Magnetic Methods PDF

Shell Intensive Training Programme 5.0 Magnetic Methods 5.1 Principles and Theoretical Background Magnetic surveying investigates the subsurface geology of an area by detecting magnetic anomalies within the Earth’s magnetic field, which are caused by the magnetic properties of the underlying rocks....

Shell Intensive Training Programme 5.0 Magnetic Methods 5.1 Principles and Theoretical Background Magnetic surveying investigates the subsurface geology of an area by detecting magnetic anomalies within the Earth’s magnetic field, which are caused by the magnetic properties of the underlying rocks. Most rock-forming minerals are non- magnetic but a few rock types contain sufficient amounts of magnetic minerals, which can impart a magnetism to their host rocks and thus produce detectable magnetic anomalies. Rock magnetism has both magnitude and direction, the latter being determined by the host rocks position relative to the past and present magnetic poles of the Earth. This is the olderst geophysical prospecting method as Von Wrede used variations in the earth’s field to locate deposits of magnetic ore in 1843. Earlier on, in the 16th century, Sir Gilbert found that the earth behaves like a North- South permanent Bar magnet. 5.1.1 Principles of Magnetism A bar magnet is surrounded by a magnetic flux, which flows from one end of the magnet to the other and can be mapped from the directions assumed by a small compass needle suspended within it. The points within the magnet where the magnetic flux converges are the poles of the magnet. Common magnets exhibits a pair of poles and are therefore referred to as dipoles. A freely suspended bar magnet will align itself parallel to the local flux of the Earth’s magnetic north pole termed the north-seeking or positive pole and this is complimented by a south-seeking or negative pole at the other end of the magnet. All substances are magnetic at an atomic scale. The phenomenon in which a material acquires a magnetization when placed within a magnetic field but loses it when it is removed from the field is termed induced magnetization or magnetic polarization. It results from the alignment of the elementary dipoles within the material parallel to the direction of the external field, but of opposite polarity. The Page 1 of 22 © Univation Shell Intensive Training Programme intensity of induced magnetization, Ι is proportional to the strength of the external field, H: Ι= χH (5.1) where χ is a dimensionless proportionality constant for the particular magnetic material and is termed susceptility. As shown in Table 1, the susceptility of a single rock type can be very variable. These ranges reflect the different amounts of magnetic minerals present in different samples of the same rock type (fig. 5.1). Table 1. Ranges and averages of magnetic susceptibility of some common rock types Rock Susceptibility (x 106 emu) Range Sediments and sedimentary rocks sandstone Shale Page 2 of 22 © Univation Shell Intensive Training Programme Limestone All very low (< 1) Gypsum Coal oil Igneous rocks Granite 0 - 4000 200 Basalt 20 - 14500 6000 Gabbro 80 - 7200 6000 Peridotite 7600 - 15600 13000 Metamorphic rocks quartzite - 350 schist 25 - 240 120 slate 0 - 300 50 gneiss 10 - 2000 - There are 3 types of magnetic materials classified according to the extent to which they can be magnetised when exposed to an external field (i. e depending on the values of their magnetic suspceptibility:). Materials with : χ = low and –ve are Diamagnetic χ = low and +ve are Paramagnetic χ = high and +ve are Ferromagnetic Page 3 of 22 © Univation Shell Intensive Training Programme (i) Diamagnetics – All electron shells are full and no unpaired electrons. When exposed to an external magnetic field, they produce magnetic field oppositeto that of the applied magnetic field. Hence they are only weakly susceptible and negative. (ii) Paramagnetics – They have incomplete electorn shells and so when exposed to an external magnetic field, the orbital paths of the unpaired electrons rotate to produce a field in same direction as that of the applied field. They are then positively susceptible to magnetization (iii) Ferromagnetic – These are metallic elements that allow electron exchange between adjacent atoms of the element (such as Ni, Co, Fe). The exchange-energy produces very strong molecular field and aligns the atomic magnetic moments exactly parallel and so produces a spontaneous magnetization. In the presence of an external field, they give rise to a strong magnetic behaviour called Ferromagnetism. If a rock containing thousands of tiny ferromagnetic mineral grains is placed in a strong magnetic field, the individual magnetic moments become aligned with the applied magnetic filed. If the magnetizing field is reduced to zero, a ferromagnetic material retains part of the induced magnetization. This RESIDUAL magnetization is termed remanence or isothermal remanent magnetization. When a ferromagnetic material is heated, its spontaneous magnetization disappears at the ferromagnetic curie temperature or curie point. On cooling down below the curie point, spontaneous magnetization reappears. 