FISHER LAKE AREA. Ontario Airborne Geophysical Surveys Magnetic Data Geophysical Data Set 1232

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Ontario Airborne Geophysical Surveys Magnetic Data

Geophysical Data Set 1232

Ontario Geological Survey

Ministry of Northern Development, Mines and Forestry Willet Green Miller Centre

933 Ramsey Lake Road Sudbury, Ontario, P3E 6B5 Canada



















List of accountabilities and responsibilities:

Jack Parker, Senior Manager, Precambrian Geoscience Section, Ontario Geological Survey (OGS), Ministry of Northern Development, Mines and Forestry (MNDMF) – accountable for the airborne geophysical survey projects, including contract management

Tom Watkins, Data Manager, Information and Marketing Services Section, Ontario Geological Survey, MNDMF – managed the project-related hard-copy products

Desmond Rainsford, Geophysicist, Precambrian Geoscience Section, Ontario Geological Survey, MNDMF – managed the project-related digital products

CGI Controlled Geophysics Inc., Thornhill, Ontario – data compilation and products

Firefly Aviation Ltd., Calgary, Alberta – data acquisition and original data processing.


To enable the rapid dissemination of information, this digital data has not received a technical edit. Every possible effort has been made to ensure the accuracy of the information provided;

however, the Ontario Ministry of Northern Development, Mines and Forestry does not assume any liability or responsibility for errors that may occur. Users may wish to verify critical information.


Information from this publication may be quoted if credit is given. It is recommended that reference be made in the following form:

Ontario Geological Survey 2010. Ontario airborne geophysical surveys, magnetic data, grid and profile data (ASCII and Geosoft® Formats) and vector data, Fisher Lake area—Purchased data; Ontario Geological Survey, Geophysical Data Set 1232.


Chief and Council of First Nation communities within and adjacent to this survey area request that you contact the closest First Nation community if you carry out any work in these areas. You are also encouraged to determine that the area you are working in does not represent an area of overlap where traditional activities are carried out by more than one community. Contact information for First Nation communities is obtained at the following website sponsored by Indian and Northern Affairs Canada: http://pse2-



As part of an on-going program to acquire high-quality, high-resolution airborne geophysical data across the Province of Ontario, the Ontario Ministry of Northern Development, Mines and Forestry (MNDMF) does, from time to time, issue Requests For Data (RFD) in order to purchase existing proprietary data held by mining companies. Purchase of existing data complements new surveys commissioned by the MNDMF. The Fisher Lake survey is part of the Request for Data process.

The purchase of data is attractive due to the low cost of acquisition relative to flying new surveys.

The money used to purchase the data can be reinvested in exploration. The data are assessed for quality prior to purchase and are reprocessed to meet the common formats and standards of other Ontario geophysical data. Once reprocessed these data are then made public.

Ranking and valuation of submitted airborne geophysical survey data sets were based on the following criteria:

date of survey – recent surveys were favoured over older surveys because of improved acquisition technology, greater data density and improved final products.

survey method – magnetometer surveys, without supplementary radiometrics or VLF, were given the lowest rating in this category; AEM and magnetometer were given the highest; the objective was to acquire data that complements what is already available in the public domain, with emphasis on exploration rather than mapping.

location of area:

data sets occurring within areas already surveyed or scheduled for survey were selected only if they added significantly to the acquired data sets,

proximity or coincidence of the survey block with areas having restricted land use designations affected the value assigned to that survey,

consideration was given to data sets that were collected in remote areas where logistical costs are very high.

line spacing – detailed surveys were normally accorded a higher rating than reconnaissance surveys.

quality of data – data quality, processed products, and adherence to correct survey specifications had to be up to normal industry standards.

survey size – data sets comprising less than 1000 line-km were selected only if they fell in very strategic locations.

other criteria – factors such as apparent mineral significance, previous exploration activity and land availability were also considered in making the final selection.

The Fisher Lake survey (originally named Warclub) is a fixed-wing high resolution aeromagnetic survey carried out for Western Warrior Resources Inc. by Firefly Aviation Ltd. of Calgary, Alberta, between September 28th and October 9th, 2004. A total of 4347.9 line km were acquired and processed for this release. The survey base was the Kenora, Ontario, airport.



The Fisher Lake survey, located southeast of Kenora in western Ontario (Figure 1a), covers an area underlain mostly by the Archean rocks of the Kakagi–Rowan Lakes greenstone belt (see Figure 1b). The greenstone belt, which lies within the western Wabigoon region, comprises mafic and felsic to intermediate volcanics with minor amounts of gabbroic intrusions located in the southeastern part of the area. A synform-antiform fold pair, whose axes are oriented

northeastward, are developed in the volcanic geology. A small granitic pluton, known as the Flora Lake Stock, is exposed along the axis of the antiform. A second, possibly related, stock (the Hope Lake Stock) is located along the same antiformal axis, near the southern limit of the survey area. The volcanic rocks are in contact with a region of metasediments that are located close to the long axis of the survey area. The contact between the volcanic and metasedimentary rocks is defined by the regional-scale, northeast-oriented Wabigoon fault.

