Section 3: Magnetics Surveying

Introduction

Magnetics surveys measure the magnitude and orientation of the Earth’s magnetic field.

Magnetic field at Earth’s surface depends on field generated in Earth’s core, magnetic mineral content of surface materials, and remnant magnetisation of surface rocks.

Magnetic susceptibility, k, is physical parameter to which magnetic surveys are sensitive.

Applications

• Location of metal objects: Pipes, cables, military ordnance.

• Mapping near-surface: Archeological sites, concealed mine shafts, igneous intrusions.

• Mineral Exploration: Identification of metalliferous deposits, for example massive sulphides

• Geological Bedrock Mapping: Identification of faults and geological boundaries, especially beneath sediment cover.

History of Magnetics

• 2^nd Century BC: Chinese used lodestone (rock rich in magnetite) for direction finding.

• 12^th Century AD: Magnetic compass in use in Europe.

• 1600: First scientific analysis of Earth’s magnetic field by William Gilbert in book De Magnete.

• 1640: First use of magnetic measurements to locate iron ore deposits in Sweden.

• 1870: Development of instrumentation for rapid, accurate measurement of magnetic field by Thalén and Tiberg.

• 1915: Development of balance magnetometer by Adolf Schmidt.

• WWII: Rapid development of magnetic surveying technology for mine and submarine detection.

• 1960s: Sensitive optical absorption magnetometers allow airborne magnetics surveying.

• 1970s: Development of magnetic gradiometers to measure field difference between two sensors.

Example of Magnetic Force, Flux, and Field

A field exists if an object placed in that field experiences a force.

• Iron filings spread over bar magnet or around wire carrying electric current orient in direction of magnetic field.

• Magnetic field said to exist around bar magnet or wire, and exerts a magnetic force that aligns the iron filings.

• Magnetic flux, corresponds to closeness of field lines, and converges at magnetic poles near ends of magnet.

• Suspended magnet aligns itself along Earth’s magnetic field. North-seeking (positive) pole of magnet will point towards Earth’s north pole.

Definition of Magnetic Force

Magnetic poles always exist as dipoles, pairs of opposite polarity, poles. If one pole sufficiently distant so does not affect other, it is said to be a monopole.

Magnetic Force is defined in terms of monopoles:

• If two magnetic poles of strength m₁ and m₂ are separated by distance r, the magnitude of force between them is given by:

where m is magnetic permeability of medium .

• Force is repulsive if poles have same sign, attractive if opposite sign.

Magnetic Field Strength H

• Magnetic field strength vector H is defined as force that would act on a unit positive pole placed in field:

• Magnitude of H represents "closeness" of flux lines. Direction of H along flux lines. (assuming no magnetic materials present).

• Magnetic field strength also defined in terms of current flowing through a loop of wire. By Biot-Savart’s Law, the magnetic field produced is equivalent to magnetic dipole at centre of loop.

Induced Magnetisation and Magnetic Susceptibility

Orbital motions of electrons around atoms’ nucleus constitute circular electric currents, causing atoms to behave like magnets.

Intensity of Magnetisation J

A body placed in a magnetic field can become magnetised as atoms and molecules align. Net external field as if bar magnet.

• Magnetic field is induced body, which is called the intensity of magnetisation J. (Also called magnetic polarisation).

• If J has same amplitude and direction throughout body, body is said to be uniformly magnetised.

Magnetic Susceptibility k

For low magnetic fields, magnetisation J is proportional to the magnetising field H:

J = k H

where k is called the magnetic susceptibility.

• Susceptibility is fundamental rock parameter of magnetics prospecting.

• Magnetic response of rocks determined by amounts and susceptibilities of constituent minerals.

Total Magnetic Field B

The Total Magnetic Field B represents the sum of the magnetising field strength and the magnetisation of the medium:

B = m₀(H + J) = m₀(H + k H) = m_rm₀ H = m H

where m₀ is magnetic permeability of free space (4p x10^-7 H/m)

m_r is relative magnetic permeability

m is absolute magnetic permeability

Clearly, m_r = m / m₀

B is also called the magnetic flux density or magnetic induction.

• Magnitude of B represents "closeness" of flux lines. Direction of B along flux lines.

• Magnetic field measured in volt. s /m² = weber/m² = teslas (T) in SI units.

• m measured in weber/ (amp.m) = henry/m (H/m)

• In c.g.s system, magnetic flux measured in gauss or gamma 1 g = 10^-5 gauss = 1 nT.