5.1.2 Rock Magnetism Introduction A rock can be regarded as a heterogeneous assemblage of minerals, most of which will be diamagnetic in character. It is only the very small content of ferromagnetic- type minerals, which will contribute to the remanent magnetic properties of a rock and determine the magnetic properties of a rock, which are geologically and geophysically significant. The most important factors influencing rock magnetism are the type of ferromagnetic-type mineral, its grain size and the manner in which it acquires a remanent magnetization. Page 4 of 22 © Univation Shell Intensive Training Programme The most important magnetic minerals are oxides of Iron and Titanium which are naturally occurring ferrites. It is the small concentration of ferromagnetic-type minerals in a rock (ferrites), which determine its magnetic properties, most importantly its ability to acquire a remanent magnetization or remanence. The untreated remanence is called its natural remanent magnetization (NRM). It may be made up of several components acquired in different ways and at different times. The geologically important types of remanence are acquired at known times in a rock’s history, such as the time of its formation or subsequent alteration. The remanence of a rock can be very stable against change and be preserved over long periods of geological time. The remanence acquired at or clsoe to the time of rock formation is termed primary magnetization. Examples of primary remanence are thermoremanent magnetization, which an igneous rock acquires as it cools, and the remanent magnetizations acquired by a sediment during or closely after deposition. A remanence acquired at a later time is called secondary magnetization. Secondary remanence may be caused by chemical changes during diagnesis or weathering or by samples and laboratory procedures. As rocks acquire their magnetic property or behaviour from the Earth’s magnetic field, this phenomenon is usually used to predict the Earth’s field strength and polarity. However, the magnetic rocks which display Remanences (as discussed earlier) show magnetic filed strength and direction unrelated to the present disposition of the Earth’s field. Hence, a detailed study of magnetic rocks especially the ones with remanent magnetization would show the previous or earlier disposition of the Earth’s field. This study which makes it possible to analyse past dispositions of the Earth’s magnetic field in rocks (as in Igneous rocks) or of the deposition of rocks (as in sedimentary rocks). This is known as PALEOMAGNETIC studies. 5.1.2(a) Magnetic Induction of Rocks The magnetic induction B of a magnetic material such as a rock with +ve magnetic susceptibility is given by the total field within the rock material which includes: (i) the strength of the Earth’ magnetic field, H that the rock material is exposed to Page 5 of 22 © Univation Shell Intensive Training Programme (ii) the strength of the magnetic field in the rock material itself, H arising from its being polarised from the exposure. The field H1 is a product of the in the intensity of magnetization Ι and a solid angle 4π. H1 = 4π Ι B = H + H1 = H + 4π Ι B = H + 4π χΙH B = H(1 + 4π χ) (5.2) The ratio of the magnetic induction B to the magnetising field H is referred to as the magnetic permeability. B/H = 1 + 4πχ = µ ---(5.3) B = µH ---(5.4) µ is the magnetic permeability of the medium between the magnetising field H and the rock material. 5.1.3 The Gamma The magnetic field H at a point is given by magnetic force per unit pole strength H = F/Po = P/µr2 F = P0 P /µr2) P0 & P are strength of 2 poles of magnet r is the distance between them The unit of H is given in dynes per pole strength OR in the density of lines of force per area. 1 dyne/unit pole = 1 line/cm2 = One Oersted In S.I. units, 1 Tesla = 104 Oe (Oersted) The Earth’s magnetic field = ½ Oe. Page 6 of 22 © Univation Shell Intensive Training Programme Therefore, this unit is too large in relation to the natural Earth’s magnetic field and much too large for the expected variations contributable from the subsurface magnetic rocks. The unit of Gamma (γ ) is then introduced as a much smaller unit or measurement. 1 gamma = 10-5Oe = 10-9 T Hence, total average magnetic field of the earth is 50,000 gamma but it varies from the equator to the poles to the order of about 20000 to 70000 gamma. 