</td>
<td>0.1</td>
<td>30</td>
<td>3.6</td>
<td>36.6</td>
<td>19838</td>
<td>-13.2</td>
<td>5 Quartz</td>
</tr>
<tr class="even">
<td>56,000</td>
<td>CP</td>
<td>6.3</td>
<td>0.1</td>
<td>30</td>
<td>-1.1</td>
<td>38.3</td>
<td>81832</td>
<td>-25.7</td>
<td>10 Epidote</td>
</tr>
<tr class="odd">
<td>56,000</td>
<td>CP</td>
<td>6.3</td>
<td>0.1</td>
<td>30</td>
<td>-10.4</td>
<td>42.3</td>
<td>76620</td>
<td>-25.8</td>
<td>20 Granite</td>
</tr>
<tr class="even">
<td>56,000</td>
<td>CP</td>
<td>6.3</td>
<td>0.1</td>
<td>30</td>
<td>-19.7</td>
<td>46.9</td>
<td>71550</td>
<td>-26.0</td>
<td>30 Diabase</td>
</tr>
<tr class="odd">
<td>56,000</td>
<td>CP</td>
<td>6.3</td>
<td>0.1</td>
<td>30</td>
<td>-28.7</td>
<td>52.0</td>
<td>66787</td>
<td>-26.1</td>
<td>40 Gabbro</td>
</tr>
<tr class="even">
<td>102,000</td>
<td>CP</td>
<td>7.86</td>
<td>0.1</td>
<td>30</td>
<td>32.5</td>
<td>117.2</td>
<td>9409</td>
<td>-0.3</td>
<td>1 Air</td>
</tr>
<tr class="odd">
<td>102,000</td>
<td>CP</td>
<td>7.86</td>
<td>0.1</td>
<td>30</td>
<td>11.7</td>
<td>127.2</td>
<td>25956</td>
<td>-16.8</td>
<td>5 Quartz</td>
</tr>
<tr class="even">
<td>102,000</td>
<td>CP</td>
<td>7.86</td>
<td>0.1</td>
<td>30</td>
<td>-14.0</td>
<td>141.6</td>
<td>97064</td>
<td>-26.5</td>
<td>10 Epidote</td>
</tr>
<tr class="odd">
<td>102,000</td>
<td>CP</td>
<td>7.86</td>
<td>0.1</td>
<td>30</td>
<td>-62.9</td>
<td>176.0</td>
<td>83995</td>
<td>-26.8</td>
<td>20 Granite</td>
</tr>
<tr class="even">
<td>102,000</td>
<td>CP</td>
<td>7.86</td>
<td>0.1</td>
<td>30</td>
<td>-107.5</td>
<td>215.8</td>
<td>73320</td>
<td>-27.0</td>
<td>30 Diabase</td>
</tr>
<tr class="odd">
<td>102,000</td>
<td>CP</td>
<td>7.86</td>
<td>0.1</td>
<td>30</td>
<td>-147.1</td>
<td>259.2</td>
<td>64875</td>
<td>-27.2</td>
<td>40 Gabbro</td>
</tr>
</tbody>
</table>

Methods have been developed (Huang and Fraser, 2000, 2001) to correct
apparent resistivities for the effects of permittivity and permeability.
The corrected resistivities yield more credible values than if the
effects of permittivity and permeability are disregarded.

##### Recognition of Culture {#recognition-of-culture .unnumbered}

Cultural responses include all EM anomalies caused by man-made metallic
objects. Such anomalies may be caused by inductive coupling or current
gathering. The concern of the interpreter is to recognize when an EM
response is due to culture. Points of consideration used by the
interpreter, when coaxial and coplanar coil-pairs are operated at a
common frequency, are as follows:

1\. Channels CXPL and CPPL monitor 60 Hz radiation. An anomaly on these
channels shows that the conductor is radiating power. Such an indication
is normally a guarantee that the conductor is cultural. However, care
must be taken to ensure that the conductor is not a geologic body that
strikes across a power line, carrying leakage currents.

2\. A flight that crosses a \"line\" (e.g., fence, telephone line, etc.)
yields a centre-peaked coaxial anomaly and an m-shaped coplanar
anomaly.[^7] When the flight crosses the cultural line at a high angle
of intersection, the amplitude ratio of coaxial/coplanar response is 2.
Such an EM anomaly can only be caused by a line. The geologic body that
yields anomalies most closely resembling a line is the vertically
dipping thin dike. Such a body, however, yields an amplitude ratio of 1
rather than 2. Consequently, an m-shaped coplanar anomaly with a CXI/CPI
amplitude ratio of 2 is virtually a guarantee that the source is a
cultural line.

3\. A flight that crosses a sphere or horizontal disk yields
centre-peaked coaxial and coplanar anomalies with a CXI/CPI amplitude
ratio (i.e., coaxial/coplanar) of 1/8. In the absence of geologic bodies
of this geometry, the most likely conductor is a metal roof or small
fenced yard.[^8] Anomalies of this type are virtually certain to be
cultural if they occur in an area of culture.

4.  A flight that crosses a horizontal rectangular body or wide ribbon
    yields an m-shaped coaxial anomaly and a centre-peaked coplanar
    anomaly. In the absence of geologic bodies of this geometry, the
    most likely conductor is a large fenced area.^5^ Anomalies of this
    type are virtually certain to be cultural if they occur in an area
    of culture.

