Geology 228/378 Environmental Geophysics Lecture 10 Electromagnetic Methods (EM) I And frequency EM (FEM)
Lecture Outline Introduction Principles Systems and Methods Case Histories
Introduction Many EM systems available EM has a wide application Mineral exploration Ground water contamination Salt water intrusion Mapping geology and soil Locating buried objects ( pipes, barrels, tanks, walls) Archeology Locating frozen permafrost Locating Gravel Locating cavities (caves, abandon mines)
Advantages relative to other electrical methods EM is based on induction Does not require electrodes in ground. Faster surveys over larger areas. Can be used from: Land, Airborne, seaborne. Downhole
Disadvantages EM is based on induction Fixed depth of investigation depending on frequency used and Tx-Rx separation. More sophisticated interpretation skill.
Types of EM Systems 1. TEM vs FEM Time-domain (TEM) Measurements as a function of time Frequency-domain (FEM) Measurements at one or more frequencies 2. Passive vs Active Passive: Uses natural ground signals (e.g., magnetotellurics), sources are lightning, magnetosphere activities, etc. Active: use transmitter to induce ground current Near-field ( ground conductivity meters) Far-field (VLF uses very low frequency signals used to communicate with submarines ).
Types of EM Systems Inductive Small loop Most FEM (EM 31, EM 34, etc.) but some TEM Most widely used in environmental investigations Large loop (5 m to 100 m loops) Many TEM systems ( esp. airborne) Mineral exploration, environmental investigations Plane wave (VLF, Magnetotelluric) Mineral exploration, deep geologic structure
Small Loop Systems FEM ( frequency domain EM) Pole, two small coils, one transmitter and one receiver, separated by a constant spacing moved along a survey transect. Geonics EM31
Small loop systems Two coils( transmitter and receiver) connected by wires that permit several different separations and configurations Geonics EM34
Loop configurations HCP (horizontal co-planer) VCP (vertical co-planer) VCA (vertical Coaxial) Others
How does EM Induction work? Magnetism Magnetic lines of force ( owing to alignment of atoms, the H-field)
EM Theory (1) In 1820, Hans Oersted discovered that a magnetic compass could be deflected from its resting position if a wire carrying electric current were placed near the compass. Magnetic Field Any wire in which an electric current is flowing is surrounded by an invisible force field called a magnetic field. This phenomenon is described as the Ampere s law. H = Idl r 2πr 0 2
EM Theory (2) Electromagnetism The term electromagnetism is defined as the production of a magnetic field by current flowing in a conductor. Coiling a current-carrying conductor around a core material that can be easily magnetized, such as iron, can form an electromagnet. The magnetic field will be concentrated in the core. This arrangement is called a solenoid. The more turns we wrap on this core, the stronger the electromagnet and the stronger the magnetic lines of force become.
Right hand being used to find the polarity of the magnetic field around a coil of wire (the thumb is pointing towards the North pole) when you know the direction of the current around the coil (the fingers are wrapping around the coil in the same direction as the current). Notice that all of the lines of force pass through the center of the coil material, regardless of how they extend outside the coil of wire.
EM Theory (3) The magnetic field that surrounds a currentcarrying conductor is made up of concentric lines of force. The strength of these circular lines of force gets progressively smaller the further away from the conductor. if a stronger current is made to flow through the conductor, the magnetic lines of force become stronger. the strength of the magnetic field is directly proportional to the current that flows through the conductor. H = Idl r 2πr 2 0
EM Theory (4) The term field intensity is used to describe the strength of the magnetic field. We have now seen that if electrical current is flowing in a conductor, there is an associated magnetic field created around the wire. In a similar manner, if we move a wire inside a magnetic field there will be an electrical current that will be generated in the wire. This is described as the Faraday s law.
EM Theory (5) Induction Current is produced in a conductor when it is moved through a magnetic field because the magnetic lines of force are applying a force on the free electrons in the conductor and causing them to move. The direction that the induced current flows is determined by the direction of the lines of force and by the direction the wire is moving in the field. If an AC current is fed through a piece of wire, the electromagnetic field that is produced is constantly growing and shrinking due to the constantly changing current in the wire. This growing and shrinking magnetic field can induce electrical current in another wire that is held close to the first wire. The current in the second wire will also be AC and in fact will look very similar to the current flowing in the first wire.
If we move a wire in a magnetic field, the movement will create a current in the wire. Essentially, as we cut through the magnetic lines of force, we cause the electrons to move in the wire. The faster we move the wire, the more current we generate. Again, the right hand helps determine which way the current is going to flow. If you hold your hand as is shown in the diagram below, point your index finger in the direction of the magnetic lines of force (N to S...) and your thumb in the direction of the movement of the wire relative to the lines of force, your middle finger will point in the direction of the current.
Principles of EM Surveying Generate EM field by passing an AC through a wire coil ( transmitter) EM field propagates above and below ground. If there is conductive material in ground, magnetic component of the EM wave induces eddy currents (AC) in conductor. The eddy currents produce a secondary EM field which is detected by the receiver. The receiver also detects the primary field (the resultant field is a combination of primary and secondary which differs from the primary field in phase and amplitude). After compensating for the primary field (which can be computed from the relative positions and orientations of the coils), both the magnitude and relative phase of the secondary field can be measured. The difference in the resultant field from the primary provides information about the geometry, size and electrical properties of the subsurface conductor.