5.1.4 Geomagnetic Field 5.1.4(a) Origin of the Field The Earth’s magnetic field is believed to originate in the core. Although the mechanism is uncertain, it is believed to be created by magnetohydrodynamic processes(self – excited Dynamo theory). It is currently believed that the geomagnetic field is generated by shearing and twisting fluid motions within the liquid, electrically conducting outer core, the result being a dynamo, which maintains and regenerates the Earth’s magnetic field. It is believed that only 1% of the Earth’s magnetic field is due to sources outside the earth (atmosphere). The geomagnetic field is usually described as being like that of a dipole. That is, it looks like the field that would be generated by a bar magnet at the core, with the lines of magnetic force looping from the south magnetic pole to the north magnetic pole. In practice, it is a little more complex with some multipolar patterns, but as a first approximation the dipolar model is convenient. While the N-S magnetic polar axis is currently inclined at an angle of about 100 to the Earth’s axis of rotation, it is thought that the magnetic and geographic poles would coincide when averaged over a long period of time. 5.1.4(b) Geomagnetic Elements The Earth’s magnetic field is a vector; that is, it has both magnitude and direction. The magnitude of this field is relatively weak, with a maximum intensity near the Page 7 of 22 © Univation Shell Intensive Training Programme magnetic poles. The geomagnetic elements (fig. 5.2) are taken to be components parallel to the geographic north and east directions. From a geological point of view, there are two important types of measurement relating to the Earth’s magnetic field: Ø The angle of declination (D) which is the angle between the observed compass reading (magnetic North) and the geographic longitude (true North). Ø The angle of inclination (I) which is the angle of dip of a magnetized needle relative to the horizontal. This angle varies from 900 (vertical) at the magnetic poles to 00 (horizontal) at the magnetic equator. Inclination (I) is thus mathematically related to latitude (λ): Tan I = 2 tan λ ---(5.5) From the orientation of the geomagnetic elements, we obtain that H = F cos I , Z = Fsin I, Z/H = tan I X = H cos D , Y = Hsin D, Y/x = tan D H2 + Z2 = F2 , X2 + Y2 = H2 ∴ X2 = Y2 + Z2 = F2 ---(5.6) Page 8 of 22 © Univation Shell Intensive Training Programme The magnetic field observed on the surface is a resultant of the field associated with a buried source superimposed on that of the earth which is a vector with both magnitude and direction and is called Total Field F. When I = 900 , the total field F = Z This means that at the poles, the total field is completely vertical and has no horizontal component. When I = 0 (at the equator) F = H ie total field is completely horizontal. Lines joining points of I = 0 define the magnetic equator and this runs roughly along the geographic equator. Magnetic poles differ from geographic poles by about 180 of latitude and the poles are not diametrically opposite one another. 5.1.4 Variations of the Geomagnetic Field In the dynamo theory of the origin of the geomagnetic field, the circulation of charged particles in coupled convective cells within the outer fluid core has no fixed patterns and so the circulation patterns change slowly with time. This is reflected in a slow progressive, temporal change in all the geomagnetic elements, (field magnitude and direction) known as Secular Variation. I and D have changed some 100 and 350 respectively over a 400 yr period. The contribution (from outside the earth’s interior) to the Geomagnetic field which is about 1% cause the earth’s field to vary on a daily basis to produce Diurnal Variations. This is caused by the interactions of the sun and the moon with ionospheric currents. The solar activities account for about 30 to 50 gamma and occurs in a 24 hourly period while that of the moon accounts for about 2 to 10 gamma every 25 hr – period. There are certain periods however, when large unpredictable disturbances occur and may cause variations of about 1000 gamma known as magnetic storms. As this is not predictable and is large, it is almost impossible to correct for it in a magnetic data assembly. Magnetic surveying must then be discontinued during the occurrence of such storms. Page 9 of 22 © Univation Shell Intensive Training Programme 5.2 Instrumentation The earliest instruments used in measuring the disposition of the earth’s magnetic field were the mariner’s compass (magnitude of the dip) and the magnetic variometers. Later developments brought magnetometers whose sensitivity required 1 to about 10 gamma. They are: Fluxgate Nuclear precession (proton) Rubidium vapour (Optical pump) Schmidt-Type Balance All the above have been in use for more than a decade but the Nuclear precession (proton) is presently the most commonly used magnetometer. They all are designed