5\. EM anomalies that coincide with culture, as seen on the camera film
or video display, are usually caused by culture. However, care is taken
with such coincidences because a geologic conductor could occur beneath
a fence, for example. In this example, the fence would be expected to
yield an m-shaped coplanar anomaly as in case #2 above. If, instead, a
centre-peaked coplanar anomaly occurred, there would be concern that a
thick geologic conductor coincided with the cultural line.

6\. The above description of anomaly shapes is valid when the culture is
not conductively coupled to the environment. In this case, the anomalies
arise from inductive coupling to the EM transmitter. However, when the
environment is quite conductive (e.g., less than 100 ohm-m at 900 Hz),
the cultural conductor may be conductively coupled to the environment.
In this latter case, the anomaly shapes tend to be governed by current
gathering. Current gathering can completely distort the anomaly shapes,
thereby complicating the identification of cultural anomalies. In such
circumstances, the interpreter can only rely on the radiation channels
and on the camera film or video records.

**Magnetic Responses**

The measured total magnetic field provides information on the magnetic
properties of the earth materials in the survey area. The information
can be used to locate magnetic bodies of direct interest for
exploration, and for structural and lithological mapping.

The total magnetic field response reflects the abundance of magnetic
material in the source. Magnetite is the most common magnetic mineral.
Other minerals such as ilmenite, pyrrhotite, franklinite, chromite,
hematite, arsenopyrite, limonite and pyrite are also magnetic, but to a
lesser extent than magnetite on average.

In some geological environments, an EM anomaly with magnetic correlation
has a greater likelihood of being produced by sulphides than one which
is non-magnetic. However, sulphide ore bodies may be non-magnetic (e.g.,
the Kidd Creek deposit near Timmins, Canada) as well as magnetic (e.g.,
the Mattabi deposit near Sturgeon Lake, Canada).

Iron ore deposits will be anomalously magnetic in comparison to
surrounding rock due to the concentration of iron minerals such as
magnetite, ilmenite and hematite.

Changes in magnetic susceptibility often allow rock units to be
differentiated based on the total field magnetic response. Geophysical
classifications may differ from geological classifications if various
magnetite levels exist within one general geological classification.
Geometric considerations of the source such as shape, dip and depth,
inclination of the earth\'s field and remanent magnetization will
complicate such an analysis.

In general, mafic lithologies contain more magnetite and are therefore
more magnetic than many sediments which tend to be weakly magnetic.
Metamorphism and alteration can also increase or decrease the
magnetization of a rock unit.

Textural differences on a total field magnetic contour, colour or shadow
map due to the frequency of activity of the magnetic parameter resulting
from inhomogeneities in the distribution of magnetite within the rock,
may define certain lithologies. For example, near surface volcanics may
display highly complex contour patterns with little line-to-line
correlation.

Rock units may be differentiated based on the plan shapes of their total
field magnetic responses. Mafic intrusive plugs can appear as isolated
\"bulls-eye\" anomalies. Granitic intrusives appear as sub-circular
zones, and may have contrasting rings due to contact metamorphism.
Generally, granitic terrain will lack a pronounced strike direction,
although granite gneiss may display strike.

Linear north-south units are theoretically not well-defined on total
field magnetic maps in equatorial regions due to the low inclination of
the earth\'s magnetic field. However, most stratigraphic units will have
variations in composition along strike that will cause the units to
appear as a series of alternating magnetic highs and lows.

Faults and shear zones may be characterized by alteration that causes
destruction of magnetite (e.g., weathering) that produces a contrast
with surrounding rock. Structural breaks may be filled by
magnetite-rich, fracture filling material as is the case with diabase
dikes, or by non-magnetic felsic material.

Faulting can also be identified by patterns in the magnetic total field
contours or colours. Faults and dikes tend to appear as lineaments and
often have strike lengths of several kilometres. Offsets in narrow,
magnetic, stratigraphic trends also delineate structure. Sharp contrasts
in magnetic lithologies may arise due to large displacements along
strike-slip or dip-slip faults.

**APPENDIX D**

**FLIGHT LOGS**

![](media/image6.png){width="5.992361111111111in"
height="6.652777777777778in"}

![](media/image7.png){width="5.992361111111111in"
height="6.652777777777778in"}

![](media/image8.png){width="5.992361111111111in"
height="6.652777777777778in"}

![](media/image9.png){width="5.992361111111111in"
height="6.652777777777778in"}

![](media/image10.png){width="5.992361111111111in"
height="6.652777777777778in"}

![](media/image11.png){width="5.992361111111111in"
height="6.652777777777778in"}

![](media/image12.png){width="5.992361111111111in"
height="6.652777777777778in"}

**Appendix E -- Tests and Calibrations**

**Altimeter Test -- Performed December 14, 2003 over landing site at
Camp Stanley**

![](media/image13.wmf)

-barometric and GPS data zeroed at the first point to match radar and
laser data

-radar data has 27.7 metres removed from raw to represent bird altitude.

##  {#section-6 .unnumbered}

##  {#section-7 .unnumbered}

##   {#section-8 .unnumbered}

**EM Calibrations**

**CALIBRATION OF THE DIGHEM DIGITAL (DSP) SYSTEM**

The calibration method used in the digital Dighem^V^ has been developed
as a significant improvement over the practices described by Fitterman
(1998), which practices were originally developed through
experimentation and consultation between Fugro and the United States
Geological Survey. The problems defined by Fitterman, including jig
calibration and conductive ground response, are obviated by calibration
at high altitude using internal, rigidly mounted, automatically
triggered and measured calibration coils.

Calibration of the system during the survey will use the Fugro AutoCal™
automatic, internal calibration process. At the beginning and end of
each flight, and at intervals during the flight, the system will be
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) will be 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.

