Preliminary Results from the Sable Island Geomagnetic ... · However, the low-frequency portion of the airborne data looks very much like the Sable Basestation total-field in the

Defence R&D Canada – Atlantic

DEFENCE DÉFENSE&

Preliminary Results from the Sable IslandGeomagnetic Coherence Experiment

J. Bradley Nelson

Technical Memorandum

DRDC Atlantic TM 2006-004

January 2006

Copy No.________

Defence Research andDevelopment Canada

Recherche et développementpour la défense Canada

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Preliminary Results from the Sable Island Geomagnetic Coherence Experiment

J. Bradley Nelson

Defence R&D Canada – Atlantic Technical Memorandum DRDC Atlantic TM 2006-004 January 2006

Abstract Previous experiments suggested that the geomagnetic field is coherent over distances of hundreds of kilometres at high-altitude over the ocean, but not at low-altitude where most magnetic anomaly detection (MAD) flights are conducted. The reason for this is not understood but computer simulations suggest that excess magnetic noise due to ocean dynamics or variations in the seafloor conductivity along the flight path could cause the effect. Detailed models are required to predict the magnitude of the effect in specific areas. Flights were conducted near Sable Island in August 2005 to gather a data set on which detailed computer simulations can be performed. The flights were conducted in areas where bathymetric and conductivity data exists, the magnetic properties of the seafloor are well known, a basestation could be set up relatively close by, and ocean dynamic processes can be modelled. This report describes the processing steps required to obtain the data set, and the preliminary results which will be compared to NRL Stennis computer simulations of ocean dynamic noise and conductivity anomaly effects.

Résumé Des expériences antérieures ont suggéré que le champ géomagnétique est cohérent sur des distances de centaines de kilomètres à haute altitude au-dessus de l’océan, mais non aux faibles altitudes auxquelles s’effectuent la plupart des vols de détection des anomalies magnétiques (MAD). Les causes du phénomène ne sont pas comprises, mais des simulations sur ordinateur suggèrent qu’il pourrait être engendré par un bruit magnétique en excès attribuable à la dynamique des océans ou des variations de la conductivité du fond marin le long des lignes de vol. Des modèles détaillés sont nécessaires pour prévoir l’ordre de grandeur de cet effet dans des régions spécifiques. Des vols ont été effectués près de l’île de Sable en août 2005 afin de recueillir un jeu de données avec lequel mener des simulations par ordinateur détaillées. Les vols ont été exécutés dans des régions où une station de base pouvait être installée relativement proche, pour lesquelles on dispose de données sur la bathymétrie et la conductivité, dont on connaît bien les propriétés magnétiques du fond marin et pour lesquelles on pouvait modéliser les processus dynamiques de l’océan. Dans le présent rapport on décrit les étapes du traitement nécessaire pour l’obtention du jeu de données et les résultats préliminaires qui seront comparés aux simulations par ordinateur du bruit engendré par la dynamique de l’océan et des effets des anomalies de conductivité menées au centre Stennis du NRL.

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Executive summary Background: Next-generation MAD systems are will employ geomagnetic noise reduction using reference sensors. Previous experiments suggested that the geomagnetic field is coherent over hundreds of kilometres at high-altitude over the ocean, but not at low-altitude where most MAD flights are conducted. The reason for this is not understood but computer simulations suggest that magnetic noise due to ocean dynamics or variations in the seafloor conductivity along the flight path could cause the effect. Detailed models are required to predict the magnitude of the effect in specific areas. Flights were conducted near Sable Island in August 2005 to gather a data set on which detailed computer simulations can be performed. The flights were conducted in areas where bathymetric and conductivity data exists, the magnetic properties of the seafloor are well known, a basestation could be set up relatively close by, and ocean dynamic processes can be modelled. Results: The lack of coherence between magnetic signals measured at a basestation and in a low-altitude aircraft was not immediately apparent in this experiment. However, the geomagnetic signals were not very large for the low-altitude portions of this experiment so it is difficult to say conclusively whether the poor coherence was caused by excess noise or conductive effects. However, the low-frequency portion of the airborne data looks very much like the Sable Basestation total-field in the cases where a significant geomagnetic signal was present. When the geomagnetic signals were large, the coherence between the Sable Island basestation and the airborne measurements was often >0.9, no matter what the altitude was. When the geomagnetic signals were small, and the coherence was smaller, it was probably due to excess noise from imperfect geological noise cancellation or ocean dynamics because conductive effects would not yield better coherence when the geomagnetic signals were larger. There may be some small phase and amplitude mismatches between the basestation and airborne data collected over the Gully. These could be due to imperfect geological noise removal, ocean dynamics, or, although less likely for the reasons described above, conductive effects. Forward modelling by NRL should determine if the latter two effects are of the correct magnitude to account for these results. Significance: When the geomagnetic noise and coherence are large, some of the geomagnetic noise can be cancelled. The remaining noise level is ~ 50-100 pT/√Hz at 0.05 Hz. In cases where the geomagnetic noise is smaller than the other magnetic noise, there is little cancellation. Regardless of the source of the excess noise, if it cannot be modelled and removed, then it is unlikely that either magnetic basestation or using a high-altitude magnetometer-equipped aircraft as a geomagnetic reference station will result in significant increases in MAD detection ranges. Future Work: Conductivity and ocean dynamics effects will be modelled by NRL Stennis and compared to the results found here. Future experiments will address the issues of imperfect geological noise reduction and additional noise due to ocean dynamics.

Nelson, JB. 2006. Preliminary Results from the Sable Island Geomagnetic Coherence Experiment. DRDC Atlantic TM 2006-004. Defence R&D Canada – Atlantic.

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Sommaire Contexte Les systèmes de détection des anomalies magnétiques (MAD) de la prochaine génération devraient comporter un type ou un autre de capteur de référence pour la réduction du bruit géomagnétique. Des expériences antérieures ont montré que le champ géomagnétique est cohérent sur des distances de centaines de kilomètres à haute altitude au-dessus de l’océan, mais non aux faibles altitudes auxquelles s’effectuent la plupart des vols de détection des anomalies magnétiques (MAD). Les causes du phénomène ne sont pas comprises, mais des simulations sur ordinateur suggèrent qu’il pourrait être engendré par un bruit magnétique en excès attribuable à la dynamique des océans ou des variations de la conductivité du fond marin le long des lignes de vol. Des modèles détaillés sont nécessaires pour prévoir l’ordre de grandeur de cet effet dans des régions spécifiques. Des vols ont été effectués près de l’île de Sable en août 2005 afin de recueillir un jeu de données avec lequel mener des simulations par ordinateur détaillées. Les vols ont été exécutés dans des régions où une station de base pouvait être installée relativement proche, pour lesquelles on dispose de données sur la bathymétrie et la conductivité, dont on connaît bien les propriétés magnétiques du fond marin et pour lesquelles on pouvait modéliser les processus dynamiques de l’océan. Résultats L’absence de cohérence entre les signaux magnétiques mesurées à l’emplacement d’une station de base et à bord d’un aéronef volant à basse altitude n’était pas immédiatement apparente lors de cette expérience et il est ainsi difficile de dire sans l’ombre d’un doute que la mauvaise cohérence était attribuable à un bruit en excès plutôt qu’aux effets de conductivité. Cependant, dans les basses fréquences, les données aériennes ressemblent beaucoup à celles sur le champ total mesuré à la station de base de l’île de Sable où est mesuré un important signal géomagnétique. Lorsque l’intensité des signaux géomagnétiques était grande, la cohérence entre les mesures effectuées par la station de base à l’île de Sable et celles effectuées à bord de l’aéronef était souvent > 0,9, quelle que soit l’altitude à laquelle les données étaient acquises. Lorsque l’intensité des signaux géomagnétiques était faible, et que la cohérence était moindre, cela était probablement attribuable à un bruit en excès résultant d’une annulation imparfaite du bruit géologique ou du bruit causé parla dynamique de l’océan parce que les effets de conductivité n’engendreraient pas une meilleure cohérence pour des signaux géomagnétiques de plus grande intensité. Il peut exister de faibles écarts de correspondance pour la phase et l’amplitude entre les données de la station de base et les données aériennes recueillies au-dessus du Goulet. Ils pourraient être attribuables à une suppression imparfaite du bruit géologique et du bruit engendré par la dynamique de l’océan ou, ce qui est moins probable pour les raisons exposées ci-haut, aux effets de conductivité. Une modélisation prédictive par le NRL devrait permettre de déterminer si ces deux derniers effets sont du bon ordre de grandeur pour expliquer ces résultats. Importance Quelle que soit la source du bruit en excès, s’il ne peut être modélisé et supprimé il est peu vraisemblable que l’utilisation des données d’une station magnétique de base ou d’un aéronef volant à haute altitude avec un magnétomètre permettront une réduction adéquate du bruit géomagnétique.