• In geophysics, magnetic fields are small and measured in nT. Earth’s magnetic field varies between 20,000 to 60,000 nT.

B vs H

There is often confusion between B and H. In practice, this mostly doesn’t matter, because for measurements in air m_r = 1 (i.e. k = 0, can’t magnetise air or a vacuum), and B = m₀ H.

Induced and Remnant Magnetisation

Induced Magnetisation

Induced Magnetisation J_i is produced within a rock in response to an applied external magnetic field.

Remnant Magnetisation

Magnetic field may exist within rock even in absence of external field due to permanently magnetic particles. This is remnant or permanent magnetisation.

Interpretation of magnetic data complicated as magnetic field due to a subsurface body results from combined effect of two vector magnetisations that may have different magnitudes and directions.

Diamagnetism and Paramagnetism

All atoms have a magnetic moment due to orbit of electrons around nucleus and spin of elections moment (i.e. behave like a small bar magnet).

According to quantum theory, two electrons can exist in same electron shell if they have opposite spins. Magnetic moment of paired electrons will cancel out

In most materials, no overall magnetisation exists in absence of external field, because the magnetic moments of adjacent atoms are randomly distributed and cancel.

Diamagnetism

• In a diamagnetic material such as halite, all electron shells are complete and no unpaired electrons exist.

• In an external magnetic field, electrons orbit to produce a weak magnetic field that opposes applied field.

• Magnetic susceptibility is weak and negative.

Paramagnetism

• In minerals such as olivine, unpaired electrons in incomplete electron shells produce unbalanced magnetic moments.

• In an external field, magnetic moments align themselves in same direction, producing a weak magnetic field aligned with external.

• Magnetic susceptibility is weak and positive, but usually an order of magnitude stronger than in diamagnetic materials.

Ferromagnetism

In metals such as cobalt, nickel and iron, unpaired electrons are coupled magnetically due to strong interaction between adjacent atoms and overlap of electron orbits.

• Groups of atoms that couple together magnetically are called magnetic domains, around 1 micron in size.

• Magnetic domains can be oriented to produce a spontaneous magnetic field in absence of external field.

• Magnetic susceptibility is large, but depends on temperature and strength of applied field. All domains oriented in same direction.

• Ferromagnetism disappears if material heated to Curie Temperature, T_C, as inter-atomic couping restricted and domains cannot exist.

• Behaves as paramagnetic above Cure temperature.

Magnetic Domains

Unmagnetised Domains

• Each domain possesses a random magnetic field orientation.

Induced Non-Permanent Ferromagnetism

• Domains aligned with external field grow at expense of others as external magnetic field increases.

Induced Permanent Ferromagnetism

• Electrons in domains rotate to align with external field as it increases.

Antiferromagnetism and Ferrimagnetism

Antiferromagnetism

• In minerals such as hematite, magnetic domains form, but are aligned in antiparallel fashion.

• Magnetic fields cancel out, but crystal lattice defects cause small net field in response to applied external field.

• Magnetic susceptibilities are large and positive.

Ferrimagnetism

• In minerals such as magnetite, titanomagnetite, and ilmenite, magnetic domains are antiparallel, but of unequal magnitude.

• Net magnetisation produced with applied external field.

• Magnetic susceptibilities are very large and positive.

• Domains can be permanently aligned, producing spontaneous magnetisation that exists after removal of external field.

• Ferrimagnetism disappears above Cure temperature.

Magnetisation of Ferromagnetic Materials

Magnetisation

• For an unmagnetised material with magnetic domains in absence of external magnetic field H, domain orientation related to crystal axes and magnetisation cancels out.

• When external field H is applied, domains reorient themselves in discrete steps to parallel H.

• Point is reached where no further increase in magnetisation, as all domains aligned with external field H. Material is saturated.

Hysteresis Loop in Magnetisation

• After magnetisation, ferro/ferri-magnetic material retains magnetisation when H decreased to zero,

• Remnant magnetisation can be removed by external reversing magnetic field.

• Magnitude of reversed magnetic field H_C , the coercivity, needed to eliminate remnant magnetisation is measure of its "hardness" or permanence.

• Same process can be applied in reverse to return to full positive magnetisation. This process is called a hysteresis loop.

Note: Small loop is hysteresis without saturation.