The northwestern part of the survey area is underlain by the felsic intrusive rocks of the Dryberry batholith. Gold, copper and nickel mineralization, primarily associated with mafic volcanic and intrusive rocks, has been noted in the southern and eastern parts of the area.


Figure 1a. Fisher Lake survey area flown with the Firefly Aviation Ltd. magnetic system is shown in black outline.


Figure 1b. Bedrock geology of the Fisher Lake area (excerpted from Blackburn 1981) showing the outline of the survey area. Granitic rocks are shown in pink, mafic volcanics in green, metasediments in grey and mafic intrusive rocks in blue.

The Fisher Lake survey boundary is defined by the following polygon (NAD 27 Zone 15U):

Point East North

1. 443570 E 5469400 N

2. 436600 E 5476400 N

3. 458220 E 5498460 N

4. 465200 E 5491400 N

The area is centered at about 9342'W, 4931'N. For an elevation of 456 m and a mean survey date of October 4, 2004 (2004.76132), the geomagnetic field had the following characteristics:


Mean Survey Location IGRF

93.69693°W (-93°41m48.95s) Total Field 58,404.86 nT

49.52162°N (49°31m17.83s) Inclination 75.65°N (75°38m48s) 456.13 m Above Sea Level Declination 1.13°E of North (1°08m51s)

Survey Specifications

The airborne survey and noise specifications in the Fisher Lake survey area were as follows:

a) traverse line spacing and direction

nominal line spacing is 100 m

nominal line direction 135°/315°

maximum deviation from the nominal traverse line location did not exceed 33 m over a distance flown greater than 1000 m.

b) control line spacing and direction

nominal line spacing 500 m

nominal line direction 045°/225°.

c) terrain clearance of the aircraft

nominal terrain clearance is 50 m in a drape mode

all lines were within altitude tolerance of the planned drape surface, except in areas of severe topography.

d) aircraft speed

nominal aircraft ground speed is 50 m/sec

nominal magnetic sample spacing is 5 m on the ground.

e) magnetic diurnal variation

Flown per specifications, with data not acquired during magnetic storms or short term disturbances which exceeded survey specifications.

f) magnetometer noise envelope

in-flight noise envelope could not exceed the noise specification for straight and level flight.

g) re-flights and turns

Any lines with channels or portions of channels missing from the database were reflown.


2) AIRCRAFT, EQUIPMENT AND PERSONNEL Aircraft and Geophysical On-Board Equipment

Aircraft: Cessna U206G

Operator: Firefly Aviation Ltd.

Registration: C-GWAS Nominal Survey Speed: 110 knots / 203.7 kph / 56.6 m/s

Magnetometers: Single Geometrics G-822 optically pumped caesium vapour magnetometer, sensitivity of +/- 0.005 nT,sampling rate= 0.1 sec., ambient range 15 000 to 100 000 nT, and 4th difference of 0.02 nT, mounted in a tail boom.

Tri-Axial Magnetic Field Sensor (for compensation, mounted in the tail boom proximal to the G-822 pod) Billingsley Magnetics model TFM 1000, Internal Noise at 1 Hz - 1 kHz;

0.6 nT rms, Bandwidth 0 to 1 kHz maximally flat, -12

dB/octave roll off beyond 1 kHz, Frequency Response 1 HZ - 100 Hz: +/- 0.5%, 100 Hz - 500 Hz: +/- 1.5%, 500 Hz - 1 kHz: +/- 5.0%, Calibration Accuracy: +/- 0.5%,

Orthogonality +/- 0.5% worst case, Package Alignment +/- 0.5% over full temperature range, Scaling Error absolute: +/- 0.5%, between axes: +/- 0.5%

Digital Acquisition: RMS Instruments DGR33A with chart recorder employing RMS4183A microprocessor, up to 128 MB on board memory via SCSI Compact Flash Interface, 5 AT and 3 PC compatible I/O slots, electro – luminescent 640 x 400 pixels display, User interface is scrolling analog chart simulation with up to 5 windows operator selectable and freeze display capability to hold image for inspection, 128 MB SCSI Compact Flash Drive recorder, 1 Hz sampling rate, 32 differential analog inputs, two RS-232/RS422 serial ports, and 4-channel Serial I/O; 4-channel ARINC parallel ports.

Barometric Altimeter: Sensym LX18001AN, sensitivity 1 foot, 1 sec. recording interval.

Radar Altimeter: King KRA-10A, 1% calibrated accuracy, 5% accuracy up to 5000 feet.