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Travaux futurs Les effets de conductivité et de la dynamique des océans seront modélisés par le centre Stennis du NRL et comparés aux résultats ici exposés.

Nelson, JB. 2006. Résultats préliminaires de l’expérience sur la cohérence du champ géomagnétique à l’île de Sable. RDDC Atlantique TM 2006-004. R et D pour la défense Canada – Atlantique.

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Table of contents Abstract………………………………………………………………………………………......... i Executive summary……………………………………………………………………...……...... iii Sommaire………………………………………………….…………………………….……...... iv Table of contents…………………………………………………………………...………..…..... vi List of figures………………………………………………………………...……………..…..... vii 1. Introduction…………………………………………………………………………..………... 1 2. The Experiment…………………....................................................................…...................... 2 2.1 Magnetic basestations………………………………………………………………….. 2 2.2 Flights near Sable Island……………………………………………………………….. 3 2.3 Flights near the Gully…………………………………………………………………... 5 3. Data Pre-Processing……........................................................................................................... 6 4. Basestation Analysis…………………………………………………………………............... 7 4.1 Sable Island magnetic field components…………..………………………………….... 7 4.2 Correlation between Sable Island and Greenwood basestations…………..………….. 10 5. Separating Geological and Geomagnetic Noise..........………………………...……………... 14 5.1 Read data, define desired track, correct for offset from desired track………………... 14 5.2 Basestation correction……………………………………………………………….... 16 5.3 Combining several lines……………………………………………………………..... 17 5.4 Re-sampling and reverse-gradient correction back to the original sampling position... 18 5.5 High-pass filter the original TF and geological noise estimate……………………..... 18 5.6 Re-compensate the filtered residual…………………………………………………... 19 5.7 Plotting the results………………………………………………….…………………. 19 6. Results…………………………………………………………………………………….….. 21 6.1 Sable Island North/South lines………………………………………………………... 21 6.2 Sable Island East/West lines………………………………………………………….. 31 6.3 Gully North/South lines……………………………………………………………..... 39 6.4 Gully East/West lines………………………………………………………………..... 46 7. Conclusions…………………………………………………………………………………... 53 8. Future Work.....................……………………..........………………………...…………….... 55 9. References........................……………………..........………………………...…………….... 56 10. Distribution list……………………………………………………………………………..... 57

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List of figures Figure 1. Locations for the experiment…………………….………………………………………………………….2 Figure 2. Flight lines near Sable Island………………..……………………………………………………………..4 Figure 3. Sable Island photographed from the Convair 580 while flying on one of the North/South flight lines……………………………………………………………………………………..…………..…4 Figure 4. Actual lines flown near the Gully at altitudes of 1000, 2000, 5000, and 10,000 ft (~300,

600, 1500, and 3000 m).…………………………………………………………………………..………..5 Figure 5. Flow chart describing the data pre-processing steps.……………………………………….………….6 Figure 6. Geomagnetic field components (Black = North, Blue = West, Red = Up) recorded at Sable

Island on August 6, 2005……………………………………………………………………………..........7 Figure 7. PSD of geomagnetic field components (Black = North, Blue = West, Red = Up) recorded

at Sable Island on August 6………………………………………………………………………………..8 Figure 8. TFS (Black), TFvector (Blue), and TFvector’ (Red) measured at Sable Island on August 6,

2005. DC values have been removed and TFS is offset by 2 nT for display purposes.……............9 Figure 9. PSD of TFS (Black), TFvector (Blue), TFS-TFvector (Green, just barely visible beneath the Red

trace), and TFS-TFvector’ (Red) of the time series data shown in Figure 8…..……………...……….10 Figure 10. Total-field measured at Greenwood (Black) and Sable Island (Blue) on August 10.

The DC values have been removed for display purposes.………………………………………..…11 Figure 11. PSD’s of time series data shown in Figure 10: Sable Island (Blue) and Greenwood (Black).………………………………………………………………………………………………........12 Figure 12. Coherence of the Sable Island and Greenwood basestation TF data on August 10.

Data were high-pass filtered with a 2nd-order high-pass Butterworth digital filter with a 3-dB point at 0.004 Hz ……………………………………………………………………………....13

Figure 13. Flow chart of geological noise modelling and removal. Detailed description of various

processes are given in the Sections shown in red.………………………………..………………...15 Figure 14a. Comparison of the raw total-field (TF) and the gradient- and basestation-corrected

total-field (TF”) along the three North/South lines near Sable Island at each altitude (Black vs, Blue)………………………………………………………………………………………...21

Figure 14b. Horizontal gradient correction applied to each North/South line at 500’ near Sable

Island, based on IGRF gradients. Blue=ΔNorth x GNorth; Red=ΔEast x GEast…………………..22 Figure 14c. Horizontal gradient correction applied to each North/South line at 1000’ near Sable

Island, based on IGRF gradients. Blue=ΔNorth x GNorth; Red=ΔEast x GEast………………..…22 Figure 14d. Horizontal gradient correction applied to each North/South line at 2000’ near Sable

Island, based on IGRF gradients. Blue=ΔNorth x GNorth; Red=ΔEast x GEast…………………..22 Figure 14e. Horizontal gradient correction applied to each North/South line at 5000’ near Sable

Island, based on IGRF gradients. Blue=ΔNorth x GNorth; Red=ΔEast x GEast…………………..22 Figure 14f. Comparison of geology estimates along the three North/South lines near Sable Island

at each altitude (Black, Blue, Green) vs the average (Red). TFgeo is set to the average………23 Figure 14g. Effect of extra compensation along the 500’ North/South lines near Sable Island:

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ResidHP (Black) vs ResidHPC (Blue)…………………………………………………………….…24 Figure 14h. Effect of extra compensation along the 1000’ North/South lines near Sable Island:

ResidHP (Black) vs ResidHPC (Blue)………………………………………………………………..24 Figure 14i. Effect of extra compensation along the 2000’ North/South lines near Sable Island:

ResidHP (Black) vs ResidHPC (Blue)………………………………………………………………..24 Figure 14j. Effect of extra compensation along the 5000’ North/South lines near Sable Island:

ResidHP (Black) vs ResidHPC (Blue)………………………………………………………………..24 Figure 14k. Upper Trace: Comparison of signals measured along the three North/South lines near Sable Island at 500’: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper trace). Middle Trace: Coherence between ResidHPC and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black), and DIF2 (red)……26 Figure 14l. Upper Trace: Comparison of signals measured along the three North/South lines near Sable Island at 1000’: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper trace). Middle Trace: Coherence between ResidHPC and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black), and DIF2 (red)……27 Figure 14m. Upper Trace: Comparison of signals measured along the three North/South lines near Sable Island at 2000’: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper trace). Middle Trace: Coherence between ResidHPC and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black), and DIF2 (red)……28 Figure 14n. Upper Trace: Comparison of signals measured along the three North/South lines near Sable Island at 5000’: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper trace). Middle Trace: Coherence between ResidHPC and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black), and DIF2 (red)……29 Figure 14o. DIF1 vs Latitude for North/South lines near Sable Island………………………………………….30 Figure 15a. Comparison of the raw total-field (TF) and the gradient- and basestation-corrected

total-field (TF”) along the three East/West lines near Sable Island at each altitude (Black vs, Blue)…………………………………………………………………………………………31

