GERALDTON-TASHOTA AREA Ontario Airborne Magnetic and Electromagnetic Surveys Geophysical Data Set 1032 - Revised Processed Data and Derived Products Archean and Proterozoic “Greenstone” Belts Ontario Geological Survey Ministry of Northern Development and Mines Willet Green Miller Centre 933 Ramsey Lake Road Sudbury, Ontario, P3E 6B5 Canada Report on Geraldton-Tashota Area Geophysical Survey Geophysical Data Set 1031/1032 - Revised NOTE: This is a revised version of the Geraldton-Tashota area ERLIS CD-ROM 1031 and 1032, issued September 14th 1998. In the previous release there was a positioning problem (with a maximum reaching several hundred meters) in the northwest part of the area, which has been fixed for this release. Consequently, new corrected gridded, profile and EM anomaly data are provided on ERLIS CD-ROM 1031.1 and 1032.1, issued in June 1999. Report on Geraldton-Tashota Area Geophysical Survey i Geophysical Data Set 1031/1032 - Revised TABLE OF CONTENTS Page CREDITS........................................................................................................................................1 DISCLAIMER................................................................................................................................2 CITATION......................................................................................................................................2 1) INTRODUCTION................................................................................................................3 2) FLIGHT PATH REPROCESSING......................................................................................6 2.1) Flight Path Standards.....................................................................................................6 2.2) Flight Path Reprocessing Methodology.........................................................................6 3) ALTIMETER DATA REPROCESSING.............................................................................8 4) MAGNETIC DATA REPROCESSING...............................................................................9 4.1) Editing of Magnetic Data.............................................................................................10 4.2) IGRF Removal.............................................................................................................10 4.3) Micro-Levelling...........................................................................................................10 4.4) Levelling to Ontario Master Aeromagnetic Grid.........................................................12 4.5) Gridding of Reprocessed Magnetic Data.....................................................................14 4.6) Second Vertical Derivative of the Total Magnetic Field.............................................14 4.7) Keating Correlation Coefficients.................................................................................17 5) ELECTROMAGNETIC DATA REPROCESSING...........................................................19 5.1) Reprocessing Specifications and Tolerances...............................................................19 5.2) Electromagnetic Data Reprocessing Procedures..........................................................22 5.3) EM Anomaly Picking...................................................................................................29 6) QUALITY CONTROL AND QUALITY ASSURANCE..................................................33 6.1) Flight Path....................................................................................................................33 6.2) Profile Data..................................................................................................................33 6.3) Grid Data......................................................................................................................34 6.4) EM Anomaly Data.......................................................................................................35 7) SURVEY-SPECIFIC DETAILS: GERALDTON-TASHOTA..........................................36 7.1) Flight Path Reprocessing..............................................................................................36 7.2) Altimeter Data Reprocessing.......................................................................................37 7.3) Magnetic Data Reprocessing........................................................................................37 7.4) Electromagnetic Data Reprocessing.............................................................................42 7.5) Known Data Bugs........................................................................................................43 REFERENCES.............................................................................................................................47 APPENDIX A...............................................................................................................................48 SURVEY HISTORY............................................................................................................48 APPENDIX