Curie Temperature

Cure temperature is temperature at which mineral loses its ferromagnetic behaviour, and any permanent magnetisation is lost.

• Cure temperature varies with mineral:

Titanomagnetite 100-200^o C

Titanomaghemite 150-450^o C

Magnetite 550-580^o C

Hematite 650-680^o C

• Curie temperature is below melting point of rock.

• In rock such as granite, there will be multiple Curie temperatures for the different minerals present.

Low-Temperature Oxidation

• Oxidation at less than 300^o C, tends to increase Curie temperature, as titanomagnetite oxidese towards hematite.

• Intensity of magnetisation also reduced.

• Oxidation can affect expected magnetic response of certain rocks.

Magnetic Susceptibilities of Rocks and Minerals

Magnetic susceptibility k is the physical parameter of magnetics surveying (equivalent to density in gravity).

Rocks with significant concentrations of ferri/ferro-magnetic minerals have highest susceptibilities:

Ultramafic rocks highest 95,000 – 200,000

Mafic rocks high 550 – 122,000

Felsic rocks low 40-52,000

Metamorphic low 0-73,000

Sedimentary very low 0-360

Measured Values of Magnetic Susceptibility

Primary Remnant Magnetisation

Rocks can become permanently magnetised in the Earth’s magnetic field,.

It is this that permits tracing past plate motions and locating magnetic ores.

Primary remnant magnetisation refers to permanent magnetisation created during formation of a rock.

Thermal Remnant Magnetisation (TRM)

• TRM acquired when a rock cools through the Curie temperature, e.g. colling of volcanic rock.

• At Curie temperature, ferro/ferri-magnetic minerals become magnetised in direction of Earth’s magnetic field at that time.

• TRM usually greater than induced magnetisation from present day field.

Detrital Remnant Magnetisation (DRM)

• DRM acquired when fine magnetic particles settle during formation of sedimentary rock, e.g. formation of clays

• Settling particles are oriented by Earth’s magnetic field at that time.

• DRM << TRM

Secondary Remnant Magnetisation

Secondary remnant magnetisation refers to magnetisation acquired later in a rock’s history by alteration processes.

Chemical Remnant Magnetisation (CRM)

• CRM acquired in situ when magnetic minerals grow or are chemically altered to another form below Curie temperature, e.g. growth of iron oxide in sandstone.

• Most common in sedimentary and metamorphic rocks.

Viscous Remnant Magnetisation (VRM)

• VRM produced by long exposure to ambient field in uniform environment.

• Buildup of VRM logarithmic with time.

Königsberger Ratio

Remnant magnetisation may be much greater than that induced by Earth’s field today, e.g. with TRM.

Königsberger Ratio Q is measure of ratio of intensity of remnant to induced magnetisation:

• Q ~ 30-50 for rapidly quenched volcanic rocks

• Q ~10 for volcanic rocks in general

• Q~1 for slowly crystallised igneous anmd thermally metamorphosed continental rocks

• Q<1 in sedimentary and metamorphic rocks when iron not present

Earth’s Distant Magnetic Field

Near the Earth’s surface: magnetic field originates largely from currents flowing in the liquid outer core, and the magnetisation of surface rocks.

Away from the surface: magnetic field is affected by currents caused by the movement of charged particles associated with Van Allen radiation belts.

Some of these charged particles are responsible for the Aurora Borealis near the poles.

At great distance: the magnetic field is due to charged particles from the sun, the solar wind.

Earth’s Dipole Field

Earth’s magnetic field roughly appears as if it originated from a large bar magnet located at the centre of the Earth oriented at 11.5^o to the axis of rotation.

Earth’s Magnetic Field

Geomagnetic Pole: the position on Earth’s surface intersected by the axis of the dipole that fits best the Earth’s magnetic field.

o North: Hayes Peninsula in northern Greenland

o South: Vostock research station in Antarctica

Magnetic Pole (or Dip Pole): the position where the magnetic field is vertical

o North: North of Bathurst Island in Canadian Arctic

o South: 150 km offshore off Adelie coast of Antarctica

Geomagnetic and Magnetic Poles differ slightly because Earth’s magnetic field is not quite a dipole.

Generation of Earth’s Magnetic Field

Exact mechanism responsible for generation of Earth’s magnetic field is not known.

Believed to be associated with electrical eddy currents induced within the liquid outer core by its slow internal convection.