Figure 15b. Comparison of geology estimates along the three East/West lines near Sable Island at each altitude (Black, Blue, Green) vs the average (Red). TFgeo is set to the average…………32

Figure 15c. Upper Trace: Comparison of signals measured along the three East/West lines near Sable Island at 500’: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper trace). Middle Trace: Coherence between ResidHPC and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black), and DIF2 (red)……34 Figure 15d. Upper Trace: Comparison of signals measured along the three East/West lines near Sable Island at 1000’: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper trace). Middle Trace: Coherence between ResidHPC and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black), and DIF2 (red)……35

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Figure 15e. Upper Trace: Comparison of signals measured along the three East/West lines near Sable Island at 2000’: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper trace). Middle Trace: Coherence between ResidHPC and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black), and DIF2 (red)……36 Figure 15f. Upper Trace: Comparison of signals measured along the three East/West lines near Sable Island at 5000’: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper trace). Middle Trace: Coherence between ResidHPC and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black), and DIF2 (red).…...37 Figure 15g. DIF1 vs Longitude for East/West lines near Sable Island………………………………………….38 Figure 16a. Comparison of the raw total-field (TF) and the gradient- and basestation-corrected

total-field (TF”) along the three North/South lines near the Gully at each altitude (Black vs, Blue)………………………………………………………………………………………………….39

Figure 16b. Comparison of geology estimates along the three North/South lines near the Gully at each altitude (Black, Blue, Green) vs the average (Red). TFgeo is set to the average………………..40

Figure 16c. Upper Trace: Comparison of signals measured along the three North/South lines near the Gully at 1000’: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper trace). Middle Trace: Coherence between ResidHPC and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black), and DIF2 (red)……41 Figure 16d. Upper Trace: Comparison of signals measured along the three North/South lines near the Gully at 2000’: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper trace). Middle Trace: Coherence between ResidHPC and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black), and DIF2 (red)……42

Figure 16e. Upper Trace: Comparison of signals measured along the three North/South lines near the Gully at 5000’: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper trace). Middle Trace: Coherence between ResidHPC and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black), and DIF2 (red)……43 Figure 16f. Upper Trace: Comparison of signals measured along the three North/South lines near the Gully at 10,000’: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper trace). Middle Trace: Coherence between ResidHPC and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black), and DIF2 (red)……44 Figure 16g. DIF1 vs Latitude for North/South lines near the Gully……………………………………………...45 Figure 17a. Comparison of the raw total-field (TF) and the gradient- and basestation-corrected

total-field (TF”) along the three East/West lines near the Gully at each altitude (Black vs, Blue)………………………………………………………………………………………………….46

Figure 17b. Comparison of geology estimates along the three East/West lines near the Gully at each altitude (Black, Blue, Green) vs the average (Red). TFgeo is set to the average………………..47

Figure 17c. Upper Trace: Comparison of signals measured along the three East/West lines near the Gully at 1000’: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace);

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ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper trace). Middle Trace: Coherence between ResidHPC and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black), and DIF2 (red)……48 Figure 17d. Upper Trace: Comparison of signals measured along the three East/West lines near the Gully at 2000’: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper trace). Middle Trace: Coherence between ResidHPC and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black), and DIF2 (red)……49 Figure 17e. Upper Trace: Comparison of signals measured along the three East/West lines near the Gully at 5000’: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper trace). Middle Trace: Coherence between ResidHPC and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black), and DIF2 (red)……50 Figure 17f. Upper Trace: Comparison of signals measured along the three East/West lines near the Gully at 10,000’: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper trace). Middle Trace: Coherence between ResidHPC and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black), and DIF2 (red).…...51 Figure 17g. DIF1 vs Latitude for East/West lines near the Gully………………………………………………...52

List of tables Table 1. Basestation Information……………………………………………………………………………………...3

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1. Introduction Previous experiments (Refs 1, 2) have suggested that the geomagnetic field is coherent over distances of hundreds of kilometres at high-altitude over the ocean, but not at low-altitude where most magnetic anomaly detection (MAD) flights are conducted. The reason for this loss of coherence is not understood, but two possibilities have been suggested:

1) excess magnetic noise due to ocean dynamics, or 2) variations in the seafloor conductivity along the flight path will affect the

local phase and amplitude of the geomagnetic variations. Computer simulations performed by Will Avera at NRL-Stennis (Ref 3) have suggested that both of these possibilities could be the source of the loss of coherence, although detailed models are required to predict the magnitude of the effect in specific areas. A series of flight tests were conducted near Sable Island in August 2005 to gather a data set on which detailed computer simulations could be performed. The flights were conducted in areas where bathymetric and conductivity data exists, the magnetic properties of the seafloor are well known, a basestation could be set up relatively close-by, and ocean dynamic processes can be modelled. This report details the experimental setup, processing steps, and preliminary results which NRL Stennis computer simulations will be compared to.

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2. The Experiment The flights were conducted near Sable Island and the Gully, as shown in Figure 1. Two magnetic ground stations were set up – one on Sable Island and one at CFB Greenwood on mainland Nova Scotia (separation ~ 400 km). A GPS basestation was set up in Dartmouth Nova Scotia to provide differential GPS positioning for the aircraft. The NRC Convair 580 research aircraft was used to gather the flight data. The Convair’s aeromagnetic instrumentation has been described elsewhere (Ref 4) so it will not be repeated here. Sections 2.1-2.3 describe the basestation instrumentation and the series of flights conducted.

CFB Greenwood

Figure 1. Locations for the experiment.

2.1 Magnetic basestations The magnetic basestation at CFB Greenwood consisted of a Geometrics G822 Caesium-vapour total-field magnetometer and associated electronics, a GT200 time interval analyser board to convert the Larmor signal from the magnetometer into a measurement of the magnetic field, a GPS receiver to tag the raw measurements with UTC time, and a desktop computer running Labview data acquisition software. Unfortunately the CFB Greenwood basestation stopped collecting data after only 3 days, but this was enough data to base conclusions regarding the coherence of the geomagnetic field between the two basestations. The magnetic basestation at Sable Island consisted of a Geometrics G822 Caesium-vapour total-field magnetometer and associated electronics, a GT658 time interval analyser board to convert the Larmor signal from the magnetometer into a measurement of the magnetic field, a Billingsley DFM100GX vector magnetometer and NRC-built 5 Hz low-pass anti-alias filter, a


National Instruments PCI-4472 24-bit A/D board, a GPS receiver to tag the raw measurements with UTC time, and a desktop computer running Labview data acquisition software. Unfortunately the anti-alias filter malfunctioned after 2 days and only the total-field and the northward component of the geomagnetic field were recorded during the flights. However, there were 2 days of data where all three components of the geomagnetic field were recorded and a correlation analysis was performed on these data. Table 1 gives a detailed description of the sensors and conditions at each basestation site.

Table 1. Basestation Information.

PARAMETER CFB GREENWOOD (Gr) SABLE ISLAND (S) Location 44°57.8’ N; 64°55.2’ W 43°55.9’ N; 60°00.4’ W

Magnetic Sensors Geometrics G822 (TF) Geometrics G822 (TF), Billingsley DFM100GX (3

components, 24 bit) Sample Rate 8 8

Time-tags UTC from GPS UTC from GPS Distance from Man-Made Noise

Sources 500 m, but located at airbase ~ 70 m from buildings

Soil 1 m above non-magnetic sediments

0.5 m above sandy non-magnetic soil

Distance from Ocean 20 km 500 m

2.2 Flights near Sable Island One flight consisted of a series of North/South lines flown off the western tip of Sable Island. All lines went from 43° 42’N to 44° 09’N along 60° 09’W and there were 3 lines at each of 500, 1000, 2000, and 5000 feet ASL (approximately 150, 300, 600, and 1500 m). Each of these lines was approximately 50 km in length the separation between the aircraft and the Sable Island basestation varied from ~ 16 to 31 km. The water depth was only 25-100 m along these flight lines near Sable Island. The second flight consisted of a series of East/West lines flown off the southern edge of Sable Island. All lines went from 59° 40’W to 60° 18’W along 43° 55.25’N, and there were 3 lines at each of 500, 1000, 2000, and 5000 feet ASL (approximately 150, 300, 600, and 1500 m). Each of these lines was approximately 50 km in length the separation between the aircraft and the Sable Island basestation varied from ~ 1 to 25 km. In both cases the flight lines were chosen to avoid the infrastructure associated with the gas fields and drilling platforms that are situated near Sable Island. Appendix A gives information on the platform locations and the pipelines that run between them and the Nova Scotia mainland. Figure 2 shows the actual flight lines flown superimposed on a map of Sable Island. Figure 3 shows a photograph of Sable Island taken from the NRC Convair during the experiment.


