Real data, Hyde Park Could interpret as 2 or 3 layers. 2 layers – top 140 ± 15 Ωm bottom ~10 ± 2 Ωm (blue line) 3 layers – top 125 ± 15 Ωm, middle 150 ± 15 Ωm, bottom ~10 ± 2 Ωm (yellow line) Calculate misfit --- normalized (observed-expected) 2 Iterate to minimize misfit
Lecture 6 Magnetic data Theory Data collection Interpretation Applications Problems 10 and 11
Magnetic anomalies Detect changes in Earth’s field Signal from near-surface magnetic rocks
Rock typeClassificationSusceptibility (SI) GraniteIgneous0.0001-0.08 BasaltIgneous0.0003-0.1 AndesiteIgneous0.003-0.2 MetasedimentMetamorphic0.0002-.02 GranuliteMetamorphic0.0004-0.03 Shale/sandstoneSedimentary0.00001-0.001 MineralCompositionSusceptibility (SI) MagnetiteFe 3 O 4 1.0-5.7 PyrrhotiteFe 7 S 8 3.2 HematiteFe 2 O 3 0.001-0.08 Induced magnetic field = susceptibilty x Earth’s field J i = k x H Induced field is in same direction as Earth’s field. Both H and J i are vectors The measured magnetic field = Earth’s field + Induced field B = J i + H (This is a vector addition)
Units -- cgs, SI, mks Henry’s, nanoTeslas (nT), Gammas (γ), Gauss, Oersted, Webers … In this lecture: Magnetic field strength/intensity: nT (J, B, H) Susceptibility: SI (dimensionless) (k) 1 nT = 1 γ = 10 -5 oersteds
Inclination and strength of Earth field (H) Arrows are vectors At poles, magnetic field strength ~65,000 nT At equator, magnetic field strength ~35,000 nT Induced field will be greatest at poles and least at equator [ J i = kH ] The majority of the field (90%) can be described in terms of a magnetic dipole placed at the centre of the Earth.
Field strength (H) varies significantly with latitude, and less so with longitude The reference field IGRF
Some definitions Inclination is the angle between the Earths field and the horizontal Declination is the angle between magnetic north and true north. [Not same H]
Due to changes in convection within the outer core, the Earth’s field changes slowly with time (Secular variation) The position of the magnetic pole changes and occasionally the north and south pole reverse There is also a daily change in the Earth’s field due to the solar winds and the rotation of the earth (Diurnal variation)
Diurnal variations – largest at midday Magnetic storm – changes very rapid Diurnal variation 20-80 nT Magnetic storm ± 100 – 200 nT
Magnetic anomaly = observed – expected magnetic field Magnetic anomaly = B – H Hence anomaly should be a direct measure of J i – the induced field Expected field is likely to be value at a base station that is some distance away from the magnetic body of interest, or could be the IGRF reference value for H
There may also be a remanent magnetic field J rem, in which case the anomaly is the vector addition of J i and J rem Usually J i >> J rem If anomaly is small then it is important to make diurnal corrections Return regularly to base and the expected magnetic field is the field at the base station at the same time as the reading at the survey point [similar to drift correction in gravity)
Magnetic surveys – sea, land, air Accuracy ~1-2 nT Measured anomalies are a few – to thousands nT
Most common hand-held field equipment is the proton magnetometer Pass ac current in a coil that is surrounded by a proton-rich fluid Protons are magnetised and align in the direction of the coil axis When the current is turned off the protons “precess” and align themselves with the Earth’s field. The speed of precession is a direct measure of B (the total field J i + H).
Airborne magnetic surveys, parallel, equally spaced flight lines. Normally oriented E-W and N-S, but may be oriented perpendicular to geologic strike direction. Survey typeLine spacingFlight heightArea covered Reconnaissance 1-2 km 300-500 m Large >10 4 km 2 Detailed100-200 m50-100 m Small <10 3 km 2 Survey points can be spaced close together or 100’s metres apart Depends on target Very quick Pretty cheap Uninvasive Excellent reconnaissance technique
Total magnetic field over the same area of the Canadian shield, near La Ronge, Saskatchewan at different flight path spacings Clear improvement as line spacing decreases
Interpretation magnetic anomalies Much less straightforward than gravity The shape of the anomaly across a magnetic body varies with: latitude the shape of the body the magnetic properties of the body the direction of the survey the depth of the body any remanent magnetic field in the body Several parameters can vary – need other constraints to come up with a good model of the subsurface
Variation with latitude south-north profile across magnetic body at 60 deg south If J i is in the same direction as the Earth’s field the anomaly is positive
Variation with profile direction, body orientation and inclination Note the change in shape of the anomaly with change in profile direction and different inclinations. There can be quite large magnetic anomalies at the edge of magnetic bodies. Away from the poles, likely to have positive and negative anomaly values
Change of anomaly with survey height (or increasing depth of burial) As flight path height increases, anomaly appears broader and the max/min heights decrease. Exactly the same would happen for an increasing depth of burial of the magnetic bodies. In each case, the bodies will become undetectable at some point.
As seen in the last slide, the width of the anomaly increases with depth of burial, and the height of the anomaly decreases. Thus can use magnetic data to estimate depth of burial of target body Theoretical curves can help us estimate the magnetic properties of the rocks and the depth of burial of the magnetic body In this case: Width of anomaly depends on z Height of anomaly depends on z and m
Removal of the regional gradient As in gravity data, there might be more than one component giving rise to the magnetic anomaly Can filter the data to remove the regional gradient
Aeromagnetic data, USA In terms of total in km 2, there is more magnetic survey data than any other geophysical data Colour = Geology Contours = magnetic anomaly
The Geology is displaced on the aeromagnetic surface (USA) Sharp spikes = volcanic intrusives 3D visualization Magnetic anomaly can be a good indicator of rock type
Magnetic field can mirror depth to basement. Used by oil industry to assess sediment thickness and “likelyhood” of oil being recovered. Used in the discovery of the Bass basin gas deposits between Australia and Tazmania Use magnetic data to interpret depth to top subducting slab, Japan Magnetic anomaly Gravity anomaly Oil prospecting – offshore Morocco and Gulf of Mexico
Reconnaissance for minerals is probably the widest use Example, iron ore body (haematite) in Australia (solid black) Magnetic anomaly offset from body – high concentration magnetite not coincident with the haematite
0 800 m 0 200 Archeaological investigations The example above shows a Fenland topsoil magnetic susceptibility map which revealed the location of a medieval kiln. The strong linear feature represents the course of a medieval ditch which contains magnetically enhanced deposits derived from the brickworks.
: A Neolithic and later hill top enclosure Neolithic enclosure, ditches have small magnetic anomaly 0 – 5 nT
Geothermal applications Used magnetic data from Yellowstone Park to estimate depth to magnetized crust. Interpreted as Curie point isotherm They discovered a strong concentration between a shallow depths of the Curie point isotherms and hydrothermal activity