Secular Variation: Magnetic field is slowly changing due to core processes, e.g. location of south magnetic pole:

Description of Earth’s Magnetic Field

A compass needle free to move in 3-D will point along the magnetic field, i.e. it will point down where field points into Earth.

Geomagnetic field can be described by the declination D, the inclination I, and total field vector F.

Can calculate (magnetic) latitude, l, from inclination:

Earth’s Non-Dipolar Field

90% of Earth’s magnetic field can be represented by a dipole.

Difference between the actual magnetic field and that of the best-fitting dipole is called the non-dipolar field.

Features in non-dipolar field with magnitudes of 20,000 nT extending for 1000s km.

Non-dipolar field can be represented as 8-12 small dipoles locate radially close to liquid core, simulate cores eddy currents.

International Geomagnetic Reference Field

Geomagnetic field can be represented mathematically, and international standard is called International Geomagnetic Reference Field (IGRF).

Total field is recalculated every 5 years because of secular variation. Year of calculation is called the epoch.

Total field intensity of the IGRF epoch 1980:

• A true dipole field would have two maxima, but Earth’s magnetic field actually has four.

• IGRF excludes effects of near-surface rocks.

Variations in Earth’s Magnetic Field

Geomagnetic Reversals

Earth’s magnetic field flips polarity unpredictably on geological time scale due to sudden changes in fluid motions in core.

Secular Variations

• Observations of Earth’s magnetic field made over 400 years show a gradual change in position of the magnetic pole.

• Due to slow movement of eddy currents in Earth’s core.

Diurnal Variations

• Daily changes in field due to changes in currents of charged particles in ionosphere.
• Changes are smooth and average around 50 nT.

Magnetic Storms

• Short term disturbances in magnetic field associated with sun spot activity and streams of charged particles from sun.
• Can be up to 1000 nT in magnitude, and make magnetic surveying impossible.

Torsion and Balance Magnetometers (Obsolete)

Magnetometers measure the total magnetic field F_T or the horizontal and/or vertical components of magnetic field, F_H and F_Z respectively.

First magnetometers devised in1640 essentially comprised:

• a magnetic needle suspended on a wire (Torsion type), or
• a magnetic needle balanced on a pivot (Balance type)

Needle oriented in direction of magnetic field at station location.

Adolf Schmidt Variometer

Magnetic beam asymmetrically balanced on agate knife edge, and zeroed at base station.

• Different magnetic field at another station caused displacement of beam, which was measured using collimating telescope.

• Had to be oriented perpendicular to magnetic meridien to remove horizontal component of Earth’s field. (Use compass?)

• Calibrated to read vertical magnetic field component.

Fluxgate Magnetometer

Measures component of magnetic field parallel to cores with accuracy of 1-10 nT.

Comprises two parallel cores of high m ferromagnetic material.

Primary coil wound on two cores in series in opposite directions. Secondary coils also wound, but in opposite direction to primary.

Operation of Fluxgate Magnetometer

• An alternating current at 50-1000 Hz is passed through primary coils, producing magnetic field that drives each core to saturation through a magnetisation hysteresis loop.

• With no external magnetic field, cores saturate every half cycle.

• Voltages induced in secondary coils have opposite polarity as coils wound in opposite directions. So zero net voltage.

• In Earth's magnetic field, component of field parallel to cores causes one core to saturate before the other, and voltages in secondary coils do not cancel.

Principle of Operation of Fluxgate Magnetometer

• Principle behind operation is Faraday’s Law of Induction (twice)

• Voltage induced in secondary coil proportional to magnetic field generated in ferromagnetic core.

• When core saturated, magnetic field does not change, and no voltage is induced in secondary coil.

Proton Precession Magnetometer

Uses sensor consisting of bottle of proton-rich liquid, usually water or kerosene, wrapped with wire coil.

Two sensors indicates a gradiometer

• Protons have a net magnetic moment, and so are oriented by Earth’s magnetic field or an applied field.

• Measures precession as protons reorient to Earth’s field.

• Precession frequency proportional to total field strength.

• As sensor bottle 15 cm long, accuracy of measurement is reduced in areas of high magnetic field gradient.

• Measures total field strength, so instrument orientation not important, unlike fluxgate.

• Overhauser Effect adds electron-rich fluid to enhance polarisation effect, and increase accuracy.

Principle of Operation of Proton Magnetometer

A. In ambient field, majority of protons aligned parallel to field, remainder antiparallel.

B) Current in coil generates strong magnetic field at right angles to Earth’s field, causing all protons to align.