Magnetic Basestation

Figure 2. Flight lines near Sable Island.

Magnetic Basestation

East

North

Figure 3. Sable Island photographed from the Convair 580 while flying on one of the North/South flight lines.


2.3 Flights near the Gully One flight consisted of a series of roughly North/South lines flown down the axis of the Gully. All lines went from 43° 41.1’N; 58° 51.6’W to 44° 07.5’N; 59° 07.1’W and there were 3 lines at each of 1000, 2000, 5000 and 10,000 feet ASL (approximately 300, 600, 1500, and 3000 m). Each of these 12 lines was approximately 50 km in length the separation between the aircraft and the Sable Island basestation roughly was 80 km for all the lines. The water depth varies from 100-1500 m along the flight lines near the Gully. The second flight consisted of a series of roughly East/West lines flown across the axis of the Gully. All lines went from 44° 0.8’N; 58° 42.4’W to 43° 50.6’N; 59° 9.6’W, and there were 3 lines at each of 1000, 2000, 5000, and 10000 feet ASL (approximately 300, 600, 1500 and 3000 m). Each of these 12 lines was approximately 50 km in length the separation between the aircraft and the Sable Island basestation varied from ~ 68 to 104 km. Figure 4 shows the actual flight lines flown near the Gully.

Figure 4. Actual lines flown near the Gully at altitudes of 1000,

2000, 5000, and 10,000 ft (~300, 600, 1500, and 3000 m).


3. Data Pre-Processing The details of the DRDC Atlantic aircraft noise removal process has been described elsewhere and will not be repeated here. Because the basestation data were sampled at a different rate (8 Hz vs 32 Hz for the aircraft), they had to be re-sampled up to 32 Hz prior to any filtering to ensure that there were no phase delays introduced between the airborne and basestation data sets. Figure 5 shows the flow chart for the data pre-processing. The aeromagnetic flights were performed in conjunction with DRDC Val Cartier hyperspectral infra-red experiments. Analysis of previous flight data had indicated that there was no magnetic effect from running that equipment. However, DRDC Val Cartier installed a cryo-cooler on the system prior to these flight trials and the cryo-cooler introduced very small DC steps into the magnetic data. These DC steps were < 0.1 nT in height, lasted for only a fraction of a second, and occurred roughly every 5 seconds. These DC steps had to be removed manually as a reliable automated algorithm could not be found. In addition to the cryo-cooler steps, DC steps were also introduced by VHF radio transmissions. These were removed manually. The East-West flight lines near Sable Island went directly over 4 wrecks that produced significant signals that caused problems for the next stage of processing. These signatures were modelled by magnetic dipoles and, using the aircraft track data, the best-fit dipole signatures were removed from the profile data. Although there was some residual signature from the wrecks, it was much smaller in amplitude than the original wreck signatures and it had little effect on the subsequent processing.

Greenwood Base

TFGr 8 Hz

Sable Base TFS, North, 8 Hz

Aircraft Raw DataGPS 4 Hz

Others 32 Hz

GPS Base Raw Data4 Hz

Inspect & Correct

Inspect & Correct

Inspect & Correct

Inspect & Correct

DGPS Processing, Interpolate to 32 Hz,

Low-Pass filter at 1 Hz, sub-sample

to 4 Hz

Standard Compensation,Adaptive Compensation,Low-Pass filter at 1 Hz

Sub-sample to 4 Hz

Interpolate to 32 Hz, Low-Pass filter at 1 Hz, sub-sample

To 4 Hz

Time-align, write a single output file for each flight, 4 Hz

Interpolate to 32 Hz, Low-Pass filter at 1 Hz, sub-sample

To 4 Hz

Figure 5. Flow chart describing the data pre-processing steps.


4. Basestation Analysis

4.1 Sable Island magnetic field components Figure 6 shows the time series of the three components of the magnetic field (North, West, Up) as recorded at the Sable Island basestation site on August 6, and Figure 7 shows the corresponding power spectral density (PSD) plots. The DC value has been removed for display purposes. It is clear that the vertical component has much less high-frequency activity than either the North or the West component. This suggests that Sable Island is a very good electrical conductor, in agreement with Ref 5 which indicates that the conductivity of Sable Island is about the same conductivity as the surrounding seawater. There is essentially no power in the frequency band of interest (0.01-0.5 Hz) in the vertical component of the geomagnetic field, so only the North and West components are relevant to this analysis.

Figure 6. Geomagnetic field components (Black = North, Blue = West, Red = Up) recorded at Sable Island on August 6, 2005.


Figure 7. PSD of geomagnetic field components (Black = North, Blue = West, Red = Up) recorded at Sable Island on August 6,

2005.

We can also compare the total-field measurements made with the Caesium magnetometer to the vector magnetometer measurements to determine which vector component contributes the most to the variations in the total-field. The vector total-field (TFvector) is given by: TFvector = √(North2 + West2 + Up2) . (1) Alternatively, we can estimate the total-field from just North, Up, and mean of West. Let us denote this TFvector’. TFvector’ = √(North2 + 2 + Up2) . (2) Figure 8 shows the time series of the total-field as measured by the Caesium magnetometer (TFS), TFvector, and TFvector’. The DC values have been removed and TFS is offset by 2 nT for display purposes. Figure 9 shows the PSD of (TFS- TFvector) and (TFS- TFvector’). Clearly the total-field constructed from the measured components is much noisier than the total-field measured with the Caesium magnetometer. This may be due to residual wind or vibration of the vector sensors which were mounted on a light wooden platform. However, sensor testing at DRDC Atlantic suggests (Ref 6) that the 1/f noise for the National Instruments A/D board starts near 6 Hz and it is possible that some of the noise may be due this as well.


Figure 8. TFS (Black), TFvector (Blue), and TFvector’ (Red)

measured at Sable Island on August 6, 2005. DC values have been removed and TFS is offset by 2 nT for display purposes.

Figures 8 and 9 indicate that the geomagnetic activity seen in the West component actually contributes very little to the geomagnetic activity measured by a Caesium total-field magnetometer at Sable Island. Since it was previously shown that the Up component has no geomagnetic activity in the frequency band of interest, this implies that only the North component of the geomagnetic field variation contributes significantly to the total-field variations seen at Sable Island. Thus it is fortuitous that the North component of the geomagnetic field was recorded at Sable Island for the entire experiment, but the Up and West components were not. It also suggests that there will be very little difference between using just the total-field sensor, or both the total-field and North component sensors, for geological noise reduction.


Figure 9. PSD of TFS (Black), TFvector (Blue), TFS-TFvector

(Green, just barely visible beneath the Red trace), and TFS-TFvector’ (Red) of the time series data shown in Figure 8.