to ensure either the total field for the components (H, Z) separately (fig. 5.3). Page 10 of 22 © Univation Shell Intensive Training Programme 5.2.1 Flux-gate magnetometer The flux-gate magnetometer was developed in World War 11 as a submarine detector and was subsequently extensively used in airborne magnetic surveying. The instrument is based on a sensor consisting of two parallel strips of a special nickel-iron alloy, which combines very high magnetic susceptibility with very low remanent magnetization. Consequently a very weak field can change its magnetization, but its susceptibility is so high that the Earth’s weak magnetic field can induce a magnetization in it. It is therefore necessary to accurately orient the sensor along the direction of the filed component to be measured. For total filed measurements, three sensors are applied. The fluxgate is however temperature sensitive but robust enough for air- borne survey. 5.2.2 Proton-precession magnetometer The proton-precession magnetometer depends on the fact that the nucleus of the hydrogen atom, a single proton, has a magnetic moment proprotional to the angular momentum of its spin. Since the angular momentum obeys quantum mechanics, the proton magnetic moment can only have specified values, which are multiples of a fundamental unit called the nuclear magneton. The ratio of the magnetic moment to the spin angular momentum is called the gyromagnetic ratio γ p = 2.67513 x 108s-1T-1 The sensor of the proton-precession magnetometer consists of a flask containing a Proton-rich fluid, such as water or kerosene. A magnetizing solenoid and a detector coil are wound around the flask. When the current in the magnetizing solenoid is switched on, it creates a magnetic field, which is about 2000 times stronger than the Page 11 of 22 © Univation Shell Intensive Training Programme earth’s magnetic field. The magnetizing field aligns the magnetic moments of the protona along the axis of the solenoid, which is oriented approximately east-west at right angles to the earth’s magnetic field (fig. 5.4). After the magnetizing field is interrupted, the magnetic moment of the proton spins reacts to the couple exerted on them by the Earth’s magnetic field by precessing about the direction of the external magnetic field. They do so at a rate known as Larmor processional frequency. The motion of the magnetic moments induces a signal in the detector coil. The induced coil is amplified electronically and the precessional frequency is accurately measured by counting cycles for a few seconds. The strength F1 of the measured magnetic field is directly proportional to the frequency of the signal (f) and is given by: F1 = (2µ/γ p) f -----------5.7 The precessional frequency produced by the Earth’s magnetic field is approximately 1250-2500 Hz, which is in the audio-frequency range. The proton-precession magnetometer is simple, robust and easily portable. It provides an absolute value for the magnetic field. The sensor does not have to be accurately oriented, although preferably it should lie at a substantial angle to the field vector. Consequently, the proton-precession magnetometer is the most commonly used magnetometer for survey work and may be towed behind ships or Page 12 of 22 © Univation Shell Intensive Training Programme aircraft. Since this instrument does not give a continuous record, it surfers from the disadvantage that it may miss small anomalies. Magnetic gradiometers The magnetic gradiometer consists of a pair of magnetometers maintained at a fixed distance vertically from each other. The difference in outputs of the two instruments is recorded. If no anomalous magnetic field body is present, both magnetometers register the Earth’s field equally and the difference in output signals will be zero. On the other hand if a magnetic contrast is present in the local subsurface rocks, the lower magnetometer will detect a stronger signal than the higher instrument and there will be a difference between the combined output signals. The gradiometer emphasizeses anomalies from local shallow sources at the expense of large-scale regional variation due to deep-seated sources. Magnetic Field Procedure Magnetic surveys are generally carried out from the air, but ground and marine surveys can also be used. In a simple land survey an operator might use a portable magnetometer to measure the field at the surface of the Earth at selected points that form a grid over a suspected geological structure. This method is slow but it yields a detailed pattern of the magnetic field anomaly over the structure, because the measurements are made close to the source of the anomaly. For land surveys, inter- station spacing varies from 5m to about 50m or more depending on the dept of structure of interest. Base station must be set up and reoccupied every 