Greg Hodges, Chief Geophysicist 01/04/09

References:

Fitterman, D.V., (1998). Sources of calibration errors in helicopter EM
data. Exploration Geophysics 29, 65-70.

**[External Calibration Results]{.underline}**

  --------------- -------------- -------------- ------------ -------------- ------------ ---------------
  **Dec 11,       **Start                                                                
  2003**          calibration.                                                           
                  On site at                                                             
                  Camp Stanley**                                                         

                                                                                         

  **FREQUENCY**   **CHANNEL**    **INTERNAL**                **EXTERNAL**                **PHASE**

                                 **MEASURED**   **TARGET**   **MEASURED**   **TARGET**   **(degrees)**

  400             L400I          195.0          196.2        188.0          200.4        -1.0

                  L400Q          194.0          196.2        194.0          200.4        

  1500            L1K5I          176.0          178.4        215.0          204.2        -3.0

                  L1K5Q          176.0          178.4        191.0          204.2        

  3300            L1K7I          103.0          104.4        104.0          99.9         -0.8

                  L1K7Q          104.0          104.4        101.0          99.9         

  6200            L6K2I          829.0          823.5        196.0          207.5        -0.1

                  L6K2Q          831.0          823.5        201.0          207.5        

  25000           L25KI          509.0          504.6        174.0          192.5        0.1

                  L25KQ          505.0          504.6        173.0          192.5        

  100000          L100I          480.0          460.6        208.0          178.8        -2.0

                  L100Q          480.0          460.6        210.0          178.8        

                                                                                         

  **Dec 20,       **End                                                                  
  2003**          calibration.                                                           
                  At Fugro's                                                             
                  calibrations                                                           
                  site,                                                                  
                  Mountsburg,                                                            
                  Ontario**                                                              

                                                                                         

  **FREQUENCY**   **CHANNEL**    **INTERNAL**                **EXTERNAL**                **PHASE**

                                 **MEASURED**   **TARGET**   **MEASURED**   **TARGET**   

  400             L400I          196.0          196.2        180.0          200.4        -1.9

                  L400Q          196.0          196.2        195.0          200.4        

  1500            L1K5I          180.0          178.4        192.0          204.2        -3.5

                  L1K5Q          180.0          178.4        198.0          204.2        

  3300            L1K7I          103.0          104.4        111.0          99.9         -1.2

                  L1K7Q          103.0          104.4        110.0          99.9         

  6200            L6K2I          824.0          823.5        185.0          207.5        -1.8

                  L6K2Q          823.0          823.5        185.0          207.5        

  25000           L25KI          505.0          504.6        192.0          192.5        -1.4

                  L25KQ          505.0          504.6        182.0          192.5        

  100000          L100I          475.0          460.6        193.0          178.8        -2.2

                  L100Q          480.0          460.6        194.0          178.8        

                                                                                         
  --------------- -------------- -------------- ------------ -------------- ------------ ---------------

-phase calculated from inverse TAN of quad deflection/inphase
deflection. Variations attributed to temperature changes from the
different times and locations of the calibrations.

**Appendix F - Processing Log**

**Total Magnetic Field data**

A fourth difference was calculated from the raw total magnetic intensity
data (TMI). The raw TMI was examined in profile form along with the
fourth difference. Spikes and duplicate points were manually defaulted
and interpolated with an Akima spline. None of the defaulted areas
exceeded one second in length. The diurnal variations recorded by the
base station were edited for any cultural contamination and filtered to
remove high-frequency noise. This diurnal magnetic data was then
subtracted from the despiked TMI to provide a first order diurnal
correction. An average base value of 48517 nT was added back to the
diurnal corrected airborne total magnetic field records. The diurnal
removed magnetic field data were then gridded and compared to a grid of
the despiked magnetic data to ensure that the data quality was improved
by diurnal removal.

The lag in the magnetic data was determined and applied to the survey
data. A vertical gradient was calculated from the lagged magnetic data
and examined for evidence of lag and leveling problems. The lag of --0.5
seconds seemed appropriate for the survey data and few leveling errors
were noted. The IGRF was calculated using the latest coefficient set for
a date of 2003/12/12 with the altitude taken from the differentially
corrected height above the WGS84 spheroid. The calculated IGRF was
removed from the magnetic data prior to any tie line leveling. Tie line
leveling corrections were calculated and applied with additional manual
corrections to a few of the survey lines. Vertical gradient grids were
calculated from the magnetic grids after each leveling correction. These
grids were shadowed and examined to determine the success of the manual
correction. To remove any short wavelength residual line-to-line
discrepancies in the total magnetic field, a microleveling technique was
used to remove errors of less than 2.5 nT striking parallel to the line
direction. This microleveled channel was used to produce the final
residual magnetic field grid. The IGRF was recalculated using the latest
coefficient set for a date of 2003/12/12 with the altitude set at 425.0
m. The new IGRF correction was added to the final residual magnetic
field to produce the final total magnetic field, IGRF corrected.

**Electromagnetic and Resistivity data**

Base level corrections were picked off of the analogue rolls from the
high altitude backgrounds. These level picks were applied to the raw EM
data which were then loaded into the Geosoft database. An 11-point
median filter followed by an 11-point Hanning filter was applied to the
raw base leveled data. (13-point filters were used on the CP400 Hz EM
data). Apparent resistivity and depth calculations were done on the
filtered, base leveled EM data using Fugro's proprietary resistivity
algorithm. All in-phase, quadrature and resistivity channels were
gridded and examined, and manual leveling corrections and phase
adjustments were applied as required. The resistivity calculation and
gridding was repeated until no further corrections were required.
Additional algorithms were run on the data to determine the centroid
depths, differential resistivity and differential depths. Sengpiel-type
and differential resistivity depth sections were generated to double
check the resistivity leveling.