4.2 Correlation between Sable Island and Greenwood basestations Figure 10 shows the time series of the total-fields recorded at the Sable Island and Greenwood (TFGr) basestations on August 10, and Figure 11 shows the corresponding power spectral densities. Clearly most of the low-frequency geomagnetic activity is seen by both basestations and track each other quite closely. The very-low frequency (periods of several hours) show considerable differences at the two basestations. Closer examination shows that there are considerable time lags between the two basestations for some of the low-frequency geomagnetic signals. These lags can be 20-70 seconds and are frequency-dependent. This suggests that they may not be propagating directly from the ionosphere through the air. It is possible that some of these signals are travelling through the seawater or seafloor where the propagation speeds are much slower. Although both basestations see activity in the 0.01-0.02 Hz region, usually denoted “PC-4 pulsations”, the amplitude of that geomagnetic signal is significantly greater at Sable Island. If Sable Island is a good electrical conductor, then the vertical component of the geomagnetic field will be reduced (as shown in Figures 6 and 7) and the horizontal components will be amplified. Depending on the dip angle of the Earth’s field, this can lead to an increase in total-field anomaly. Ref 1 also found that the amplitude of the geomagnetic signals at the Greenwood and Keji basestations were smaller than at basestations near the ocean, so this result is not unexpected.


There is also evidence of 0.03-0.06 Hz pulsations in the Sable Island basestation data which are just barely discernible in the Greenwood data. Figure 12 shows the frequency-domain coherence between the Greenwood and Sable Island total-field measurements on August 10. One hundred and one averages were used in calculating the coherence. There is excellent coherence of the PC-4 pulsations (~0.85) and somewhat poorer coherence for the pulsations in the 0.03-0.06 Hz band. It should be noted immediately that the coherence of the geomagnetic field measured between the Greenwood or Keji basestation, and low-flying magnetometer-equipped aircraft at similar separations, has been much lower in the previous trials (Refs 1,2) than is seen in Figure 12. However, the important distinction here is that the Sable Island basestation is stationary, and the hypothesis for this experiment is that either the aircraft was flying through local conductivity anomalies, or through magnetic fields generated by ocean dynamics. As long as the conductivity of Sable Island is not changing on the timescale of 10-100 seconds (which is clearly unlikely), and any magnetic noise caused by ocean dynamics is smaller than the geomagnetic signals (which is probably true considering the basestation is ~ 500 m from the ocean), or on time scales greater than 100 seconds (which is probably true for large-scale processes), then one should expect a high degree of correlation between the two basestations in this experiment.

Figure 10. Total-field measured at Greenwood (Black) and Sable Island (Blue) on August 10. The DC values have been

removed for display purposes.


Figure 11. PSD’s of time series data shown in Figure 10: Sable Island (Blue) and

Greenwood (Black).


Figure 12. Coherence of the Sable Island and Greenwood

basestation TF data on August 10. Data were high-pass filtered with a 2nd-order high-pass Butterworth digital filter with a 3-dB

point at 0.004 Hz.


5. Separating Geological and Geomagnetic Noise Geological noise is a spatial noise source whereas geomagnetic noise is a temporal noise source. However, because the aircraft is moving, these two effects are mixed. The technique used here for separating these noise sources involves:

1) average the measurements along flight lines at nearby altitudes to obtain an estimate of the geology,

2) subtract this geology estimate from the original airborne measurements to yield a residual,

3) compare this residual to the geomagnetic total-field measured at a basestation. Figure 13 shows a flowchart of the processing involved in this processing. Sections 5.1-5.7 deal with each the processes shown in red in Figure 13.

5.1 Read data, define desired track, correct for offset from desired track Appendix B contains the analysis code for the August 12 flight where the North/South lines near Sable Island were flown. Similar IDL procedures were written for each flight described in Sections 2.2-2.3. A 4-Hz data file obtained from the pre-processing described in Figure 5 was read into IDL and the relevant variables were extracted. Although we intended to fly from the starting waypoint directly to the ending waypoint, the aircraft can never be flown exactly on a straight line between these two points. Thus the first step in the analysis was to define the desired track in 3-dimensional space, and the vector (ΔX, ΔY, ΔZ) from the actual track to the desired track. It is important to have the same number of data points along the desired track as there were in the original flight data. In order to estimate what the total field would be at each point along the desired track (TF’), the following simple equation is used:

TF’i = TFi + (ΔXi, ΔYi, ΔZi)·(Gxi, Gyi, Gzi) (3) where i denotes the ith point along the flight line, TFi is the measured total-field at that point, and (Gxi, Gyi, Gzi) is the 3-dimensional total-field gradient vector at each point along the flight line. For simplicity, the total-field gradient vector will be referred to simply as the gradient vector. There are several methods for estimating the gradient vector. The simplest method is to use the International Geophysical Reference Field (IGRF) model (Ref 7). The IGRF gradients vary little over the distances flown in this experiment so constant values of Gx = 0.0038 nT/m (North) Gy = - 0.0014 nT/m (East) Gz = 0.026 nT/m (Down) . (4) This technique makes sense if the geological gradients are small in comparison to the IGRF gradients.


Define “Desired track” and deviation Δx,Δy,Δz

from that track

Read 4 Hz file for each flight

Correct TF with gradients × (Δx,Δy,Δz)

yielding TF’ along desired track

Subtract Basestation TF”=TF’-Base

IGRF Gradients?

Sable TF, filtering?

Geocode & average several lines to

obtain estimate of geology TFgeo

Just 3 lines at each altitude?

5.1

5.2

5.3

5.1

5.1

5.6

Subtract TFest from TF & avoid discontinuities

= Resid

0.02 Hz High-Pass filter= ResidHP

(TFHP & TFestHP

Re-compensate ResidHP along

individual lines = ResidHPC &TFC

IGRF Gradients? Measured Gx, Gy & Estimated Gz?

18 vector mag terms Accelerometers, INS, control surfaces, etc

Plot Results Dif=ResidHPC-Sable TF

Dif vs Position

5.4

5.5

5.6

5.4

5.7

Re-sample geology along desired

track with correct # dps

Correct re-sampled geology with

gradients × (Δx,Δy,Δz)yielding TFest

along actual track

Measured Gx, Gy & Estimated Gz?

Greenwood TF, filtering?

Upward/downward continue nearby lines?

5.4

Use coherent part of geology estimate?

Figure 13. Flow chart of geological noise modelling and removal. Detailed description of various processes are given in the Sections shown in red.


The second technique for estimating the gradients is to use measured values for Galong and Gacross for the horizontal gradients. The only difficulty with this is that the actual DC values that come from the Convair’s lateral gradient measurements are highly dependent on the aircraft compensation. Small heading-dependent shifts can sometimes remain unless a very careful analysis of cross-over errors is done for proper surveys. Since these lines were only flown in two directions (either North/South or East/West on a given flight), there may be some heading error in the gradient measurements. If the geological gradients are much larger than any residual heading error (in other words there is a large geological gradient), then this technique should work much better that simply using the IGRF gradient values. However, for the areas near Sable Island and the Gully, the geological gradients should be small compared to the IGRF. The technique was tried, but it was determined that simply using the IGRF horizontal gradients yielded the best results. The Convair was not equipped with a tail magnetometer for these experiments so there was no measurement of the vertical gradient. However, a common technique for estimating the vertical gradient from measured total-field data along a profile is to assume that the sources are two-dimensional and oriented perpendicular to the flight line. Under this assumption, the vertical gradient is simply the Hilbert Transform of Galong. (The Hilbert Transform simply phase shifts a time series by 90 degrees.) While this assumption is rarely valid, the vertical gradient estimates that result from the technique are usually acceptable. This technique was tried, but it was determined that simply using the IGRF vertical gradient yielded the best results. A final technique for estimating both the horizontal and vertical gradients is to use existing aeromagnetic maps of the area. This technique hasn’t been tried as yet, but it may be included in follow-on work on the project if high-quality aeromagnetic maps of the area can be obtained. The gradient-corrected TF is denoted TF’.

5.2 Basestation correction If there are enough flight lines that are being averaged, then one can assume that the geomagnetic noise measured along the flight lines will average out to zero. In this experiment however, there were only three flight lines at each altitude. The following methods were tried for removing the geomagnetic noise prior to building up a geological noise model:

1) no basestation removal at all, 2) subtract the Sable Island basestation TF measurements directly, 3) apply a low-pass smoother to the Sable Island TF measurements,

then subtract them, 4) subtract the Greenwood basestation TF measurements directly, 5) subtract the smoothed Greenwood basestation TF measurements.

