Magnetic method Magnetic force and field strength for pole strength m’ and m

Magnetic method Magnetic poles only exist in pairs  magnetic moment M C = magnitude of couple

Magnetic method Intensity of magnetization J k is the magnetic susceptibility which describes the degree to which a body is magnetized when put in an external field H. k is the fundamental parameter in magnetic prospecting.

Magnetic method Diamagnetic minerals have low negative susceptibilities (quartz, feldspar) Paramagnetic minerals have low positive susceptibilities (olivine, pyroxene) Ferromagnetic material have strong susceptibilities (iron, nickel, cobalt) Antiferromagnetic mineral have a net (almost) zero magnetic moment (hematite) Ferrimagnetic minerals have a net magnetic moment (magnetite, illmenite)

Magnetic method

Magnetic method

Magnetic method If a magnetic material is placed in an external field H, its internal poles line up to produce a field on their own H’ producing a total field B

Earth’s Magnetic field Main field (core: mostly dipolar) Small external field, changes rapidly with time Variations of the main field due to local magnetic anomalies (targets)

Earth’s Magnetic field

Earth’s Magnetic field

Dipole equations

Measuring the magnetic field Flux-gate magnetometer measures all components of the magnetic field. Approximately 1nT precision

Measuring the magnetic field Proton-precession magnetometer only measures the intensity of the magnetic field. Approximately 1nT precision

TOTAL field anomalies

TOTAL field anomalies We measure FET which is FAT plus FEU The Earth’s field is much stronger than that of the anomaly if low susceptibility We define a body and calculate HA and ZA

Field procedures Magnetic cleanliness (watches, pens, cars, power lines….) Short-term variations in the external field of a few nT Returning to base Continuous recording at base Storms!

Field procedures Elevation correction Approximately 0.03 nT/m , normally neglected because lost in noise Horizontal correction Approximately 6 nT/km

Field procedures

Interpretation more difficult than for gravity Positive and negative poles Horizontal and vertical component

Magnetic effect of simple shapes: monopole

Magnetic effect of simple shapes: monopole

Magnetic effect of simple shapes: dipole

Fig. 7.20g top

Fig. 7.20g middle

Fig. 7.22g

Magnetic effect of simple shapes: sphere Poisson’s relation, where U gravitational potential, w direction of magnetization

Magnetic effect of simple shapes: sphere

Fig. 7.25g

Fig. 7.27g

Fig. 7.28g top

Fig. 7.28g bottom

Fig. 7.30g top

Fig. 7.30g bottom

Fig. 7.30g bottom

Fig. 7.31g

Fig. 7.33g bottom

Fig. 7.33g top

INTERPRETATION of magnetic data Difficulties No unique solution additional information Remnant magnetization Large variability and non-uniform distribution of susceptibility Total-field measurements only Dependence of anomaly on direction of magnetization

INTERPRETATION of magnetic data Advantages Low cost - high precision Orientation of Earth’s field is constant for given survey  compare to appropriate characteristic curves Large anomalies due to few rock types with high susceptibility Poisson’s relation can turn magnetic into pseudo gravity data Similar techniques to gravity

Half-maximum technique Less precise than with gravity even if we know the shape. Example: thin vertical rod (monopole) Sphere and cylinder: width at ZA2 = z Semi-infinite sheet: (xmax-xmin)/2 = z

Fig. 7.34g bottom

Fig. 7.34g top

Fig. 7.35g SLOPE methods Peters: z=d/1.6 (prism depth ~ width << length, strike infinite // meridian)

Fig. 7.35g Applications Archaeology: often iron objects (high susceptibility) associated with ancient sites, high remnant magnetism in production of bricks etc. Voids and well castings, steal objects, bombs Landfill geometry Geology

Fig. 7.38g

Fig. 7.40g