Two sets of grids were generated. The 100 metre spaced detail area was
windowed out of the block and gridded with 20 metre cells (1/5 of the
line spacing). The remaining area flown at 200-metre spacing was gridded
using 40 metre cells. A grid function was run on the 40 metre grids to
reduce the cell size to 20 metres and the two grids were merged
together, averaging the grid values in a 2 cell overlap. The merged
grids were then filtered for final presentation using the following
rectangular Hanning filter values:

RES115K 3x3

RES25K 3x3

RES6200 5x5

RES1500 11x11

RES400 19x19

The filtered grids were written back to the database as database
channels with a "\_filt" suffix. The digital grid archive also contains
unfiltered and filtered grids.

Grids for area B3 were gridded with 10 metre cells and were filtered in
a similar manner as above. B3 filtered and unfiltered grids are included
in the digital grid archive, filtered and unfiltered channels are
included in the digital data archive.

**Appendix G - GLOSSARY OF AIRBORNE GEOPHYSICAL TERMS**

Note: The definitions given in this glossary refer to the common
terminology as used in airborne geophysics.

**altitude attenuation**: the absorption of gamma rays by the atmosphere
between the earth and the detector. The number of gamma rays detected by
a system decreases as the altitude increases.

**apparent- :** the ***physical parameters*** of the earth measured by a
geophysical system are normally expressed as apparent, as in "apparent
***resistivity*"**. This means that the measurement is limited by
assumptions made about the geology in calculating the response measured
by the geophysical system. Apparent resistivity calculated with
***HEM***, for example, generally assumes that the earth is a
***homogeneous half-space*** -- not layered.

**amplitude**: The strength of the total electromagnetic field. In
***frequency domain*** it is most often the sum of the squares of
***in-phase*** and ***quadrature*** components. In multi-component
electromagnetic surveys it is generally the sum of the squares of all
three directional components.

**analytic signal**: The total amplitude of all the directions of
magnetic ***gradient***. Calculated as the sum of the squares.

**anisotropy**: Having different ***physical parameters*** in different
directions. This can be caused by layering or fabric in the geology.
Note that a unit can be anisotropic, but still **homogeneous**.

**anomaly**: A localized change in the geophysical data characteristic
of a discrete source, such as a conductive or magnetic body. Something
locally different from the **background**.

**B-field**: In time-domain **electromagnetic** surveys, the magnetic
field component of the (electromagnetic) **field**. This can be measured
directly, although more commonly it is calculated by integrating the
time rate of change of the magnetic field **dB/dt**, as measured with a
receiver coil.

**background**: The "normal" response in the geophysical data -- that
response observed over most of the survey area. **Anomalies** are
usually measured relative to the background. In airborne gamma-ray
spectrometric surveys the term defines the **cosmic**, radon, and
aircraft responses in the absence of a signal from the ground.

**base-level**: The measured values in a geophysical system in the
absence of any outside signal. All geophysical data are measured
relative to the system base level.

**base frequency**: The frequency of the pulse repetition for a
***time-domain* *electromagnetic*** system. Measured between subsequent
positive pulses.

**bird**: A common name for the pod towed beneath or behind an aircraft,
carrying the geophysical sensor array.

**calibration coil**: A wire coil of known size and dipole moment, which
is used to generate a field of known ***amplitude*** and ***phase*** in
the receiver, for system calibration. Calibration coils can be external,
or internal to the system. Internal coils may be called Q-coils.

**coaxial coils**: **\[CX\]** Coaxial coils are in the vertical plane,
with their axes horizontal and collinear in the flight direction. These
are most sensitive to vertical conductive objects in the ground, such as
thin, steeply dipping conductors perpendicular to the flight direction.
Coaxial coils generally give the sharpest anomalies over localized
conductors. (See also ***coplanar coils***)

**coil**: A multi-turn wire loop used to transmit or detect
electromagnetic fields. Time varying ***electromagnetic*** fields
through a coil induce a voltage proportional to the strength of the
field and the rate of change over time.

**compensation**: Correction of airborne geophysical data for the
changing effect of the aircraft. This process is generally used to
correct data in ***fixed-wing*** ***time-domain electromagnetic***
surveys (where the transmitter is on the aircraft and the receiver is
moving), and magnetic surveys (where the sensor is on the aircraft,
turning in the earth's magnetic field.

**component**: In ***frequency domain electromagnetic*** surveys this is
one of the two **phase** measurements -- ***in-phase or quadrature***.
In "multi-component" electromagnetic surveys it is also used to define
the measurement in one geometric direction (vertical, horizontal in-line
and horizontal transverse -- the Z, X and Y components).

**Compton scattering**: gamma ray photons will bounce off the nuclei of
atoms they pass through (earth and atmosphere), reducing their energy
and then being detected by ***radiometric*** sensors at lower energy
levels. See also ***stripping***.

**conductance**: See ***conductivity thickness***

**conductivity**: \[**σ**\] The facility with which the earth or a
geological formation conducts electricity. Conductivity is usually
measured in milli-Siemens per metre (mS/m). It is the reciprocal of
***resistivity***.