The technique which gave the best results was subtracting a lightly smoothed (5 points = 1.25 second boxcar smoother) version of the Sable Island total-field. The gradient-and-basestation-corrected TF is denoted TF’’.


5.3 Combining several lines The simplest method for estimating the geological noise is to re-sample the gradient-and-basestation-corrected-TF along each profile to common (latitude, longitude) points using Akima Splines, and compute the average. However, other techniques can be used. If one wishes to use the data collected at other altitudes (say data from the 500’ lines to calculate the geological model for 1000’), then there is a common geophysical technique known as upward/downward continuation that allows one to estimate the total-field at one altitude based on measurements at another. If a 2-dimensional survey has been conducted, then the upward/downward continuation algorithms work quite well unless one attempts to estimate the total-field quite close to the source, based on measurements taken quite far away. However, in our case, we have only a series of flight lines at various altitudes. If we again use the assumption that the magnetic sources are 2-dimensional and oriented perpendicular to the flight line, then we can not only estimate the vertical gradient along the flight line, but the 2nd and 3rd-order vertical gradients as well. A Taylor expansion can then be used to estimate the total-field along a flight line at a higher or lower altitude as follows:

TFi(z) = TFi + (zi · Gzi) + (zi2 · Gzzi)/2 + (zi3 · Gzzzi)/6 + … (5) where z is the new height relative to the old height, i denotes the ith point along the profile,

Gz = - Hilbert Transform (Galong) Gxz = d(Gz)/dx Gzz = - Hilbert Transform (Gxz) Gxzz = d(Gzz)/dx Gzzz = - Hilbert Transform (Gxzz) and (d/dx) denotes the spatial derivative of a quantity along the flight track. When applying this algorithm, there is often some smoothing applied to the higher-order vertical gradient estimates because they tend to be dominated by high-frequency noise. Lines from different altitudes can then be used in the average, thereby possibly improving the geological noise model. Once again, if a high-quality 2-dimensional total-field survey was conducted, then Gz, Gzz, and Gzzz could be calculated without the assumption of linear, two-dimensional sources perpendicular to the flight path. Finally, instead of merely calculating the average of multiple estimates of the gradient-and-basestation-corrected-TF, or multiple upward/downward continued lines, it is possible to take only the coherent part of the signals as the estimate for the geology (denoted TFgeo in subsequent text). The technique which gave the best results was simply to average the three lines from each altitude, although the other techniques gave only marginally different results.


5.4 Re-sampling and reverse-gradient correction back to the original sampling positions Once an estimate of the geological noise along each desired path has been obtained, it can be re-sampled with the same number of data points as in the original flight line. Then it is reverse-corrected with the same gradient vector (Gxi, Gyi, Gzi) and deviation from the desired path vector (ΔXi, ΔYi, ΔZi) yielding an estimate of the geological noise at the exact location where the original TF measurement was made. This geological noise estimate is denoted “TFest”. Subtracting this geological noise component from the original TF measurement gives a first-order estimate of the geomagnetic signal along each flight path. Note that up until this point, everything has been calculated right down to DC, but eventually we wish to filter the residual data in order to accentuate the geomagnetic activity in the frequency band of interest. In order to avoid large discontinuities which may cause filter ringing in later processing, the time series TFest is adjusted in the following manner:

1) during the turns between the lines, TFest is set equal to the actual flight data during these turns,

2) the DC level of TFest along each line is shifted slightly so there is no discontinuity at the first or last point of the line.

These corrections have almost no effect on the geological estimate within the frequency band of interest, but do allow subsequent processing to be carried out over the entire flight line. The difference between the original TF and the gradient-corrected geological noise estimate (TFest) is denoted “Resid”.

5.5 High-Pass filter the original TF and geological noise estimate For most of the flights performed during this experiment, the geomagnetic field was active near or above 0.02 Hz. A 2nd-order digital Butterworth high-pass filter with a 3-dB point at 0.02 Hz was applied to the original airborne TF measurements and the geological noise estimate (TFest). These quantities will be referred to as TFHP and TFestHP in subsequent text. Other filters were investigated, including a similar filter with a 3 dB point at 0.01 Hz high-pass, and multiple applications of the 0.02 Hz high-pass filter. The results were very similar so only a single application of the 0.02 Hz filter was used in subsequent analyses. The difference between the TFHP and TFestHP is denoted “ResidHP”.

5.6 Re-compensate the filtered residual Even though the standard compensation and adaptive compensation algorithms remove the vast majority of the aircraft interference, it is possible that there is some residual noise remaining. To remove this noise, a 25-term model was fit to ResidHP on each line. The model consisted of the standard 18 Leliak terms generated from the vector magnetometers, plus 7 terms from the


pitch, roll, DGPS altitude, left and right accelerometers, aileron, and (Gacross· ΔY). Each of the 25 terms was filtered with the same 0.02 Hz high-pass filter used to generate ResidHP. The result is denoted ResidHPC The effect of this re-compensation was to reduce the noise where the aircraft had done slight banks to stay on track. In these cases, the magnetometers located on the wingtips moved up, or down, through the vertical gradient in the Earth’s magnetic field. Also, the aircraft moved back and forth through the lateral gradient in the Earth’s field in response to these slight banks and that is why a term for (Gacross· ΔY) was included. Note that although a similar term was used for the original gradient correction (Section 5.1), it was mainly the very-low frequency part of the term that was important in trying to build a better geology model. In this case it is only the variations above 0.02 Hz that are important and any small DC errors in Gacross are not as important. The residual during periods where the aircraft was not banking was barely affected. It should be noted that most of the Leliak terms, aileron, pitch, and accelerometer terms had very little effect on the re-compensation. Once the re-compensation coefficients have been calculated, it is possible to apply them to the unfiltered versions of the 25-terms and perform a “wide-band re-compensation” on the original airborne TF measurements. The result is denoted TFC. Passing TFC through the 0.02 Hz Butterworth high-pass filter yields the quantity TFCHP. Care must be taken not to introduce any DC offsets during this process, and the re-compensation coefficients must only be applied to the original flight line on which they were calculated. This is because there is very little variation in some of the 25 terms and there are many colinearities between them. The entire process of building up a geological model and removing the geology from the airborne TFC measurements can then repeated. It was determined that iterating more than once did not significantly improve ResidHPC. It is worth noting that in standard aircraft compensation, the signals are bandpass filtered near 0.1 Hz instead of 0.02 Hz in order to separate the aircraft motion noise from the geological signals. In this case we have attempted to remove as much geological noise as possible and then perform a re-compensation.

5.7 Plotting the results The residual after all the processing, ResidHPC, was compared to the similarly-filtered Sable Island basestation total field (TFSHP) in the following ways:

1) overplot the two time series and their difference (DIF1). 2) perform a frequency-domain coherence analysis of ResidHPC and TFSHP

and overplot the coherence residual (DIF2). 3) plot the coherence vs frequency and the PSD of the basestation data vs

frequency to determine if the coherence is better for large geomagnetic signals.

4) plot DIF1 or DIF 2 vs latitude (or longitude depending on the flight line direction) to determine if the places where the differences are the greatest occur at the same position.

5) determine the portions of each line where the geomagnetic activity was “large” or “small” and compare the DIF1 (or DIF2) in these regions.


6) Perform a frequency-domain coherence analysis of ResidHPC vs only the North component of the geomagnetic field measured at Sable Island (NorthSHP). This did not lead to any better results than against the TFSHP and so won’t be presented here.


6. Results

6.1 Sable Island North/South Lines Figure 14a shows the raw total-field aircraft data (TF) vs the gradient- and basestation-corrected total-field (TF”) for the twelve North/South lines flown near Sable Island. In general the blue lines overlap better than the black lines, indicating that corrections have at least been applied in the correct sense.

