Published on January 5, 2008
Slide1: Chap 7 - Magnetic Method in Exploration “possibly most versatile of all the methods discussed..” deep work and shallow cost-effective (i.e., cheap) at both regional and local scales downside - interpretation more difficult than gravity (now HOW is that possible??) for shallow seds, mag not particularly helpful - why? “mag signatures depend on magnetite content”, and there is not much magnetite in seds (nor in many basement rx, either) but there are rock types that are traceable: buried ore deposits magnetic rock units beneath glacial cover (iron ranges of upper midwest a classic example here) goal of chapter - give you basics and “practical grounding” what controls mag anomalies & potential fields what affects interpretation Slide2: I. Fundamental relationships look at bar magnet - fig 7-1 note lines of the force field - where they are close together, field is strongest the magnet is a dipole…a positive and negative end Fig 7-2 shows visualization of a bar magnet as collection of many small bar magnets…one end has collection of + charge, the other end is - charge…all the points in the middle cancel each other out. A. Magnetic force Coulomb’s Law describes force between poles: F = 1 m1m2 r2 where is magnetic permeability - stronger the permeability, the less the force B. Field strength described by H = m/r2 this is a vector quantity with both magnitude and direction, felt by a dipole in a field Slide3: C. Magnetic moment - when magnet is put into a field, a couple is placed upon it (Fig 7-4) looks like shear stress, sort of..a shear “couple” … I visualize a compass needle swinging in the field, orienting it self magnitude of this can be calculated - C = 2 (ml) H sin note that when = zero (magnet aligned), then C = 0 OK, no shear stress acting on the magnet when in this position note that ml is like a torque arm, longer the “l”, the the bigger the “magnetic moment” M D. Intensity , known as I I = M / vol = m/area some intuitive sense here…bigger the area or volume, the less the “intensity” Slide4: E. Mag susceptibility - important concept I = k H where k is susceptibility, I is intensity, H is field strength so the lower the susceptibility of a material, the less intensely it will become magnetized…some practical examples?? Iron? Steel? Aluminum? Copper? Glass? Confusing terminology here 1. Diamagnetic - low negative susceptibility (qtz, feldspar) 2. Paramagnetic - low positive susceptibility (Fe-Mg silicates like olivine, pyroxene, amphibole - dark mins with some iron) paramagnetic mins further broken into (Fig 7-5) ferromagnetic - magnetic domains all point in same direction very high susceptibility, but not naturally occurring on Earth (find in meteorites) anti-ferromagnetic - equal nos of domains point in diff directions, so magnetic moment = zero very low susceptibility, like hematite Slide5: Ferrimagnetic - third type that does have susceptibility due to some preferred orientation magnetite, ilmenite, pyrrhotite for example (magnetite the most common) depending upon field strength, a ferrimagnetic min may or may not be permanently magnetized Fig 7-6 at saturation, material has been magnetized as much as it will be…. This is known as remanent magnetization (plate tectonics fans, this is how they figured out that the continents have been moving around for a long time…) little compass needles in hot basalts, frozen in place pointing at north pole at the time of cooling F. Magnetic potential magnetics, gravity, electricity, water flow…all governed by potential field theory…all about describing the work done to move a pole or charge or water molecule from one point to another, and this is path independent - that is, if a complete circuit is made, work done is zero (I have trouble conceptualizing this…I can see the potential diff as + zero, but if you move a pole from starting to ending point, seems like it would take some work…. Slide6: But the equation is: V = m/r let’s move on… II. Earth’s Mag Field A. Field elements -again, mag field is a vector, with both magnitude and direction compass needle example - needle will move into parallelism with field (both declination AND inclination - example - where is compass pointing when you’re AT the North Pole??) Fig 7-7 confusing…look at this instead…Earth as a bar magnet North Slide7: Ok so what is the non-Greek version? Declination - number of degrees away from geographic North Inclination - number of degrees down or up from horizontal intensity at magnetic North/South is strong - about 70,000 nT at magnetic equator, less strong - about 30,000 nT (remember, the lines of force bunch up at the poles, spread out at equator….where bunched together, field is strong, where apart, field is weaker). Book makes good point about mag vs grav field mag field can vary 200%, but grav field varies only 0.5% think about this….grav emanating from center of a sphere, uniform in all directions, while mag is a dipole, strong at ends, weak in middle. Makes sense. B. dipolar field of Earth -Fig 7-11 - note how mag dipole tilted about 11.5o off axis of rotation - note too that this is NOT the same as the magnetic DIP poles, which are what we call “magnetic north” great…now we have North, Magnetic North, and Geomagnetic North!! Slide8: C. variations in Earth field - maps from figs 7-9 and 7-10 show the variation - quite striking where does field come