**conductivity-depth imaging**: see ***conductivity-depth transform***.

**conductivity-depth transform**: A process for converting
electromagnetic measurements to an approximation of the conductivity
distribution vertically in the earth, assuming a ***layered earth***.
(Macnae and Lamontagne, 1987; Wolfgram and Karlik, 1995)

**conductivity thickness**: \[**σt**\] The product of the
***conductivity***, and thickness of a large, tabular body. (It is also
called the "conductivity-thickness product") In electromagnetic
geophysics, the response of a thin plate-like conductor is proportional
to the conductivity multiplied by thickness. For example a 10 metre
thickness of 20 Siemens/m mineralization will be equivalent to 5 metres
of 40 S/m; both have 200 S conductivity thickness. Sometimes referred to
as conductance.

**conductor**: Used to describe anything in the ground more conductive
than the surrounding geology. Conductors are most often clays or
graphite, or hopefully some type of mineralization, but may also be
man-made objects, such as fences or pipelines.

**coplanar coils**: **\[CP\]** The coplanar coils lie in the horizontal
plane with their axes vertical, and parallel. These coils are most
sensitive to massive conductive bodies, horizontal layers, and the
***halfspace***.

**cosmic ray**: High energy sub-atomic particles from outer space that
collide with the earth's atmosphere to produce a shower of gamma rays
(and other particles) at high energies.

**counts (per second)**: The number of ***gamma-rays*** detected by a
gamma-ray ***spectrometer*.** The rate depends on the geology, but also
on the size and sensitivity of the detector.

**culture**: A term commonly used to denote any man-made object that
creates a geophysical anomaly. Includes, but not limited to, power
lines, pipelines, fences, and buildings.

**current gathering**: The tendency of electrical currents in the ground
to channel into a conductive formation. This is particularly noticeable
at higher frequencies or early time channels when the formation is long
and parallel to the direction of current flow. This tends to enhance
anomalies relative to inductive currents (see also ***induction***).
Also known as current channelling.

**current channelling**: See current gathering.

**daughter products**: The radioactive natural sources of gamma-rays
decay from the original element (commonly potassium, uranium, and
thorium) to one or more lower-energy elements. Some of these lower
energy elements are also radioactive and decay further. ***Gamma-ray
spectrometry*** surveys may measure the gamma rays given off by the
original element or by the decay of the daughter products.

**dB/dt**: As the ***secondary electromagnetic field*** changes with
time, the magnetic field \[**B**\] component induces a voltage in the
receiving ***coil***, which is proportional to the rate of change of the
magnetic field over time.

**decay**: In ***time-domain electromagnetic*** theory, the weakening
over time of the ***eddy currents*** in the ground, and hence the
***secondary field*** after the ***primary field*** electromagnetic
pulse is turned off. In ***gamma-ray spectrometry***, the radioactive
breakdown of an element, generally potassium, uranium, thorium, or one
of their ***daughter*** products.

**decay series**: In ***gamma-ray spectrometry***, a series of
progressively lower energy ***daughter products*** produced by the
radioactive breakdown of uranium or thorium.

**decay constant**: see time constant.

**depth of exploration**: The maximum depth at which the geophysical
system can detect the target. The depth of exploration depends very
strongly on the type and size of the target, the contrast of the target
with the surrounding geology, the homogeneity of the surrounding
geology, and the type of geophysical system. One measure of the maximum
depth of exploration for an electromagnetic system is the depth at which
it can detect the strongest conductive target -- generally a highly
conductive horizontal layer.

**differential resistivity**: A process of transforming ***apparent**
**resistivity*** to an approximation of layer resistivity at each depth.
The method uses multi-frequency HEM data and approximates the effect of
shallow layer ***conductance*** determined from higher frequencies to
estimate the deeper conductivities (Huang and Fraser, 1996)

**dipole moment**: \[NIA\] For a transmitter, the product of the area of
a ***coil***, the number of turns of wire, and the current flowing in
the coil. At a distance significantly larger than the size of the coil,
the magnetic field from a coil will be the same if the dipole moment
product is the same. For a receiver coil, this is the product of the
area and the number of turns. The sensitivity to a magnetic field
(assuming the source is far away) will be the same if the dipole moment
is the same.

**diurnal**: The daily variation in a natural field, normally used to
describe the natural fluctuations (over hours and days) of the earth's
magnetic field.

**dielectric permittivity**: **\[ε\]** The capacity of a material to
store electrical charge, this is most often measured as the relative
permittivity \[ε~r~\], or ratio of the material dielectric to that of
free space. The effect of high permittivity may be seen in HEM data at
high frequencies over highly resistive geology as a reduced or negative
***in-phase***, and higher ***quadrature*** data.

**drift**: Long-time variations in the base-level or calibration of an
instrument.

**eddy currents**: The electrical currents induced in the ground, or
other conductors, by a time-varying ***electromagnetic field*** (usually
the ***primary field***). Eddy currents are also induced in the
aircraft's metal frame and skin; a source of ***noise*** in EM surveys.

**electromagnetic**: **\[EM\]** Comprised of a time-varying electrical
and magnetic field. Radio waves are common electromagnetic fields. In
geophysics, an electromagnetic system is one which transmits a
time-varying ***primary field*** to induce ***eddy currents*** in the
ground, and then measures the ***secondary field*** emitted by those
eddy currents.

**energy window**: A broad spectrum of ***gamma-ray*** energies measured
by a spectrometric survey. The energy of each gamma-ray is measured and
divided up into numerous discrete energy levels, called windows.

**equivalent** (thorium or uranium): The amount of radioelement
calculated to be present, based on the gamma-rays measured from a
**daughter** element. This assumes that the ***decay series*** is in
equilibrium -- progressing normally.