Figure 14a. Comparison of the raw total-field (TF) and the

gradient- and basestation-corrected total-field (TF”) along the three North/South lines near Sable Island at each altitude (Black vs,

Blue).

Figures 14b-e show the horizontal gradient corrections applied to each line. They are predominantly low-frequency and correspond to the aircraft manoeuvres as the pilots attempted to maintain the desired track. These corrections are typically less that 1 nT in amplitude.


Figure 14e. Horizontal gradient correction

applied to each North/South line at 5000’ near Sable Island, based on IGRF gradients.

Blue=ΔNorth x GNorth; Red=ΔEast x GEast.

Figure 14d. Horizontal gradient correction applied to each North/South line at 2000’ near

Sable Island, based on IGRF gradients. Blue=ΔNorth x GNorth; Red=ΔEast x GEast.

Figure 14c. Horizontal gradient correction applied to each North/South line at 1000’ near


Figure 14b. Horizontal gradient correction applied to each North/South line at 500’ near



Figure 14f shows the individual geology estimates and the averages for each altitude. The three geology estimates for each altitude overlap almost perfectly. There is a small amount of very-low-frequency difference between the three estimates which suggests that there may in fact be some phase lag between the very-low-frequency geomagnetic activity measured at the Sable Island basestation and that measured along the flight lines. However, this will have no effect on the final high-pass filtered geology estimate.

Figure 14f. Comparison of geology estimates along the three North/South lines near Sable Island at each altitude (Black, Blue,

Green) vs the average (Red). TFgeo is set to the average.

Figures 14g-j compare the original high-pass filter total-field measurements with the geology estimate removed (ResidHP) to the re-compensated quantity (ResidHPC). By comparing these figures to Figures 14b-e, it can be seen that the only significant changes occur during the aircraft manoeuvres where the pilots were bringing the aircraft back onto the desired track. Three things occurred at these time – the aircraft rolled a few degrees, the magnetometers in the wingpods moved up (or down) through the vertical gradient, and the aircraft moved laterally through the horizontal gradient in the ambient field. All of these effects are modelled by the 25-term model used for re-compensation. It is equally important to note that when the pilots were not manoeuvring the aircraft to get back onto the desired track, the re-compensation process does not increase the magnetic noise.


Figure 14j. Effect of extra compensation along the 5000’ North/South lines near Sable Island: ResidHP

(Black) vs ResidHPC (Blue).

Figure 14i. Effect of extra compensation along the 2000’ North/South lines near Sable Island: ResidHP


Figure 14h. Effect of extra compensation along the 1000’ North/South lines near Sable Island: ResidHP


Figure 14g. Effect of extra compensation along the 500’ North/South lines near Sable Island: ResidHP



Figures 14k-n compare the filtered re-compensated airborne measurements to the geology estimates (TFCHP vs TFestHP), the residual after subtracting the geology estimate to the Sable Island basestation (ResidHPC vs TFSHP), and the two residuals of the latter using either simple subtraction or frequency-domain coherence processing (DIF1 vs DIF2). The coherence and the PSD of the geomagnetic field as measured at the Sable Island basestation are also shown. The four figures correspond to the four different flight altitudes. Each figure contains individual plots of the three lines flown at each altitude. The first thing to note is that the geology estimate tracks the aircraft data extremely well (see top traces in each figure). This indicates that the geology estimates, and the process used to correct for the basestation and gradients is reasonable. It may not be optimum, but it is reasonable. The second thing to notice is that when the geology estimate is subtracted from the aircraft measurements, the low-frequency residual looks very much like the Sable Basestation total-field (see middle traces in each upper figure). There are a few locations where the two differ, but these are at places where the geological signal is quite large and variable between the three lines; e.g. near 43.98º, 44.05º, 44.07º and 44.10º in the 500’ and 1000’ data (Figures 14k-l). These are the locations where the simple IGRF gradient corrections are the least valid so it is not surprising that the geological model is poorest in these areas. However, the differences tend to be at a higher frequency than the geomagnetic noise. The third thing to notice is that there is very little difference between the residuals formed by simple subtraction of TFSHP from ResidHPC (DIF1) and the frequency-domain coherence processing of the two time series (DIF2). This suggests that in fact there is not a simple change in amplitude or phase of the geomagnetic field (as measured at the Sable Island basestation) and the flight lines nearby at 500-5000 feet of altitude. In general the geomagnetic activity was quite low when these lines were flown, but in the few places where the geomagnetic signals were significant in the Sable Island basestation data, they were highly correlated with the airborne data (e.g. south ends of L1 and L3 @ 500’, L2 and L3 @ 2000’, and L1 and L3 @ 5000’). The coherence is usually < 0.8, but where the geomagnetic signal is larger (L2 @ 2000’ and L3 and 5000’), the coherence is higher. This strongly suggests that the poorer coherence seen on the other lines is because there is excess noise from some other source such as imperfect geological noise cancellation or ocean dynamics. If the lack of coherence was because of something changing the local amplitude or phase of the geomagnetic signal, then the coherence would not be larger for larger geomagnetic signals. However, because we do not have large geomagnetic signals during the low altitude flights where the effect should be greatest, we cannot definitively conclude this.


Figure 14k. Upper Trace: Comparison of signals measured along the three 500’

North/South lines near Sable Island: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs

Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each

upper trace).

Middle Trace: Coherence between ResidHPC and TFSHP.

Lower Trace: Power Spectral Density of

ResidHPC (blue), TFSHP (black), and DIF2 (red).


Figure 14l. Upper Trace: Comparison of signals measured along the three 1000’ North/South lines near Sable Island: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper

trace).





Figure 14m. Upper Trace: Comparison of signals measured along the three 2000’



upper trace).





Figure 14n. Upper Trace: Comparison of signals measured along the three 5000’



upper trace).





Figure 14o is a plot of DIF1 vs latitude. It is clear that the largest residuals are at the lowest altitudes, and are clustered towards the northern end of the lines where the geological signals are the largest and most complex (compare Figures 14k and 14o).

Figure 14o. DIF1 vs Latitude for North/South lines near Sable Island.


6.2 Sable Island East/West Lines Figure 15a shows the raw total-field aircraft data (TF) vs the gradient- and basestation-corrected total-field (TF”) for the twelve East/West lines flown near Sable Island. In general the blue lines overlap better than the black lines. The amplitude of the horizontal gradient corrections and improvement obtained by re-compensation were not very different than for the North/South lines shown in Figures 14b-e and 14-g-j, so they have not been included.


gradient- and basestation-corrected total-field (TF”) along the three East/West lines near Sable Island at each altitude (Black vs,

Blue).

Figure 15b shows the individual geology estimates (TFgeo) and the averages for each altitude. The three geology estimates for each altitude are very similar but there is a small amount of very-low-frequency difference between the three estimates. Again this suggests that there may in fact be some phase lag between the very-low-frequency geomagnetic activity measured at the Sable Island basestation and that measured along the flight lines even though they are separated by less than 25 km. However, this will have no effect on the final high-pass filtered geology estimate.


Figure 15b. Comparison of geology estimates along the three East/West lines near Sable Island at each altitude (Black, Blue, Green)

vs the average (Red). TFgeo is set to the average.