C.When current turned off protons precess back to orientation of Earth’s field.

• Protons are charged particles, and create magnetic field, which alternates as proton precesses.
• Current induces alternating voltage in coil at precession frequency.

C.Measuring frequency of current in coil gives magnitude of Earth’s total magnetic field as it is proportional to precession frequency.

D.Measuring current frequency to 0.004 Hz gives field to ±0.1nT.

Airborne and Seaborne Magnetometers

Proton precession magnetometers are used extensively in marine and airborne surveys:

• At sea: sensor bottle is towed in a "fish" 2-3 ship’s length astern to remove it from magnetic field of ship

• In air: sensor is towed 30 m behind aircraft or placed in a "stinger" on nose, tail or wingtip.

Often active compensation for magnetic effect of aircraft is calculated. Effectiveness of compensation is called Figure of Merit (FOM).

Advantage:

Aeromag is rapid, cost-effective method for covering large areas.

Magnetic Gradiometers

Gradiometers use two sensors separated by fixed distance to measure gradient of the Earth’s magnetic field:

• In airborne work, separation is 2-5 m for stinger, up to 30 m for bird.

• In ground work, separations of 0.5 m are common.

Example of 3-axis gradiometer system:

Advantages:

• No correction for diurnal variation required as measurement is difference off two magnetic sensors.

• Vertical gradient measurements emphasise shallow anomalies and suppress long wavelength features.

Magnetic Surveying

Ground Surveys

• Ideally lines should be perpendicular to strike, with a few along strike tie-lines.

• Establish base-station to monitor diurnal variations every 0.5-1.0 hours.

• Avoid readings near metal objects such as railway tracks, cars...

• Avoid wearing metal objects, such as watch, geological hammer.

Airborne Surveys

• Estimate line spacing to avoid significant signal aliasing for aircraft height.

• Approximate rule of thumb for maximum line spacing for particular application:

Note that h is flight height above magnetic basement, not Earth’s surface.

Reduction of Magnetic Survey Data 1

Magnetics data reduction is usually simpler than with gravity, comprising:

1. Diurnal Correction

2. Geomagnetic Correction

3. Elevation/Terrain Correction (occasionally)

Diurnal Variation

• Similar to tidal correction in gravity

• Reading is recorded at base station during survey, and then corrections applied to survey data.

• Difficult to return to base station in airborne work: possible to estimate diurnal correction from line intersections especially with additional tie lines

Reduction of Magnetic Survey Data 2

Geomagnetic Correction

Similar to latitude correction in gravity: produces "anomaly" data

Earth’s total magnetic field varies from 25,000 nT at equator to 69,000 nT at poles

Three possible correction methods:

1) Subtraction of IGRF: Earth’s theoretical magnetic field is removed from survey data by subtracting IGRF

2) Linear approximation to IGRF: Earth’s field is approximated by linear variation across survey area, and subtracted:

For example, in UK IGRF is approximated by 2.13 nT/km north, and 0.26 nT/km west.

3. Regional correction: With large surveys, regional trend can be estimated and removed to leave residual anomaly, as with gravity data.

Terrain Correction

• There are no elevation corrections (equivalent to Free-air and Bouguer corrections) with magnetic data as gradient is only 0.035 nT/m at poles, 0.015 nT/m at equator.

• Terrain corrections can be applied, but are complicated. Require estimate of ground susceptibility, and topography.

Shape of Magnetic Anomalies

Interpretation more complicated than gravity data because:

1. Earth’s magnetic field is dipolar: single body can appear as peak and trough

Example

Vertical component of magnetic field induced in body inclined at 60^o parallel to Earth’s magnetic field (no remnant magnetisation)

• Earth’s field removed by IGRF correction.
• Induced field strength maximum in direction of Earth’s field.
• Sign convention: downwards is positive, negative upwards.
• Body has negative anomaly on side towards pole

1. Remnant magentisation unknown: strength and direction J_r can distort anomaly shape

Qualitative Interpretation of Magnetic Anomalies

General inferences can be made from magnetic anomaly shapes

Example

• Anomaly B is same form as A, but has longer wavelength, so must be deeper.
• Amplitude of B greater than A, so B has greater magnetisation.

Summary

Qualitative Profile Interpretation

Identify zones with different magnetic properties:

• Zones with little variation, "magetically quiet", associated with rocks of low susceptibility
• Sources in subsurface in "magnetically noisy" areas.