B...............................................................................................................................58 CONTENTS OF PROFILE, GRID AND ELECTROMAGNETIC ANOMALY DATABASES.......................................................................................................................58 APPENDIX C...............................................................................................................................64 2002 DIGITAL ARCHIVE RE-FORMATTING NOTES...................................................64 Report on Geraldton-Tashota Area Geophysical Survey ii Geophysical Data Set 1031/1032 - Revised CREDITS The overall project management, scientific authority and quality control was provided by Vinod K. Gupta, Ontario Geological Survey, Sudbury. The following is a list of the organisations that have participated in this project, and their various responsibilities: Paterson, Grant & Watson Limited, Toronto - Prime contractor -project management -magnetic data processing Geoterrex, a division of CGG Canada Ltd., Ottawa - Sub-contractor -flight path correction -electromagnetic data processing (time-domain surveys) Dighem/I-Power, a division of CGG Canada Ltd., Mississauga - Sub-contractor -flight path correction -electromagnetic data processing (frequency-domain surveys) CGI Controlled Geophysics Inc., Thornhill - Sub-contractor - quality control of data - data merging and preparation of final products Geopak Systems, Toronto - Independent contractor - developed viewing and geophysical data exporting Windows TM “Centurion” software accompanying this CD-ROM Report on Geraldton-Tashota Area Geophysical Survey 1 Geophysical Data Set 1031/1032 - Revised DISCLAIMER Every possible effort has been made to ensure the accuracy of the information provided on this CD-ROM; however, the Ontario Ministry of Northern Development and Mines does not assume any liability or responsibility for errors that may occur. Users may wish to verify critical information. CITATION Information about the Archean and Proterozoic “greenstone belts” airborne geophysics digital data set may be quoted if credit is given. It is recommended that reference be made in the form shown in this example: Ontario Geological Survey 2003. Ontario airborne geophysical surveys, magnetic data, Geraldton-Tashota area; Ontario Geological Survey, Geophysical Data Set 1031/1032 - Revised. Report on Geraldton-Tashota Area Geophysical Survey 2 Geophysical Data Set 1031/1032 - Revised 1) INTRODUCTION High resolution airborne magnetic and electromagnetic surveys, over major greenstone belts, were initiated in 1975 by the Ontario Department of Mines (currently known as the Ontario Geological Survey) to aid geological mapping and mineral exploration. Between the period 1975 to 1992, thirty-two airborne surveys were flown and processed by different survey contractors and subcontractors. The two earlier surveys, Matachewan and Bamaji-Fry Lakes, were acquired in analog form and the remaining thirty surveys were recorded digitally. The surveys were flown at a nominal flight line spacing of 200m with the exception of James Bay Cretaceous Basin survey, which was flown at a flight line spacing of 1000m. The flight directions for the surveys, including individual survey blocks, were chosen to transect the predominant regional structural trends of the underlying rocks. The result of the surveys were published on 1:20,000 semi-controlled photo mosaic paper maps, showing total magnetic field contours onto which picked electromagnetic conductor anomalies were superimposed in symbol form. There are significant differences in quality of data acquisition, and original processing, of older and newer surveys. The surveys flown recently were designed and flown based on the state-of- the-art specifications, equipment and technology, while some of the older surveys, though conducted to the then industry prevailing standards, were poorly processed. In many cases, on older surveys, the local map coordinates were registered in map inches of uncontrolled to semi- controlled photo mosaics. The vast amount of digital data, collected by the survey contractors, was archived on 9-track tapes of different sizes and densities, in numerous incompatible data formats and file structures, many of which were difficult to access. For this reason the archival digital data largely remained inaccessible to the mining industry. To alleviate many of these problems, and to bring the archival data set of all thirty-two airborne magnetic and electromagnetic (AMEM) surveys to modern data storage, digital processing and Report on Geraldton-Tashota Area Geophysical Survey 3 Geophysical Data Set 1031/1032 - Revised interpretation standards, the present recompilation and reprocessing project was initiated under the Northern Ontario Development Agreement (NODA). This includes approximately 450 000 line-km of AMEM data, which was recompiled and reprocessed to correct any errors in the original data sets, to compute new derived products and to produce a revised electromagnetic anomaly database, using state-of-the-art geophysical data processing and imaging techniques. The major objectives of this project were to: 1) Provide a single, well-defined, common data format for all thirty-two AMEM survey data sets on CD-ROM, allowing easy access to the data on a PC platform. 