Electronic Navigation: Novatel ProPak LB Plus 3D GPS receiver, CDGPS


differential source, Continuous tracking, L1 frequency, C/A code (SPS), 12 channel (independent), 2 Hz sampling rate, accuracy of ~1.0 m, differentially corrected position (SA implemented) 100 m, (no SA) 30 m, velocity 0.1 knot, time recovery 1 pps, 100 nsec pulse width, and all GPS data and positional data logged by onboard DGR33A on compact flash.

Navigation Interface: AG-NAC Inc. model P141, Real time processing of GPS output data, Pilot Readout of Left/Right indicator/forward line projection screen, Operator Readout Screen modes: map, survey and line, all data recorded in real time on Compact Flash disk via DGR33A.

Magnetic compensation for aircraft and heading effects is done in real time. Raw magnetic values are also stored and thus if desired, compensation with different variables can be run at a later time.

Magnetic Compensation System: RMS Instruments Model AADCII, input from 1 to 4 high sensitivity magnetometers, Input Frequency Range 70 kHz to 350 kHz, Magnetic Field Range 20,000 to 100,000 nT, Front End Counter 100 MHz, Resolution 1 pT, Compensation Improvement ratio 10 to 20 typical for total field, accuracy of Compensation 0.035 nT standard deviation for the entire aircraft flight envelope in the bandwidth 0 to 1 Hz typical, Data Output Rate of 10 Hz maximum, Internal System Noise less than 1 pT, Vector Magnetometer 3-Axis Fluxgate over sampled with 16 bit resolution, three Serial RS232C output ports (max rate 19.2 Kbaud), Magnetometer data output, Direct Interface with GR33A, parallel output port, 16 bit with full handshaking and four analog outputs with 12 bit


Base Station Equipment

Magnetometer: GEM GSM-19 Overhauser magnetometer located in a magnetically quiet area, measuring the total intensity of the earth’smagnetic field inunits of 0.1 nT at 1 or 2 Hz.

GPS Receiver: Novatel OEM 2 GPS receiver, Continuous tracking, L1 frequency, C/A code (SPS), 10 channel WAAS Enabled, at 1 second intervals. Accuracy (with SA implemented) 100 meters, (no SA) 30 meters, velocity 0.1 knot, time recovery 1 pps, 100 nsec pulse width Data Recording all GPS raw and positional data logged by PC based data logger

Computer: PC with Logging software by GEM-Terraplus Ltd.

Compatible to PC with RS 232 input; supports real time


graphics, automatic startup, compressed data storage,

selectable start/stop times, automatic disk swapping, plotting of data to screen or printer at user selected scales, and fourth digital difference and diurnal quality flags set by user.


The following Firefly Aviation Ltd. personnel were involved in the project.

The field crew was:

Bruce Evans Survey Pilot

Travis Reed Equipment Operator

Jeremy Weber Field Data Processor

The processing crew was:

Bruce Evans Project Manager

Jeremy Weber Senior Processor, Quality Control

Christopher Campbell (Intrepid Geophysics) Final Processing and Map Production.



Field operations were conducted between September 28 and October 9, 2004. The aircraft and crew mobilized to the project on September 28, and conducted initial calibration and

compensation flights September 29, 2004. The aircraft and crew demobilized from the project area on October 9, 2004, and arrived back at the Calgary base on the same day. The final

acquisition flight was completed on October 8, 2004. There were a total of 11 acquisition flights conducted.

The main base of operations for the project was the airport located at Kenora (CYQK). The base station magnetometer and GPS equipment were located in a magnetically quiet location at the airport.

Fuel for the aircraft was purchased on site at the airport. Accommodations for the field crew were at the town of Kenora.

The satellite navigation system was used to ferry to the survey site and to survey along each line using UTM coordinates. The survey coordinates of the survey outline for navigation purposes and flight path recovery were calculated from the project area coordinates listed above.

The navigation accuracy is variable depending on the number and condition of the satellites;

however, with use of the real time differential 3D GPS navigation it is generally less than 5 m and typically in the 1 to 3 m range. Post-flight differential correction of the flight path, which corrects for satellite range errors, improves the accuracy of the flight path recovery to

approximately within 1 to 3 m.

After each mission the flight data was fully field processed and quality-checked. Each line of data was viewed on-screen, displaying raw mag, compensated mag, ground mag, noise, radar altitude, latitude/longitude, flight path, and in-grid/out-of-grid. These, with the digital review, were the basis for the data QC. Any flight lines that exceeded the survey specifications due to aircraft positioning, diurnal variations or noise were noted for reflight, and forwarded to the flight crew for re-collection.

The generalized processing procedure during the survey consisted of the following:

1) Import all flight and base data into Geosoft®.

2) Edit diurnal channel to remove any uncharacteristic spikes and linearly interpolate across any gaps.

3) Establish table of mean terrain clearances at intersection locations from tie line data to provide elevation guidance for survey line navigation. Grid differences in elevations at intersections of tie and survey lines to provide quality check on elevation control and tag any for reflight.

4) Edit flight path channels to remove any false spikes and linearly interpolate gaps.