2 to 3 hrs to keep track of diurnal variations and drift. Stations should not be near railroads, power lines, wire fences, culverts, vehicles, belt buckles, knives, Jewelry etc. Large surveys are covered with air-borne work. In practice, the surveying of magnetic anomalies is most efficiently carried out from an aircraft, since this is both cheap and rapid. The magnetometer must be removed as far as possible from the magnetic environment of the aircraft. This may be achieved by mounting the instrument on a fixed boom several metres long. Alternatively, the instrument may be towed behind the aircraft in an aerodynamic casing at the end of a cable between 30 and 150m long. The ‘’bird’’ Page 13 of 22 © Univation Shell Intensive Training Programme Page 14 of 22 © Univation Shell Intensive Training Programme containing the magnetometer then flies behind and below the aircraft (fig.5.5a). Airborne magnetic surveying is an economical method of surveying a large territory (fig. 5.5c) in a short time and has become a routine part of the initial stage of the geophysical exploration of an unexplored region. A disadvantage of airborne surveying is related to the speed at which it is done, since a small error in heading or speed measurement produces a large error in the calculated position (fig.5.5d), although the use of the Global Positioning Satellite data gets around this problem. The magnetic field in the marine environment may also be surveyed from the air. However, most of the marine magnetic data has been obtained by ship-borne surveying. In the marine application a proton-precession magnetometer mounted in a waterproof ‘’fish’’is towed behind the ship at the end of a long cable. To minimize the large magnetic disturbance caused by towing vessel, the tow-cable must be about 100-300m in length. At this distaance the ‘’fish’’will be located well below the water surface. At a typical survey speed of 10km hr-1 its operation depth is about 10-20m. Magnetic Data Analysis All causes of magnetic variation must be removed from the observations other than those arising from the effects of the subsurface. This process is termed magnetic reduction. In comparison with the reduction of gravity data, magnetic survey data requires very few corrections. Diurnal variation correction The effects of diurnal variation may be corrected by installing a constantly recording magnetometer at a fixed base station within the survey area or the records of a geomatic observatory may be used, provided it is not too distant from the survey area. Also the method of reoccupying an established base station for repeated readings and plotting the data takes care of the diurnal and instrumental drift correction Page 15 of 22 © Univation Shell Intensive Training Programme Geomagnetic (secular variation) correction If a survey is made over a very large area, the variation in the Earth’s internal magnetic field must be taken into account. This variation can be expressed as a simple mathematical function and the expected value at any point must be derived from national or world magnetic contour maps. The expected value of the field at any point on the Earth’s surface is taken to be that of the International Geomagnetic Reference Field (IGRF). Geomagnetic correction removes the effect of geomagnetic reference field from the survey data. However , in small survey areas the necessary corrections are so small and are usually ignored. Unlike gravity anomalies, corrections for elevation and topography do not have to be applied. Magnetic Anomalies The magnetometer observations, after the application of diurnal and, if necessary, secular/geomagnetic corrections, should represent magnetic field variations due entirely to the contrast in magnetization when rocks of different magnetic properties are adjacent to each other. These variations are referred to as magnetic anomalies. Positive and negative anomalies, expressed in units called gamma, record the magnetism of rocks in which the alignment of magnetic poles shows opposite orientations. Magnetic anomalies can be summarised graphically in the form of vertical profiles (fig. 5.6) or as contour maps (fig. 5.7). Page 16 of 22 © Univation Shell Intensive Training Programme Page 17 of 22 © Univation Shell Intensive Training Programme 5.4 Magnetic Data Interpretation The magnetic anomaly profile or contour obtained and displayed at this stage is rightly assumed to be due ONLY to the response from the magnetic properties dispalyed by subsurface rocks. Whatever deductions that are made or calculated are then the interpretation of the presence or absence of magnetic rocks. Qualitative Interpretation For most purposes, the magnetic anomaly profiles are generally interpreted in the form of: (i) Noting and describing areas of high (+ve) and low (-ve) magnetic anomalies (fig. 5.6) (ii) The amplitude of these anomalies and their extent along measurement profile line. (a) In the case of the magnetic anomaly contour presentation, it is necessary to: (i) Note the areas of high magnetic gradient (closely packed contour lines) (fig. 5.7) and areas of low magnetic gradient (sparse distribution of contour lines – fig 5.5c and 5.7). (ii) Identify the trend (attitude/pattern) of the contour and their closures (if any). (iii) Describe the trend noted and figure whether this trend so identified agrees with known geology or suspected feature. 