**fiducial, or fid**: Timing mark on a survey record. Originally these
were timing marks on a profile or film; now the term is generally used
to describe 1-second interval timing records in digital data, and on
maps or profiles.

**fixed-wing**: Aircraft with wings, as opposed to "rotary wing"
helicopters.

**footprint**: This is a measure of the area of sensitivity under the
aircraft of an airborne geophysical system. The footprint of an
***electromagnetic*** system is dependent on the altitude of the system,
the orientation of the transmitter and receiver and the separation
between the receiver and transmitter, and the conductivity of the
ground. The footprint of a ***gamma-ray spectrometer*** depends mostly
on the altitude. For all geophysical systems, the footprint also depends
on the strength of the contrasting ***anomaly***.

**frequency domain**: An ***electromagnetic*** system which transmits a
***primary field*** that oscillates smoothly over time (sinusoidal),
inducing a similarly varying electrical current in the ground. These
systems generally measure the changes in the ***amplitude*** and
***phase*** of the ***secondary field*** from the ground at different
frequencies by measuring the **in-phase** and **quadrature** phase
components. See also ***time-domain***.

**full-stream data**: Data collected and recorded continuously at the
highest possible sampling rate. Normal data are stacked (see
***stacking***) over some time interval before recording.

**gamma-ray**: A very high-energy photon, emitted from the nucleus of an
atom as it undergoes a change in energy levels.

**gamma-ray spectrometry**: Measurement of the number and energy of
natural (and sometimes man-made) gamma-rays across a range of photon
energies.

**gradient**: In magnetic surveys, the gradient is the change of the
magnetic field over a distance, either vertically or horizontally in
either of two directions. Gradient data is often measured, or calculated
from the total magnetic field data because it changes more quickly over
distance than the ***total magnetic field***, and so may provide a more
precise measure of the location of a source. See also ***analytic
signal***.

***ground effect***: The response from the earth. A common calibration
procedure in many geophysical surveys is to fly to altitude high enough
to be beyond any measurable response from the ground, and there
establish ***base levels*** or ***backgrounds***.

**half-space**: A mathematical model used to describe the earth -- as
infinite in width, length, and depth below the surface. The most common
halfspace models are ***homogeneous*** and ***layered earth***.

**heading error**: A slight change in the magnetic field measured when
flying in opposite directions.

**HEM**: Helicopter ElectroMagnetic, This designation is most commonly
used to helicopter-borne, ***frequency-domain*** electromagnetic
systems. At present, the transmitter and receivers are normally mounted
in a ***bird*** carried on a sling line beneath the helicopter.

**herringbone pattern**: a pattern created in geophysical data by an
asymmetric system, where the **anomaly** may be extended to either side
of the source, in the direction of flight. Appears like fish bones, or
like the teeth of a comb, extending either side of centre, each tooth an
alternate flight line.

**homogeneous**: This is a geological unit that has the same ***physical
parameters*** throughout its volume. This unit will create the same
response to an HEM system anywhere, and the HEM system will measure the
same apparent ***resistivity*** anywhere. The response may change with
system direction (see ***anisotropy***).

**in-phase**: the component of the measured ***secondary field*** that
has the same phase as the transmitter and the ***primary field***. The
in-phase component is stronger than the ***quadrature*** phase over
relatively higher ***conductivity***.

**induction**: Any time-varying electromagnetic field will induce
(cause) electrical currents to flow in any object with non-zero
***conductivity***. (see ***eddy currents***)

**infinite**: In geophysical terms, an "infinite' dimension is one much
greater than the ***footprint*** of the system, so that the system does
not detect changes at the edges of the object.

**International** **Geomagnetic Reference Field**: **\[IGRF\]** An
approximation of the smooth magnetic field of the earth, in the absence
of variations due to local geology. Once the IGRF is subtracted from the
measured magnetic total field data, any remaining variations are assumed
to be due to local geology. The IGRF also predicts the slow changes of
the field up to five years in the future.

**inversion**, or **inverse modeling**: A process of converting
geophysical data to an earth model, which compares theoretical models of
the response of the earth to the data measured, and refines the model
until the response closely fits the measured data (Huang and Palacky,
1991)

**layered earth**: A common geophysical model which assumes that the
earth is horizontally layered -- the ***physical parameters*** are
constant to ***infinite*** distance horizontally, but change vertically.

**magnetic permeability**: **\[μ\]** This is defined as the ratio of
magnetic induction to the inducing magnetic field. The relative magnetic
permeability \[μ~r~\] is often quoted, which is the ratio of the rock
permeability to the permeability of free space. In geology and
geophysics, the ***magnetic susceptibility*** is more commonly used to
describe rocks.

**magnetic susceptibility**: **\[k\]** A measure of the degree to which
a body is magnetized. In SI units this is related to relative
***magnetic permeability*** by k=μ~r~-1, and is a dimensionless unit.
For most geological material, susceptibility is influenced primarily by
the percentage of magnetite. It is most often quoted in units of 10^-6^.
In HEM data this is most often apparent as a negative ***in-phase***
component over high susceptibility, high ***resistivity*** geology such
as diabase dikes.

**noise**: That part of a geophysical measurement that the user does not
want. Typically this includes electronic interference from the system,
the atmosphere (***sferics***), and man-made sources. This can be a
subjective judgment, as it may include the response from geology other
than the target of interest. Commonly the term is used to refer to high
frequency (short period) interference. See also ***drift***.

**Occam's inversion**: an ***inversion*** process that matches the
measured ***electromagnetic*** data to a theoretical model of many, thin
layers with constant thickness and varying resistivity (Constable et al,
1987).