5) Edit raw MAG channel to remove any false spikes and linearly interpolate gaps.

6) Create new diurnally corrected channel as MAGDC = (MAG1 - diurnal) + base constant (59656 nT).

7) Perform lag correction and heading correction to MAGDC channel.

8) Perform tie line leveling using all the survey line data to level the tie lines.

9) Perform preliminary survey line leveling using the leveled tie lines.

10) All data were viewed on the screen on a line-by-line basis using the interactive Geosoft® Oasis Montaj database to inspect for quality, required tolerances and data integrity.

11) Produce preliminary flight path map and gridded magnetic intensity map including shadowing.

12) Plot survey line and tie line flight paths and profiles for quality control inspection.



The Fisher Lake data set was reprocessed and reformatted in order to conform with other

airborne data sets, held by the Ministry, prior to making them available to the public. To this end the Ministry retained CGI Controlled Geophysics Inc. of Thornhill, Ontario, for reprocessing and creation of publication-ready products. The steps carried out and the products generated are described below.

Base maps

Base maps of the survey area, supplied by the Ontario Ministry of Northern Development, Mines and Forestry, were derived from the Ontario Land Information Warehouse, Land Information Ontario, Ontario Ministry of Natural Resources, scale 1:50 000.

Projection description

Datum: NAD83 Local Datum: (4 m) Canada Ellipsoid: GRS80 Projection: UTM (Zone 15N) Central Meridian: 93°W

False Northing: 0 m False Easting: 500,000 m Scale factor: 0.9996

Flight Path, Terrain Clearance, and Magnetic Data Review

Upon receipt of the data from MNDMF, the Geosoft® database navigation and geophysics data channels were inspected on a line-by-line basis, the flight path plotted at 1:50 000 scale, and the Total Magnetic Intensity (TMI) gridded at the required 20 m resolution to ensure completeness of the archive and to reveal any impediments to advancing the processing.

Examinations included differences between the supplied raw and final TMI channels, sensor terrain clearance consistency, and detection of flight path gaps, back-tracks, or other

inconsistencies. Sample images of these initial products were generated and forwarded to MNDMF as part of the progress reporting.

The database lines were found to be untrimmed to a survey polygon, the radar altimeter data were missing, and the IGRF channel, although present, was undescribed. A suitable polygon outline was defined and applied. Line ends that crossed other survey lines were manually edited out.

The supplied IGRF channel was compared with a new IGRF created using the expected parameters. A match was obtained using the following parameters below.


The IGRF at the actual survey centre and average aircraft elevation on the mean survey date of October 4, 2004 (2004.76132), was:

Lon 93.69693°W -93°41′48.95″ T.F. 58,404.86 nT Lat 49.52162°N 49°31′17.83″ => Incl. 75.6467°N (75°38′48″)

Elev 456.13 m ASL Decl. 1.1307°E (1°08′51″)

The missing radar data was located and imported; however, it represented uncalibrated mV values. To create a new final radar channel, an artificial radar channel was generated from the provided GPS elevation and the Space Shuttle digital elevation model. This was cross-correlated with the raw radar channel and calibration formula derived of:


A digital elevation model was made from the newly calibrated radar channel and gridded for inspection. Several flights exhibited a herring-bone pattern indicating that those data had not been corrected for parallax. The lag-correction was applied where indicated and a new acceptable DEM produced. The final radar data are included in the database.

In preparation for later flight path plotting, a new Fid channel was created from the existing time-based fiducial channel. By using (FLOOR(gtime*10)/10.0-49990.0)/10.0, the number of significant digits required to be drawn was reduced.

Microlevelling of Magnetic Data

Levelling errors are a major source of noise in aeromagnetic data sets and can be recognized by visual inspection of coloured or shadowed images of the total magnetic field or through similar inspection of computed first and second vertical derivatives of the magnetic field.

Aside from possible processing errors, the main sources of TMI levelling errors are inadequate characterization and removal of diurnal variations and/or insufficient or poor quality tie line intersections and/or inconsistent terrain clearance between adjacent lines. Standard TMI levelling minimizes all of these problems, but cannot completely eliminate them. For this reason, the microlevelling method was developed (see Minty 1991).

The method used entailed preparation of a high-quality grid of the TMI using minimum-

curvature gridding. The standard Geosoft® gridding defaults were altered to stress quality rather than performance. The parameters employed were:

Grid Cell Size 20 m

Desample Factor 1 Blanking Radius 450 m Maximum Search Radius 1280 m Order of Weighting Function 2 Weighting Slope 0 Edge extend cell number 0


Tolerance Limit in Iteration 0.01 nT Points Passed Tolerace Limit 100%

Maximum Iteration Times 300 Internal Tension Parameter 0 Coarse Grid Factor 16

Next, via a 2-D FFT transform, the TMI grid was operated on using a filter designed to extract signal oriented only parallel to the nominal flight path, and only of a wavelength range consistent with levelling differences between adjacent lines. The resulting grid contained only the levelling errors plus a variable amount of geological signal that happened to align with the flight path (common in Achaean terrains). The parameters of the operator were a 400 m high-pass 8th-order Butterworth filter combined with a Directional Cosine filter of degree 1.2 to pass features closely parallel to N 135° E.