2) Digitize survey data acquired in analog form, obtain missing or bad data from digital or paper archives, and check the validity of all data including units, conversion factors, etc. 3) Analyse and correct errors in the flight path to account for distortions in the photomosaics and provide a digital flight path referenced to Universal Transverse Mercator (UTM) coordinates. 4) Link and level all thirty-two surveys total magnetic field data to the Single Master Aeromagnetic Grid for Ontario, which was prepared from the Geological Survey of Canada’s magnetic database under an earlier project. 5) Reprocess the magnetic data to provide image quality total magnetic field grids and profiles for each survey. 6) Create image quality second vertical derivative grids of the total magnetic field. Report on Geraldton-Tashota Area Geophysical Survey 4 Geophysical Data Set 1031/1032 - Revised 7) Compute image quality apparent resistivities from a selected single frequency of the frequency-domain electromagnetic data, which have been fully corrected for signal to noise enhancement and accurately levelled. 8) Compute image quality decay constant and resistivity values from the corrected and levelled data channels of time-domain electromagnetic data (INPUT, GEOTEM I and GEOTEM II systems). Provide de-herringboned (i.e. correct for directional effect) decay constant and resistivity grids for all time-domain surveys. 9) Re-pick electromagnetic anomalies from the surveys in a consistent manner, and store anomaly parameters, including a unique identifier, in a new digital anomaly database. The resulting profile and grid data in digital form, along with the second vertical derivative of the total magnetic field, the apparent resistivity and decay constant values, and a comprehensive EM anomaly database, will become valuable tools for orebody detection, and enhanced lithological and structural mapping of the geology. The original profile data are also included in the profile database for reference. A full description of the digital databases are provided in Appendix B. In addition, MNDM has archived a number of interim processed channels, which can be made available to users on request. Report on Geraldton-Tashota Area Geophysical Survey 5 Geophysical Data Set 1031/1032 - Revised 2) FLIGHT PATH REPROCESSING 2.1) Flight Path Standards The goals of the flight path processing were to satisfy the three criteria below: 1) The database coordinates of picked fiducial points corresponding to identifiable topographic features on the photo mosaic should match the position of these features on published topographic maps within 100 metres on average. 2) The appearance of the final levelled total magnetic field and apparent resistivity data is consistent with the above standard of accuracy, i.e. continuity of linear features on adjacent flight lines to within 100 metres on average. 3) Speed checks run on the profile data do not show discontinuities or improbable values for the survey aircraft flight speed. 2.2) Flight Path Reprocessing Methodology The following steps were taken to ensure that the final flight path data is in correct UTM coordinates: 1) Determine, if necessary, a regional correction comprising scaling, rotation and translation based on position of flight path relative to identifiable topographic features on the uncontrolled photo-mosaics. 2) Apply a regional correction to the digital data. Report on Geraldton-Tashota Area Geophysical Survey 6 Geophysical Data Set 1031/1032 - Revised 3) Plot the digital flight path (with regional correction) at the 1:20,000 scale. 4) Pick visual control points from the published survey maps at identifiable topographic features. Transfer these pick points to a scale stable 1:20,000 scale topographic map (Ontario Base Map), or if not available, to a 1:50,000 scale topographic map (National Topographic System). 5) Identify the pick points on the digital flight path. Digitise the vectors joining the pick points on the digital flight path to the visual pick points on the topographic maps. These vectors represent the required corrections at each pick point. 6) Generate a polynomial surface through the corrections for each of the easting and northing components of the digitised vectors. 7) Correct the regionally accurate digital flight path for local distortions by adding the polynomial surfaces, which define the corrections, to the digital data. 8) Plot the final corrected flight path at 1:20,000 and confirm that all control points have been properly corrected. 9) Verify the flight path positioning by correlating the geophysical responses to any identifiable cultural features. The error of the final flight path position is within the specification of less than 100m. The primary source of error is the visual picking of the control points. The final corrected flight path is provided in the archive. Report on Geraldton-Tashota Area Geophysical Survey 7 Geophysical Data Set 1031/1032 - Revised