**off-time**: In a ***time-domain electromagnetic*** survey, the time
after the end of the ***primary field pulse***, and before the start of
the next pulse.

**on-time**: In a ***time-domain electromagnetic*** survey, the time
during the ***primary field pulse***.

**phase**: The angular difference in time between a measured sinusoidal
electromagnetic field and a reference -- normally the primary field. The
phase is calculated from tan^-1^(***in-phase** / **quadrature***).

**physical parameters**: These are the characteristics of a geological
unit. For electromagnetic surveys, the important parameters for
electromagnetic surveys are ***conductivity**, **magnetic**
**permeabilit*****y** (or ***susceptibility***) and ***dielectric
permittivity***; for magnetic surveys the parameter is magnetic
susceptibility, and for gamma ray spectrometric surveys it is the
concentration of the major radioactive elements: potassium, uranium, and
thorium.

**permittivity**: see ***dielectric permittivity***.

**permeability**: see ***magnetic permeability***.

**primary field**: the EM field emitted by a transmitter. This field
induces ***eddy currents*** in (energizes) the conductors in the ground,
which then create their own ***secondary* fields**.

**pulse**: In time-domain EM surveys, the short period of intense
***primary*** field transmission. Most measurements (the ***off-time***)
are measured after the pulse.

**quadrature**: that component of the measured ***secondary field***
that is phase-shifted 90° from the ***primary field***. The quadrature
component tends to be stronger than the ***in-phase*** over relatively
weaker ***conductivity***.

**Q-coils**: see ***calibration coil***.

**radiometric**: Commonly used to refer to ***gamma ray*** spectrometry.

**radon**: A radioactive daughter product of uranium and thorium, radon
is a gas which can leak into the atmosphere, adding to the
non-geological background of a gamma-ray spectrometric survey.

**resistivity**: **\[ρ\]** The strength with which the earth or a
geological formation resists the flow of electricity, typically the flow
induced by the ***primary field*** of the electromagnetic transmitter.
Normally expressed in ohm-metres, it is the reciprocal of
***conductivity***.

**resistivity-depth transforms**: similar to ***conductivity depth
transforms***, but the calculated ***conductivity*** has been converted
to ***resistivity***.

**resistivity section**: an approximate vertical section of the
resistivity of the layers in the earth. The resistivities can be derived
from the ***apparent resistivity***, the ***differential***
***resistivities***, ***resistivity-depth transforms***, or
***inversions***.

**secondary field**: The field created by conductors in the ground, as a
result of electrical currents induced by the ***primary field*** from
the ***electromagnetic*** transmitter. Airborne ***electromagnetic***
systems are designed to create, and measure a secondary field.

**Sengpiel section**: a ***resistivity section*** derived using the
***apparent** **resistivity*** and an approximation of the depth of
maximum sensitivity for each frequency.

**sferic**: Lightning, or the ***electromagnetic*** signal from
lightning, it is an abbreviation of "atmospheric discharge". These
appear to magnetic and electromagnetic sensors as sharp "spikes" in the
data. Under some conditions lightning storms can be detected from
hundreds of kilometres away. (see ***noise***)

**signal**: That component of a measurement that the user wants to see
-- the response from the targets, from the earth, etc. (See also
***noise***)

**skin depth**: A measure of the depth of penetration of an
electromagnetic field into a material. It is defined as the depth at
which the primary field decreases to 1/e of the field at the surface. It
is calculated by approximately 503 x √(resistivity/frequency ). Note
that depth of penetration is greater at higher ***resistivity*** and/or
lower ***frequency**.*

**spectrometry**: Measurement across a range of energies, where
***amplitude*** and energy are defined for each measurement. In
gamma-ray spectrometry, the number of gamma rays are measured for each
energy ***window***, to define the ***spectrum***.

**spectrum**: In ***gamma ray spectrometry***, the continuous range of
energy over which gamma rays are measured. In ***time-domain***
***electromagnetic*** surveys, the spectrum is the energy of the
**pulse** distributed across an equivalent, continuous range of
frequencies.

**spheric**: see ***sferic***.

**stacking**: Summing repeat measurements over time to enhance the
repeating ***signal***, and minimize the random ***noise***.

**stripping**: Estimation and correction for the gamma ray photons of
higher and lower energy that are observed in a particular ***energy
window***. See also ***Compton scattering***.

**susceptibility**: See ***magnetic susceptibility***.

**tau**: \[***τ***\] Often used as a name for the ***time constant***.

**TDEM**: ***time domain electromagnetic***.

**thin sheet**: A standard model for electromagnetic geophysical theory.
It is usually defined as thin, flat-lying, and ***infinite*** in both
horizontal directions. (see also ***vertical plate***)

**tie-line**: A survey line flown across most of the ***traverse
lines***, generally perpendicular to them, to assist in measuring
***drift*** and ***diurnal*** variation. In the short time required to
fly a tie-line it is assumed that the drift and/or diurnal will be
minimal, or at least changing at a constant rate.

**time constant**: The time required for an ***electromagnetic*** field
to decay to a value of 1/e of the original value. In ***time-domain***
electromagnetic data, the time constant is proportional to the size and
***conductance*** of a tabular conductive body. Also called the decay
constant.

**Time channel**: In ***time-domain electromagnetic*** surveys the
decaying ***secondary field*** is measured over a period of time, and
the divided up into a series of consecutive discrete measurements over
that time.

**time-domain**: ***Electromagnetic*** system which transmits a pulsed,
or stepped ***electromagnetic*** field. These systems induce an
electrical current (***eddy current***) in the ground that persists
after the ***primary field*** is turned off, and measure the change over
time of the ***secondary field*** created as the currents ***decay***.
See also ***frequency-domain***.