The 2-D FFT algorithm requires a filled, regular-shaped grid and a piecewise continuous surface from one edge to its opposite. Therefore, the process included grid extrapolation and trend removal. The Geosoft® defaults are not always optimal; therefore, the error grid was inspected to ensure that no artefacts were created during the FFT, and parameters altered if necessary. The final error grid was retained for QA/QC purposes.

The error grid was next read into a new channel in the database from which the TMI was created and inspected for every survey line. The tie lines were not included in the process. In order to avoid removal of genuine geological signal, the error channel was amplitude-limited and filtered to extract only long-wavelength features. Normally, if the data have been properly levelled, a single universal amplitude limit is applicable. However, individual lines can be treated separately if warranted, especially if a localized linear geological feature is present which must be retained.

Both the long width, zero-amplitude nonlinear filter and the shorter width low-pass filter approaches were applied and compared. The nonlinear filter did not remove sufficient noise adjacent to high-gradient anomalies and the low-pass filter experienced too much ringing in the same areas. In the end, a transformation was applied to the error channel to retain low amplitude levelling errors and reduce the contribution of the high-amplitude anomalies in the error channel, and the low-pass filter method was employed. The standard deviation of the final error channel was +/- 5.6 nT. Finally, the approved error channel was subtracted from the TMI to produce a microlevelled TMI channel (ML). The results were inspected both in profile form and by preparing the ML grid from which shadows and derivatives are made.

The approved ML channel was gridded using high accuracy settings to produce an uncompressed Geosoft® grid in both binary GRD format and ASCII GXF format.

Calculation of Magnetic Second Vertical Derivative Grid

The second vertical derivative of the total magnetic field (2VD) was computed to enhance small and weak near-surface anomalies and as an aid to delineate the contacts of the lithologies having


contrasting susceptibilities. The location of contacts or boundaries is usually traced by the zero contour of the second vertical derivative map.

Via a 2-D FFT transform, the TRF grid was operated on using a filter designed to calculate the 2nd vertical derivative while minimizing grid aliasing effects in the total residual field, which are emphasized by the second vertical derivative. To avoid introducing high frequency noise in the 2VD, an upward continuation operator using a distance equivalent to one cell size (i.e., 20-m) was employed.

The 2-D FFT algorithm requires a filled, regular shaped grid and a piecewise continuous surface from one edge to its opposite. Therefore, the process included grid extrapolation and trend removal. The Geosoft® defaults are not always optimal, therefore the 2VD grid was inspected to ensure that no artefacts were created during the FFT, and parameters altered if necessary.

The final approved 2VD grid was read into a channel in the magnetic database and delivered as an uncompressed Geosoft® grid in both binary GRD format and ASCII GXF format.

For the published map, the 2VD grid was shadowed using artificial sun illumination of 30°

elevation and 0° azimuth.

Keating Correlation Coefficients

Possible kimberlite targets are identified from the residual magnetic intensity data, based on the identification of roughly circular anomalies. This procedure is automated by using a known pattern recognition technique (Keating 1995), which consists of computing, over a moving window, a first-order regression between a vertical cylinder model anomaly and the gridded magnetic data.

Only the results where the absolute value of the correlation coefficient is above a threshold of 75%

were retained. On the magnetic maps, the results are depicted as circular symbols, scaled to reflect the correlation value. The most favourable targets are those that exhibit a cluster of high-amplitude solutions. Correlation coefficients with a negative value correspond to reversely magnetized sources.

First, an artificial kimberlite anomaly grid was prepared using parameters provided by MNDMF.

The cylinder model parameters are as follows:

Cylinder diameter: 200 m

Cylinder length: infinite

Overburden thickness: 4 m (average)

Magnetic inclination: 75.65N

Magnetic declination: 1.08W

Window size: 24 x 24 cells (480 x 480 m)

Magnetization scale factor: 100 Model window grid cell size: 20 m The model’s magnetic response is shown in Figure 2.


Figure 2. Vertical cylinder model anomaly used for Keating correlation on the Fisher Lake survey. Grid cell size is 20 m and contour interval is 5 nT. Top of cylinder outlined in blue.

The model was cross-correlated with the final ML grid and a database of coefficients created.

These were plotted in a symbol format and reviewed for correctness. The databases have been provided in Geosoft®.GDB format (uncompressed) and comma delimited ASCII CSV format. The databases fields are described in the Appendixes.

It is important to be aware that other magnetic sources may correlate well with the vertical cylinder model, whereas some kimberlite pipes of irregular geometry may not. The user should study the magnetic anomaly that corresponds with the Keating symbols, to determine whether it does resemble a kimberlite pipe signature, reflects some other type of source or even noise in the data, e.g., boudinage (beading) effect of the minimum curvature gridding. All available geological information should be incorporated in kimberlite pipe target selection.