Figures 15c-f compare the filtered re-compensated airborne measurements to the geology estimates (TFCHP vs TFestHP), the residual after subtracting the geology estimate to the Sable Island basestation (ResidHPC vs TFSHP), and the two residuals of the latter using either simple subtraction of frequency-domain coherence processing (DIF1 vs DIF2). The coherence and the PSD of the geomagnetic field as measured at the Sable Island basestation are also shown. The four figures correspond to the four different flight altitudes. Each figure contains individual plots of the three lines flown at each altitude. Just as was seen in the North/South results, the geology estimate tracks the aircraft data extremely well (see top traces in each figure). This indicates that the geology estimates, and the process used to correct for the basestation and gradients are reasonable. The quantity ResidHPC looks very much like the Sable Basestation total field (see middle traces in each figure). There are a few locations where the two differ, but these are at places where the geological signal is quite large and variable between the three lines; e.g. near -60.02º, -59.95º, and -59.81º in the 500’ and 1000’ data (Figures 15c-d). Again this is consistent with the


results from the North/South flight line data. These differences also tend to be at a higher frequency than the geomagnetic noise. Once again there is very little difference between the residuals formed by simple subtraction of ResidHPC and TFSHP (DIF1) and the frequency-domain coherence processing of the two time series (DIF2). This suggests that in fact there is no appreciable change in amplitude or phase of the geomagnetic field variations near 0.02-0.05 Hz as measured at the Sable Island basestation and the nearby East/West flight lines at 500-5000 feet of altitude. (Remember there does appear to be some phase lag at much lower frequencies because the various estimates for TFgeo do have some very-low-frequency variation in them.) The geomagnetic activity was quite variable when the East/West lines were flown, but in the places where the geomagnetic signals were large in the Sable Island basestation data, they were highly correlated with the airborne data (e.g. West end of L1 & L2 @ 500’; L2 and East end of L3 @ 1000’; L1 @ 2000’). The coherence, even at low altitude, is >0.8 when there is a substantial geomagnetic signal and smaller when there is less geomagnetic activity. If we compare the same geographic area on lines flown at the same altitude, but at times when the geomagnetic field was quiet vs active (East ends of L1 & L2 @500’ in Figure 15c or the West ends of L1 & L2 @ 1000’ Figure 15d), we see that the nature of DIF1 does not change. Taken together, these two results suggest that the remaining magnetic noise seen in DIF1 at the lower altitudes for the East/West lines near Sable Island is independent of the geomagnetic field amplitude. This implies that there are no conductivity effects at low altitude. Remember, though, that the separation between the basestation and these flight lines was less than 25 km, and the water depth is less than 100 m near Sable Island.



Figure 15c. Upper Trace: Comparison of

Ea

Middle Trace: Cohe ween ResidHPC

Lower Trace: Pow al Density of R

signals measured along the three 500’ st/West lines near Sable Island: TFCHP vs

TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2

(Black vs Orange at bottom of each upper trace).

ren e betc

and TFSHP.

e Spectrr esidHPC (blue), TFSHP (black), and DIF2

(red).

Figure 15d. Upper Trace: Comparison of signals measured along the three 1000’

East/West lines near Sable Island: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2


Middle Trace: Coherence between ResidHPC

and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black), and DIF2

(red).


Figure 15e. Upper Trace: Comparison of signals measured along the three 2000’

East/West lines near Sable Island: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2


Middle Trace: Coherence between ResidHPC

and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black), and DIF2

(red).


Figure 15f. Upper Trace: Comparison of signals measured along the three 5000’ East/West lines near Sable Island: TFCHP vs TFestHP, (Black

vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper

trace).





Figure 15g is a plot of DIF1 vs longitude. It is clear that the largest residuals are at the lowest altitudes, and are clustered towards the middle of the lines where the geological signals are the largest and most complex (compare Figures 15c and 15g).

Figure 15g. DIF1 vs Longitude for East/West lines near Sable Island.


6.3 Gully North/South Lines Figure 16a shows the raw total-field aircraft data (TF) vs the gradient- and basestation-corrected total-field (TF”) for the twelve North/South lines flown near the Gully. Again the horizontal gradient correction and re-compensation improvement plots have been omitted for brevity.

.


gradient- and basestation-corrected total-field (TF”) along the three North/South lines near the Gully at each altitude (Black vs,

Blue). Figure 16b shows the individual geology estimates and the averages for each altitude. The three geology estimates for each altitude overlap almost perfectly for the three lower altitudes, but there is a significant very-low-frequency difference between the three estimates at 10,000 ft altitude. However, this will have no effect on the final high-pass filtered geology estimate.


Figure 16b. Comparison of geology estimates along the three North/South lines near the Gully at each altitude (Black, Blue, Green)

vs the average (Red). TFgeo is set to the average.

Figures 16c-f compare the filtered re-compensated airborne measurements to the geology estimates (TFCHP vs TFestHP), the residual after subtracting the geology estimate to the Sable Island basestation (ResidHPC vs TFSHP), and the two residuals of the latter using either simple subtraction of frequency-domain coherence processing (DIF1 vs DIF2). The coherence and the PSD of the geomagnetic field as measured at the Sable Island basestation are also shown. The four figures correspond to the four different flight altitudes. Each figure contains individual plots of the three lines flown at each altitude. Again the geology estimate tracks the aircraft data extremely well (see top traces in each figure). The geological noise in the Gully area is generally less than in the Sable Island area. Unfortunately the geomagnetic field was fairly quiet during the three 1000 ft altitude lines so it is difficult to draw firm conclusions on the how well ResidHPC correlates with the Sable Island Basestation signal (TFSHP) at low altitudes. The coherence is

Figure 16c. Upper Trace: Comparison of signals measured along the three 1000’

North/South lines near the Gully: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace); ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2


Middle trace: Coherence between ResidHPC

and TFSHP.

Lower Trace: Power Spectral Density of ResidHPC (blue), TFSHP (black) and DIF2

(red).


Figure 16d. Upper Trace: Comparison of signals measured along the three 2000’




and TFSHP.


(red).


Figure 16e. Upper Trace: Comparison of signals measured along the three 5000’




and TFSHP.


(red).


Figure 16f. Upper Trace: Comparison of signals measured along the three 10,000’ North/South

lines near the Gully: TFCHP vs TFestHP, (Black vs Red at the top of each upper trace);

ResidHPC vs TFSHP (Blue vs Green in the middle of each upper trace); DIF1 vs DIF2 (Black vs Orange at bottom of each upper

trace).

Middle trace: Coherence between ResidHPC and TFSHP.


ResidHPC (blue), TFSHP (black) and DIF2 (red).


Only the northern half of L3 at 2000 ft altitude has significant geomagnetic activity and the phase/amplitude match of ResidHPC and TFSHP appears better than in the segment at 1000 ft. Again, with so little data it is difficult to draw any firm conclusions. There is a great deal of geomagnetic activity in all the lines at 5000 and 10,000 ft altitude and the phase and amplitude of ResidHPC and TFSHP match very well. In all cases there is very little difference between the residuals formed by simple subtraction of ResidHPC and TFSHP (DIF1) and the frequency-domain coherence processing of the two time series (DIF2). This suggests that in fact there is no consistent change in amplitude or phase of the geomagnetic field between Sable Island and the Gully between 1000-10,000 feet of altitude. There may still be local variations at the lower altitudes as described above. Figure 16g is a plot of DIF1 vs latitude. It is clear that the largest residuals are at the lowest altitudes, and are near where the geological signals are the largest (compare Figures 16c and 16g).

Figure 16g. DIF1 vs Latitude for North/South lines near the Gully.


6.4 Gully East/West Lines Figure 17a shows the raw total-field aircraft data (TF) vs the gradient- and basestation-corrected total-field (TF”) for the twelve East/West lines flown near the Gully. The two traces for the 10,000’ lines fall almost on top of each other because there was very little geomagnetic activity at this time. The horizontal gradient correction and re-compensation improvement plots have been omitted for brevity.


gradient- and basestation-corrected total-field (TF”) along the three East/West lines near the Gully at each altitude (Black vs, Blue).

Figure 17b shows the individual geology estimates and the averages for each altitude. The three geology estimates for each altitude overlap very well at 1000 and 5000 ft altitude, but there is some very-low-frequency difference between the three estimates at 2000 and 10,000 ft. Again this will have no effect on the final high-pass filtered geology estimate.


Figure 17b. Comparison of geology estimates along the three East/West lines near the Gully at each altitude (Black, Blue, Green) vs

the average (Red). TFgeo is set to the average.

Figures 17c-f compare the filtered re-compensated airborne measureme

Documents

Preliminary Results from the Sable Island Geomagnetic ... · However, the low-frequency portion of the airborne data looks very much like the Sable Basestation total-field in the