**total energy envelope**: The sum of the squares of the three
***components*** of the ***time-domain electromagnetic secondary
field***. Equivalent to the ***amplitude*** of the secondary field.

**transient**: Time-varying. Usually used to describe a very short
period pulse of ***electromagnetic*** field.

**traverse line**: A normal geophysical survey line. Normally parallel
traverse lines are flown across the property in spacing of 50 m to 500
m, and generally perpendicular to the target geology.

**vertical plate**: A standard model for electromagnetic geophysical
theory. It is usually defined as thin, and ***infinite*** in horizontal
dimension and depth extent. (see also ***thin shee***t)

**waveform**: The shape of the ***electromagnetic pulse*** from a
***time-domain*** electromagnetic transmitter.

**window**: A discrete portion of a ***gamma-ray spectrum*** or
***time-domain electromagnetic decay***. The continuous energy spectrum
or ***full-stream*** data are grouped into windows to reduce the number
of samples, and reduce ***noise***.

Version 1.1, March 10, 2003

Greg Hodges,

Chief Geophysicist

Fugro Airborne Surveys, Toronto

####  {#section-9 .unnumbered}

####  {#section-10 .unnumbered}

####  {#section-11 .unnumbered}

#### Common Symbols and Acronyms {#common-symbols-and-acronyms .unnumbered}

**k** Magnetic susceptibility

**ε** Dielectric permittivity

**μ, μ~r~** Magnetic permeability, apparent permeability

**ρ, ρ~a~** Resistivity, apparent resistivity

**σ,σ~a~** Conductivity, apparent conductivity

**σt** Conductivity thickness

**τ** Tau, or time constant

**Ω.m** Ohm-metres, units of resistivity

**AGS** Airborne gamma ray spectrometry.

**CDT** Conductivity-depth transform, conductivity-depth imaging (Macnae
and Lamontagne, 1987; Wolfgram and Karlik, 1995)

**CPI, CPQ** Coplanar in-phase, quadrature

**CPS** Counts per second

**CTP** Conductivity thickness product

**CXI, CXQ** Coaxial, in-phase, quadrature

**fT** femtoteslas, normal unit for measurement of B-Field

**EM** Electromagnetic

**keV** kilo electron volts -- a measure of gamma-ray energy

**MeV** mega electron volts -- a measure of gamma-ray energy 1MeV =
1000keV

NIA dipole moment: turns x current x Area

**nT** nano-Tesla, a measure of the strength of a magnetic field

**ppm** parts per million -- a measure of secondary field or noise
relative to the primary.

**pT/s** picoTeslas per second: Units of decay of secondary field, dB/dt

**S** Siemens -- a unit of conductance

**x**: the horizontal component of an EM field parallel to the direction
of flight.

**y**: the horizontal component of an EM field perpendicular to the
direction of flight.

**z**: the vertical component of an EM field.

**References:**

Constable, S.C., Parker, R.L., And Constable, C.G., 1987, Occam's
inversion: a practical algorithm for generating smooth models from
electromagnetic sounding data: Geophysics, 52, 289-300

Huang, H. and Fraser, D.C, 1996. The differential parameter method for
muiltifrequency airborne resistivity mapping. Geophysics, 55, 1327-1337

Huang, H. and Palacky, G.J., 1991, Damped least-squares inversion of
time-domain airborne EM data based on singular value decomposition:
Geophysical Prospecting, v.39, 827-844

Macnae, J. and Lamontagne, Y., 1987, Imaging quasi-layered conductive
structures by simple processing of transient electromagnetic data:
Geophysics, v52, 4, 545-554.

Sengpiel, K-P. 1988, Approximate inversion of airborne EM data from a
multi-layered ground. Geophysical Prospecting, 36, 446-459

Wolfgram, P. and Karlik, G., 1995, Conductivity-depth transform of
GEOTEM data: Exploration Geophysics, 26, 179-185.

Yin, C. and Fraser, D.C. (2002), The effect of the electrical anisotropy
on the responses of helicopter-borne frequency domain electromagnetic
systems, Submitted to *Geophysical Prospecting*

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

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

[^3]: Constable et al, 1987, Occam's inversion: a practical algorithm
    for generating smooth models from electromagnetic sounding data:
    Geophysics, 52, 289-300.

[^4]: Huang H., and Palacky, G.J., 1991, Damped least-squares inversion
    of time domain airborne EM data based on singular value
    decomposition: Geophysical Prospecting, 39, 827-844.

[^5]: ^?^ Resistivity mapping with an airborne multicoil electromagnetic
    system: Geophysics, v. 43, p.144-172

[^6]: ^?^ Magnetic susceptibility and permeability are two measures of
    the same physical property. Permeability is generally given as
    relative permeability, µ~r~, which is the permeability of the
    substance divided by the permeability of free space (4 π x 10^-7^).
    Magnetic susceptibility *k* is related to permeability by
    *k*=µ^r^-1. Susceptibility is a unitless measurement, and is usually
    reported in units of 10^-6^. The typical range of susceptibilities
    is --1 for quartz, 130 for pyrite, and up to 5 x 10^5^ for
    magnetite, in 10^-6^ units (Telford et al, 1986).

[^7]: ^?^ See Figure C-1 presented earlier.

[^8]: ^?^ It is a characteristic of EM that geometrically similar
    anomalies are obtained from: (1) a planar conductor, and (2) a wire
    which forms a loop having dimensions identical to the perimeter of
    the equivalent planar conductor.