In 1989, as part of the requirements for the contract with the Ontario Geological Survey (OGS) to compile and level all existing Geological Survey of Canada (GSC) aeromagnetic data (flown prior to 1989) in Ontario, PGW developed a robust method to level the magnetic data of various base levels to a common datum provided by the GSC as 812.8 m grids. The essential theoretical aspects of the levelling methodology were fully discussed in Gupta et al. (1989), and Reford et al. (1990). The method was later applied to the remainder of the GSC data across Canada and the high-resolution AMEM surveys flown by the OGS (Ontario Geological Survey, 1996). It has since been applied to all newly acquired OGS aeromagnetic surveys.


Master grid – refers to the 200 m Ontario magnetic grid compiled and levelled to the 812.8 m magnetic datum from the Geological Survey of Canada.

GSC levelling – the process of levelling profile data to the master grid, first applied to GSC data.

Intra-survey levelling or microlevelling – refers to the removal of residual line noise described earlier; the wavelengths of the noise removed are usually shorter than tie line spacing.

Intersurvey levelling or GSC levelling – refers to the level adjustments applied to a block of data;

the adjustments are the long wavelength (in the order of tens of kilometres) differences with respect to a common datum, in this case, the 200 m Ontario master grid, which was derived from all pre-1989 GSC magnetic data and adjusted, in turn, by the 812.8 m GSC Canada wide grid.

The GSC Levelling Methodology

The GSC levelling methodology is described below, using the Vickers survey flown for OGS as an example.

As described above, microlevelling and IGRF calculation and removal were applied to the survey data prior to GSC levelling.

The steps in the GSC levelling process are as follows:

1. Create an upward continuation of the survey grid to 305 m

Almost all recent surveys (1990 and later) to be compiled were flown at a nominal terrain clearance of 100 m or less. The first step in the levelling method is to upward continue the survey grid to 305 m, the nominal terrain clearance of the Ontario master grid (Figure 3). The grid cell size for the survey grids is set at 100 m. Since the wavelengths of level corrections will be greater than 10 to 15 km, working with 100 m or even 200 m grids at this stage will not affect the integrity of the levelling method. Only at the very end, when the level corrections are

imported into the databases, will the level correction grids be re-gridded to 1/5 of line spacing.


The un-levelled 100 m grid is extended by at least 2 grid cells beyond the actual survey boundary, so that, in the subsequent processing, all data points are covered.

Figure 3. Ontario master aeromagnetic grid (Ontario Geological Survey 1999). The outline for the sample data set to be levelled (Vickers) is shown.

2. Create a difference grid between the survey grid and the Ontario master grid.

The difference between the upward continued survey grid and the Ontario master grid, re-

gridded at 100 m, is computed (Figure 4). The short wavelengths represent the higher resolution of the survey grid. The long wavelengths represent the level difference between the two grids.


Figure 4. Difference grid (difference between survey grid and master grid), Vickers survey.

3. Rotate difference grid so that flight line direction is parallel to grid column or row, if necessary.

4. Apply the first pass of a nonlinear filter (Naudy and Dreyer 1968) of wavelength on the order of 15 to 20 km along the flight line direction. Reapply the same nonlinear filter across the flight line direction.

5. Apply the second pass of a nonlinear filter of wavelength on the order of 2000 to 5000 m along the flight line direction. Reapply the same nonlinear filter across the flight line direction.

6. Rotate the filtered grid back to its original (true) orientation (Figure 5).


Figure 5. Difference grid after application of nonlinear filtering and rotation, Vickers Survey.

7. Apply a low pass filter to the nonlinear filtered grid

Streaks may remain in the nonlinear filtered grid, mostly caused by edge effects. They must be removed by a frequency-domain, low-pass filter with the wavelengths in the order of 25 km (Figure 6).

Figure 6. Level correction grid, Vickers survey.

8. Re-grid to 1/5 line spacing and import level corrections into database.


9. Subtract the level correction channel from the unlevelled channel to obtain the level corrected channel.

10. Make final grid using the gridding algorithm of choice with grid cell size at 1/5 of line spacing.

Survey Specific Parameters

The following GSC levelling parameters were used in the Fisher Lake survey:

Distance to upward continue: 230 m Difference grid rotation angle: 45°

First pass non-linear filter length: 2100 m Second pass non-linear filter length: 425 m

The small aerial extent of the Fisher Lake survey warranted shorter filter lengths to capture the differences. This also allowed the final low-pass filter step to be omitted.

The approved GSC levelled magnetic channel was gridded using high accuracy settings to produce an uncompressed Geosoft® grid in both binary GRD format and ASCII GXF format.