Example: Mineralisation in granite (Dartmoor, UK)

• Profile quiet except around mineralised zone.
• Negative on north side indicates direction J_i >>J_r as anomaly not distorted

Example: Geochemically identical dolerite dykes (Arran, Scotland)

• J_r high as no dipolar anomaly shape
• Two peaks and trough, intruded after magnetic reversal

Qualitative Map Interpretation

Magnetic data acquired on ugrids can be displayed as maps

Example: Shetland Islands, Scotland

Interpretation in terms of magnetic characteristics:

• Elongate lows correspond to gneissified semipelites

• Can also identify fault at discontinuity

Magnetic Profile Across Buried Sphere

Magnetic data are often interpreted in terms of specific geometric forms that approximate subsurface bodies.

Sphere or dipping sheet most common and no remnance assumed

Sphere

Example with F = 50000 nT, I=60^o, D=0^o, k=0.05 for sphere radius 1 m at 3 m depth located at x=15 m.

• Vertical and Horizontal components also shown in addition to total field

Magnetic Profile Across Vertical Dyke

Example

Total field over 50 m thick vertical dyke with F = 50000 nT, I=60^o, D=0^o, k=0.05

• Typical peak-trough anomaly arises when dyke strikes E-W

• Anomaly is symmetric when strike is N-S. True for all regular bodies.

Magnetic Profile Across Flat Slab

Example

Total field over 70 m thick, 400 m long flat slab located 30 m below surface with F = 50000 nT, I=60^o, D=0^o, k=0.05

• Slab striking N-S has symmetric anomaly

• Anomaly is broader than for thinner vertical dyke

Effect of Change of Position on Magnetic Profile

• Change in Depth: Anomaly will broaden and decrease in amplitude with increas in depth.

Total field over 10 m wide vertical dyke oriented E-W

• Change in Dip: Shape of anomaly is altered

Total field over 5 m wide dyke with varying dips

Effect of Change of Latitude

Unlike gravity, magnetic anomaly shape changes with latitude, because orientation and magnitude of Earth’s total field varies.

Changes induced magnetisation.

Example

5 m thick dyke dipping at 45 degrees to north with E-W strike with different magnetic inclinations

In Northern Hemisphere:

In Southern Hemisphere:

Depth Determination

Can get very approximate depth from magnetic anomalies

Sphere or half-cylinder: Depth to centre of body w is roughly equal to width of anomaly peak at half its maximum value dF_max/2.

Dipping Sheet or Prism: Depth to centre of body is roughly width of linear segment of anomaly d.

Peter’s Half-Slope Method (~theoretically-based)

• Draw tangent at point of maximum slope (line 1)
• Find two tangents to curve with half maximum slope (lines 3, 4)

• Depth to top of body is distance between tangent points

Application to Mineral Exploration

Example from Saramäki deposit, Finland

Target is massive copper in mineralised black schists, beneath 30 m glacial overburden.

• Resistivity anomalies due to black (carbon-rich) schists, not Cu

• Entire skarn zone has high density, not just Cu, so connot see subunits

• Shows up well in magnetics

Modelling of Saramäki ore body

• Drill information suggested that magnetisation of body heterogeneous, and upper part dominated anomaly.

• 2-D magnetics modelling could reproduce anomaly across body, but with incorrect dip if no remanence assumed

• Including remanence with 45^o inclination and 90^o declination in model allowed same match, but with correct dip.

Detection of Underground Iron Pipes

Possible to identify underground pipes, and sometimes joints between sections, reducing excavation required for repairs.

Each individually cast segment behaves as dipole, causing repetition of anomalies along length

Example 1: E-W oriented pipe composed of 6.3 m segments with diameter 0.5 m at 0.5 m depth

Gradiometer Data contoured at 200 nT/m

• Magnetic highs offset 0.5-1.0 m from joints along pipe

• Get typical peak-trough anomaly for E-Q oriented feature

Example 2: N-S oriented pipe with 7.6 cm diameter

Gradiometer data contoured at 50 nT/m

• More symmetric anomaly due to N-S orientation of pipe

Stanford Environmental Test Site Layout

Number of objects typical found in near surface buried in test site, which was surveyed

Magnetics Survey of Stanford Test Site

• Pipes stand out clearly due to linear trend

• Non-magnetic objects such as plastic drums or tree stumps have minimal anomaly