5.4.1 Quatitative Interpretation Magnetic anomaly data presented in a profile form can directly be interpreted although total field data is usually complex to interprete as this may include both the desired induced magnetization and the undesired remanent magnetization. Page 18 of 22 © Univation Shell Intensive Training Programme Page 19 of 22 © Univation Shell Intensive Training Programme In the case of magnetic anomaly contours, a section or sections are usually chosen across the contour in a direction usually intended to cross an identified feature perpendicularly (if possible). When the chosen section is plotted, a profile anomaly is obtained and interpreted. In the absence of other information, the interpretation of a magnetic anomaly is a process of developing graphical models of a hypothetical subsurface structure of assumed magnetic properties, calculating the magnetic anomaly that would be produced. The model is then adjusted until it fits the measured magnetic data as closely as possible (fig. 5.8). Page 20 of 22 © Univation Shell Intensive Training Programme The inverse problem Although it is relatively easy to calculate the magnetic anomaly caused by a structure of known shape and magnetic character, there is no unique solution to the reverse problem of calculating the shape and magnetic properties of a subsurface structure from the anomaly if no other information about it is available. This ambiguity represents the inverse problem of potential field interpretation, which states that although the anomaly of a given body may be calculated uniquely, there are an infinite number of bodies that could give rise to any specified anomaly (fig. 5.8). If magnetic measurements can be combined with another geophysical technique, then interpretation may become more precise. Approximate Interpretation As stated above, the calculation of the depth and dimension of a causative body directly from a magnetic anomaly is only feasible if a given geometrical shape is assumed as the causative body. If a sphere is assumed to be the causative body of a given magnetic anomaly, the Half-width method discussed in gravity data interpretation can be used to estimate the depth of burial to the centre of source (Zc). X1/2 = 0.75 Zc ---(5.8) Zc = 1.33X1/2 ---(5.9) Peter’s slope can also be utilised for these estimates of depth: Zc = 0.635 ---(5.10) S = Peter’s slope which is the horizontal distance between the +ve and –ve peaks of the anomaly. Equations 5.9 and 5.10 are valid only for the vertical components of the field (Hz). For Horizontal components (Hh) equation 5.9 and 5.10 become Page 21 of 22 © Univation Shell Intensive Training Programme Zc = X1/3 ---(5.11) X1/3 is half-width of the anomaly at the point of one third value of the anomaly maximum. Zc = 0.67S ---(5.12) 5.5 Applications Magnetic surveying is a very rapid and very cheap technique. In academic studies, magnetic surveys can be used in investigations of large-scale crustal features, although the sources of major magnetic anomalies tend to be restricted to basic or ultrabasic intrusions. It is in the search for metalliferous ore deposits that magnetic surveying has proved most valuable. It is capable of locating both massive sulphide deposits and iron ores, although the latter must contain a reasonably high proportion of magnetite for a significant anomaly to be produced. While not very widely used in hydrocarbon exploration, magnetic surveying is a very useful aid in geological mapping in areas with thick sedimentary cover and may reveal intrusives, structural features if magnetic horizons such as ferruginous sandstones, tuffs or lavas are present. Alternatively, if the basin fill contains no magnetic sediments, a magnetic survey has the ability to “see through”the cover to disclsoe the nature and form of the crystalline basement and thus the depth and character of the boundaries of the sedimentary basin. In both cases, it might reveal the location of structural traps within the sediments or features of basement topography, which influenced the development of the basin fill. Paleomagnetic studies have revealed the positions of the continents at various times in the past (earth history) such as in sea floor spreading and continental drift. Page 22 of 22 © Univation