The following products were delivered to MNDMF:

Map products at 1:50 000

colour residual magnetic field grid with contours, plotted along with flight path and Keating kimberlite coefficient anomalies on a planimetric base

shaded colour image of the second vertical derivative of the magnetics, plotted along with flight path and Keating kimberlite coefficient anomalies on a planimetric base

Profile database

Magnetic database at 10 samples/sec in both Geosoft® GDB and ASCII format Kimberlite coefficient database

Keating kimberlite coefficient anomaly database in both Geosoft® GDB and ASCII CSV format.

Data grids

Data grids, in both Geosoft® GRD and GXF formats, gridded from coordinates in NAD83 datum of the following parameters:

digital elevation model

GSC-levelled residual magnetic field

second vertical derivative of the GSC-levelled residual magnetic field GeoTIFF images of the entire survey

colour gradient-enhanced residual magnetic field on a planimetric base

colour shaded relief of the second vertical derivative of the magnetic field on a planimetric base

DXF vector files of the entire survey

flight path

Keating kimberlite coefficient anomalies

residual magnetic field contours

Project report

Provided in both Microsoft® Wordand Adobe® PDF formats.



Blackburn, C.E. 1981. Kenora–Fort Frances, Geological Compilation Series, Kenora and Rainy River districts; Ontario Geological Survey, Map 2443, scale 1:253 440.

Briggs, I. 1974. Machine contouring using minimum curvature; Geophysics, v.39, p.39-48.

Evans, B. 2004. Warclub and Eagle projects, Kenora area, Ontario, high resolution aeromagnetic survey (HRAM) logistical report for Western Warrior Resources Inc.; unpublished report.

Gupta, V., Paterson, N., Reford, S., Kwan, K., Hatch, D. and Macleod, I. 1989. Single master aeromagnetic grid and magnetic colour maps for the province of Ontario; in Summary of Field Work And Other Activities 1989, Ontario Geological Survey Miscellaneous Paper 146, p.244-250.

Gupta, V. and Ramani, N. 1982. Optimum second vertical derivatives in geological mapping and mineral exploration; Geophysics, v.47, p.1706-1715.

Gupta, V., Rudd, J. and Reford, S. 1998. Reprocessing of thirty-two airborne electromagnetic surveys in Ontario, Canada: Experience and recommendations; 68th Annual Meeting of the Society of Exploration Geophysicists, Extended Technical Abstracts, p.2032-2035.

Keating, P.B. 1995. A simple technique to identify magnetic anomalies due to kimberlite pipes;

Exploration and Mining Geology, v.4, no.2, p.121-125.

Minty, B.R.S. 1991. Simple microlevelling for aeromagnetic data; Exploration Geophysics, v.22, p.591-592.

Naudy, H. and Dreyer, H. 1968. Essai de filtrage nonlinéaire appliqué aux profiles aeromagnétiques; Geophysical Prospecting, v.16, p.171-178.

Ontario Geological Survey 1999. Single master gravity and aeromagnetic data for Ontario;

ERLIS Data Set 1036.

Reford, S.W., Gupta, V.K., Paterson, N.R., Kwan, K.C.H. and Macleod, I.N. 1990. Ontario master aeromagnetic grid: A blueprint for detailed compilation of magnetic data on a regional scale; in Expanded Abstracts, Society of Exploration Geophysicists, 60th Annual International Meeting, San Francisco, v.1., p.617-619.



Geophysical Data Set 1232 is derived from a survey using a single sensor magnetic system mounted on a fixed wing platform which was carried out by Firefly Aviation Ltd.

Data File Layout

The files for the Fisher Lake Survey, Geophysical Data Set 1232, are archived on a single DVD- ROM and sold as single product, as outlined below:

Type of data Magnetic

Grid/Vector and Profile data (DVD-R)

Format ASCII and

Geosoft® Binary 1232

The content of the ASCII and Geosoft® binary file types are identical. They are provided in both forms to suit the user’s available software. The survey data are divided as follows:

DVD - 1232

- ASCII (GXF) grids - total field magnetics

- second vertical derivative of the total field magnetics - digital elevation model

- Geosoft® Binary (GRD) grids:

- total field magnetics

- second vertical derivative of the total field magnetics

- Keating correlation (kimberlite) database (ASCII CSV format) - Keating correlation (kimberlite) database (Geosoft® GDB format) - DXF files:

- flight path

- Keating correlation (kimberlite) anomalies - total field magnetic contours

- GEOTIFF images (150 dpi) of the entire survey block - colour total field magnetics with base map

- colour shaded relief of second vertical derivative with base map - ASCII Profile data

- Profile database of magnetic data (10 Hz sampling) in ASCII (XYZ) format - Binary Profile data

- Profile database of magnetic data (10 Hz sampling) in Geosoft® GDB format - Survey report (Microsoft® Word®97and Adobe® PDF formats)

Coordinate Systems


The profile and Keating coefficient data are provided in two coordinate systems:

- Universal Transverse Mercator (UTM) projection, Zone 15N, NAD83 datum, Canada local datum

- Latitude/longitude coordinates, NAD83 datum, Canada local datum The gridded data are provided in the following UTM coordinate system:

- Universal Transverse Mercator (UTM) projection, Zone 15N, NAD83 datum, Canada local datum

Line Numbering

The line numbering convention is as follows:

Line number x 10 + part number

e.g., Line 172 part 1 is identified as 1721

The same convention is used for the labelling of the control lines.