from? “currents in fluid core” (PS - this remains speculative - no one ever took their Brunton down there and measured mag field….) we get changes over time in mag field…both intensity and direction (not unlike the changes in grav field….the need to return to a base station to build a “correction curve” is always there) secular variation occurs over long times, not to worry diurnal variation occurs over short times, that’s why we go back to the base station - these due to particle flow in ionosphere, etc…(solar winds? Magnetic storms?) what we’re going after is the induced magnetization variations in the field, much like we go after variations and anomalies in the Gravity field…same basic principle D. now to the …good stuff - dipole equations Fig 7-12 mag potential at pt P is V = m/r1 - m/r2 this leads to derivation of other equas Slide9: IV. Measuring Mag Field 2 main magnetometer types: 1. Flux-gate 2. Proton-precession we discuss advantages/disadvantages, and how we separate total-field msrmt from local anomalies 1. Flux-gate -sensor is parallel magnets, double wound wiring voltage induced in outer coil when in presence of external field…and it is proportional to the external field can measure FE (total field), ZE (vert field), HE (horiz field), but need to be carefully aligned to be accurate 2. Proton precession - sensor is H-rich liquid in container, surrounded by a coil. Power to coil creates mag field parallel to coil axis. Little hydrogen proton dipoles become aligned in the field. Power then shut off, protons “precess” around Earth total field. This movement sets up small current, proportional to total field strength. Msrmt very accurate. Easy to use, reliable too. P-p msrs total field, not components like fluxgate Slide10: P-p msrs total field, not components like fluxgate so you can only msr intensity, and not direction - nat a s easy to interpret results as when you can do just the vertical field but this device so easy to use, it is now the standard tool so we must figure out how to interpret the data, because the tool isn’t going away… A. Total field anomalies we want to determine the anomalous induced field, which is the field in a body that is a product of Earth’s mag field total field = main fld + anom fld (Fig 7-13a) FET = FEU + FAT we see that main fld is much greater than the anom, so we assume that total field vector orientation is same as the main field vector this leads to defining the effect of the anomalous field as occurring in the same direction as the undisturbed main field (again, the main field is overwhelming the anomaly) (Fig 7-13b) Slide11: Now geometric relations are developed, ultimately yield: FAT = ZA sin i + HA cos cosi - where FAT is the component of the anomaly in the direction of the total field temporarily move on to field procedure, then return… V. Field Procedures cleanse yourself of metal objects…they have an effect on your readings! (they become induced magnets in the Earth’s field…) nearby objects also present problems - cars, fences, powerlines, poles, etc. diurnal corrections also important - Fig 7-14 just like in gravity, this helps you keep track of your field fluctuations - they say best thing to do is keep a 2nd magnetometer at your base station, recording continuously. Fine if you have another 5-10 grand to blow on a 2nd magnetometer! A. elevation correction - don’t bother…but the why illustrates an interesting point of diff between grav and mag Slide12: Note here the diffs in VARIATION of these fields over the entire globe… Grav = 5200 mgal (.5% variation) Mag = 40,000 nT (57%) 983,000 mgal 70,000 nT what’s their point??? An elevation correction means something with grav data, it can radically change your interp an elev correction with mag data just gets lost in the noise…it’s a very minor component correcting for horiz position no big deal - just find the values for the corners of your area, Fig 7-15, determine gradients. Corner plus your gradient gives you what the FEU should be with no anomaly, and the magnetometer tells you FET, what the total field actually is… use FET = FEU + FAT to calc the anomaly, FAT book example …. Good exam question!! Slide13: VI. Magnetics of simple shapes (same principle as grav) disclaimer suggests that much derivation is involved - start with range of susceptibilities - quite a big diff.. K = .00005 cgs emu for sed rx (little Fe, Mg) up to k = .012 cgs emu for ultra-basics (lots Fe, Mg) iron and steel go from 1 to 10 (way high) easy to see how aluminum from aircraft would have some effect on a magnetometer…. OK now let’s briefly visit some geometries…. Single pole…..Simple but impossible..! V = m/r - equiv to a long thin rod standing vertically FAT = ZA sin i + HAx cos i table 7-1 looks at this situation, with i = 70 degrees Fig 7-17 too vert intensity highest over the rod, but FAT changes based on i value look at the trig….use some common sense…anom in z direction is high when the field in that direc is high..anom is low if field strgth is low Slide14: B. dipole effect - lot of derivation to get what looks much the same as the monopole - figs 7-19 and 7-20 show the response with various i of earth….note in 7-20 how the vertical anom becomes a positive-negative couple as the dipole becomes horizontally oriented - note the map view of total field anom due to a dipole in Fig 7-22 C. sphere - first, vertically polarized, then inclined the general curve shapes for sphere very similar to that of dipole, comparing the vert field to the total field Fig 7-25 authors note this similarity, indicate that this means that we can approximate and model spherical magnetic bodies in the subsurface by using dipoles… D. horizontal sheet (like a bed) extensive derivation, then some interesting applications for looking at Precambrian basement under sedimentary rx Fig 7-28 this example has assumption of series of negative poles on surface of basalt, and the positive poles are at depth and out of the picture ZA = Z A top - Z Abot = 2 I ( 1 - 2) where I = m/area = pole strgth/area Slide15: So look at the example where there’s a lithologic contrast betw high-mag basalt and low-mag sediments - couple things to remember here… authors forget to remind us of our terms… I = kH Intensity of magnetization (nT) = susceptibility x external field strgth (nT) thus ZA = Z top - Z bot = 2 k H (top) - 2 k H (bot) = 1286 nT (big anom) compare this to 7-28b, where there’s a slight amt of basement relief with seds draped over top, there the anom is only 84 nT, which is pretty small OK so the moral of the story here is… Mag may be consid more effective as a tool for determining lateral lith contrasts in basement, rather than as a tool for finding relief on structures with same lith E. Faulted horiz sheet - Fig 7-30 shows this; pretty clear position for edge of sheet in either of the 3 scenarios, but tough to see the diff between a faulted sheet and the case where an upper bed is present, with no lower half Slide16: F. move to analysis of polygons whose strike length is much greater than their width (like a core of a fold, or a dike) Figs 7-31a,b show the diff profliles by taking a dike and orienting it parallel to magnetic north and perpendicular to magnetic north. In latter case, horiz field causes add’l poles to be induced that create an assymmetric anomaly, vs the symmetric one produced by dike parallel to magnetic meridian. Next they incline the dike, make it look more like a bed dipping to the west. Resulting curve has a hump on east side, where the bed is close to surface (Fig 7-33b) - then an assymmetric tail to the west (this curve looks intuitively more like a buried sphere than an inclined plane if you ask me…) Summary of factors at work…they are many, and cause mag interp to be complicated - body shape body depth body orientation (strike) body dip Earth field inclination & declination relative to body here they suggest gravity doesn’t look so BAD now, compared to this! Slide17: VII. Interp of mag data - (Finally…) Disadvantages No unique solutions (like any potential field method..) magn susceptibility varies, even within a rock mass Advantages cost-effective once you’re familiar with local area field inclination, model shapes can be used for map interpretation when high values are encountered, they are likely to be from a relatively few number of rock/mineral types, constraining your interpretation ID of mag gradients and linking them to rock types, contacts, faults, etc often enough - then geologic mapping can produce the required detail for further anomaly definition Slide18: A. Quantitative interpretation techniques a. Half-maximum techniques like the grav techniques we’ve seen before….you spot the data, determine max Z value, determine half the max Z value, spot where the x position of the Z 1/2 max value is, and use some general rule of thumb relations to come up with general ideas of depth of burial 1. monopole: depth to monopole = x position at Z 1/2 max .766 or z = x 1/2 max .766 X=0 Z max Z 1/2max x 1/2max meters nT Slide19: Some other geometries and their rules of thumb: 2. sphere and cylinder: width of curve at Z 1/2 max = depth to sphere center or cylinder center z Z max Z 1/2max meters nT width Slide20: 3. Also a rule of thumb for the semi-infinite sheet depth to sheet = I/2 distance (vert dist?) from Z max to Z min (don’t worry about this one…) b. slope methods - another class of rule of thumb methods, used for PRISMS Fig 7-35 tells all 1.find max slope on your anomaly curve 2.make a line whose slope = 1/2 the max slope 3. Move the line around until it looks tangent to the anomaly curve at TWO places 4. Note the horizontal positions of the 2 tangency points 5. Horiz distance between these 2 pts is “d” 6.Calculate the depth to top of prism as z = d 1.6 definitely a good exam question or two here…. Slide21: VIII. Applications of magnetic method A. Archaeology - used frequently 1. looking for iron objects at burial sites, and also some remnant mag produced during firing of bricks, pottery, etc. They show a burned, filled pit house in Fig 7-36 a really cool one is Fig 7-37 - showing a quasi-3D view of vertical-gradient data …almost looks like little seismic lines - Bronze Age pits 2. another archaeolog app in finding tunnels - these are in volcanics (pyroclastics) with positive susceptibility, so the tunnels represent Negative contrasts. Fig 7-38 3. “Modern archaeology” - finding drums, old wellheads, casing etc….magnetics very effective for this Fig 7-39 - these represent potential sources of contaminants, so good to know where they are B. Magnetic data in landfills - old one in Kalamazoo as a training site for our class - really effective at finding lots of steel cable, things like old toilets (iron covered by porcelain), old washing machines, drums, etc. FINI!!