Profile Data

The profile data are provided in two formats, one ASCII and one binary:


ASCII XYZ file of magnetic data, sampled at 10 Hz:

- FLMAG.XYZ: data for entire survey Binary

Geosoft® OASIS Montaj binary database file (no compression) of magnetic data, sampled at 10 Hz:

- FLMAG.GDB: data for entire survey

The contents of FLMAG.XYZ/GDB (both file types contain the same set of data channels) are summarized as follows:

Channel Name Description Units

x_nad83 GPS X in UTM co-ordinates using NAD83 datum metres

y_nad83 GPS Y in UTM co-ordinates using NAD83 datum metres

lon_nad83 differentially corrected GPS X (longitude - NAD83 datum) decimal-degrees lat_nad83 differentially corrected GPS Y (latitude - NAD83 datum) decimal-degrees

radar_raw raw radar altimeter height mV

radar_final low-pass filtered radar altimeter height metres above terrain gps_z_final differentially corrected GPS Z (NAD83 datum) metres above sea level

dem digital elevation model metres above sea level

fiducial fiducial seconds

flight flight number

line_number full flight line number (flight line and part numbers) line flight line number

line_part flight line part number

time_utc UTC time seconds

date local date YYYY/MM/DD

mag_base_final corrected magnetic base station data nanoteslas


mag_edit compensated, edited, diurnally-corrected magnetic field nanoteslas

igrf local IGRF field nanoteslas

mag_lev levelled magnetic field nanoteslas

mag_igrf IGRF-corrected magnetic field nanoteslas

mag_final micro-levelled magnetic field nanoteslas

mag_gsclevel GSC-levelled magnetic field nanoteslas

fluxgate_x compensation fluxgate x component nanoteslas

fluxgate_y compensation fluxgate y component nanoteslas

fluxgate_z compensation fluxgate z component nanoteslas



The Keating kimberlite pipe correlation coefficient data are provided in two formats, one ASCII and one binary:

FLKC.CSV – ASCII comma-delimited file

FLKC.GDB – Geosoft® OASIS montaj binary database file

Both file types contain the same set of data channels, summarized as follows:

Channel Name Description Units

lon_nad83 differentially corrected GPS X (longitude - NAD83 datum) decimal-degrees lat_nad83 differentially corrected GPS Y (latitude - NAD83 datum) decimal-degrees lon_nad27 differentially corrected GPS X (longitude - NAD27 datum) decimal-degrees lat_nad27 differentially corrected GPS Y (latitude - NAD27 datum) decimal-degrees

x_nad27 GPS X in UTM co-ordinates using NAD27 datum metres

y_nad27 GPS Y in UTM co-ordinates using NAD27 datum metres

x_nad83 GPS X in UTM co-ordinates using NAD83 datum metres

y_nad83 GPS Y in UTM co-ordinates using NAD83 datum metres

corr_coeff correlation coefficient percent

norm_error standard error normalized to amplitude percent

amplitude peak-to-peak anomaly amplitude within window nanoteslas

pos_coeff positive correlation coefficient percent

neg_coeff negative correlation coefficient percent



The gridded data are provided in two formats, one ASCII and one binary:

*.gxf - Geosoft® ASCII Grid eXchange Format (revision 3.0, no compression)

*.grd - Geosoft® OASIS montaj binary grid file (no compression) The grids are summarized as follows:

FLMAG83.GRD/.GXF IGRF-corrected and GSC-levelled magnetic field in nanoteslas (UTM coordinates, NAD83 datum)

FL2VD83.GRD/.GXF second vertical derivative of the IGRF-corrected and GSC-levelled magnetic field in nanoteslas per metre-squared (UTM coordinates, NAD83 datum)

FLDEM83.GRD/.GXF digital elevation model in metres above sea level (UTM coordinates, NAD83 datum)



Geographically referenced colour images, incorporating a base map, are provided in GeoTIFF format for use in GIS applications:

FLMAG83.tif – GSC Levelled Residual Magnetic Intensity

FL2VD83.tif – Second Vertical Derivative of GSC Levelled Residual Magnetic Intensity

Vector Archives

Vector line work from the maps is provided in DXF (v12) ASCII format using the following naming convention:

FLPATH83.dxf – flight path of the survey area FLKC83.dxf – Keating correlation targets

FLMAG83.dxf – contours of the residual magnetic intensity in nanoteslas

The layers within the DXF files correspond to the various object types found therein and have intuitive names.





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