Secondary field can be converted to components in-phase and 90 out of phase with the transmitted field. The out-of-phase (or quadrature-phase ) component, using certain simplifying assumptions, can be converted to a measure of apparent ground conductivity. The in-phase component, while generally not responsive to changes in bulk conductivity, is especially responsive to discrete, highly-conductive bodies such as metal objects. The apparent conductivity measurement is the average conductivity of one or more layers in the ground in the proximity of the instrument, to a depth of investigation dependent on the coil spacing, orientation, operating frequency of the instrument, and the individual conductivity of each ground layer.
Depth of penetration Function of frequency and ground EC Skin depth (SD): amplitude of plane wave has decreased to 1/e or 37% relative to the initial amplitude (Ao) or Az = Ao e -1 SD = (2/ωσ).5 = 503(fσ).5 ω = 2πf, f = frequency (HZ), σ = EC (S/m)
FEM: General Principles of EM Operation Transmitter produces continuous EM field, secondary field is determined by nulling the primary field ( need two coils) TEM Primary field is applied in pulses ( 20-40ms) then switched off and the secondary field measured ( same coil can be transmitter and receiver, more often large coil on ground and move small coil around)
Factors influencing subsurface electrical conductivity Mineralogy Clays more conductive (relates to CEC) Moisture content Porosity EC of the subsurface water Stratigraphy Structure Temporal Changes in soil EC due to soil moisture change, water table changes, soils are frozen ( Low EC), soil temperature changes (lowers EC of soil water). Adding or subtracting soluble constituents (contaminants) source strength variations and directions of ground water flow. Presence of NAPLs
EC = 1/R crystalline rock 10 5 ohm-m <=> 10-5 S/m = 10-2 ms/m = 10µS/m = 0.1 µs/cm Clay soil 10 ohm-m <=> 0.1 S/m = 10 2 ms/m = 10 5 µs/m = 10 3 µs/cm Water EC = TDS(mg/l)/.5
Relative Response φ Horizontal dipole Vertical dipole z dz z = normalized depth: =depth/(inter-coil spacing); φ= relative contribution to Hs from a thin layer at depth z; For Vertical dipole, max contribution of layer is at.4z, not sensitive to surface conditions.
Layered Earth Models 25% of Hs below 2z or 25% of σ below 2z
Using different spacing and configurations in Modeling
Advantages Relative to DC Resistivity Less sensitive to conditions at surface of ground No problems with coupling to ground since it is inductive. Perform simple multilayered earth calcs. Easy and Rapid Measurements On plane and boat
Disadvantages relative to DC resistivity Limited dynamic range (1-1000 mmhos/m) Low EC: can t readily induce current High EC: EC not linear function of H Setting instrument to zero Ideally needs to suspending in free space Reality set to zero rel. prevailing conditions Limited vertical sounding capability
Survey Instruments
EM 31 GEONICS
EM 31 Characteristics Intercoil spacing of 3.7 m. Effective depth of exploration = 6 m (pole horizontal), 3 m (pole vertical) Detect layering by rasing and lower instrument. Procedure: Lay out survey line with a measuring tape, walk to measurement location, turn on transmitter read apparent conductivity ( in millimhos/m)
EM 34 GEONICS
EM 34 Characteristics Two person instrument Intercoil spacing of 10, 20 and 40M Intercoil spacing is measured electronically, read meter to accurately set spacing.
Survey procedure: (1) Lay out survey line with tape (2) Transmitter operator stops at measurement station. (3) The receiver operator moves coil forward and back until his meter indicates correct intercoil spacing. (4) The transmitter operator reads apparent conductivity in millimhos/m. (5) Takes 10-20 sec per reading. (6) Normally survey in horizontal dipole mode ( coils vertical) which is less subject to coil misalignment. (7) you can also use vertical dipole ( coils horizontal).
EM 31 and 34 relation of H to σ Instruments are designed to operate at: Specific fixed frequencies, Fixed inter-coil spacings and at Fixed Hp Given above instrument constraints: σ directly proportional to Hs/Hp Depth of penetration primarily function of instrument configuration
The basic GEM-3 Package consists of: a 64-cm diameter sensing head, handle boom, console and display unit, and battery charger. Standard software includes WinGEMv3, Windows-based operation software. The optional 96-cm head, due to its size, must be mounted on a cart. Programmable Operation Bandwidth 30 Hz to 24 khz Frequency domain Single, multiple, or stepping frequencies Maximum sampling rate Approx. 15 Hz at one frequency or 8 Hz at 10 frequencies
Airborne Surveying GEOTEM T and R separations 20-135 m
The World s Most Advanced HEM System Redefining Helicopter Electromagnetics Reliable, Repeatable, Precise 3D RESISTIVITY Unsurpassed Horizontal and Vertical RESOLUTION RESOLVE your Questions. SOLVE your Exploration Problems RESOLVE -a unique six frequency system with horizontal coplanar coils capable of measuring the EM response at 400Hz, 1500Hz, 6400Hz, 25kHz, 100kHz, and one coaxial coil pair at 3300Hz. Designed for the calculation of 3D earth resitivity models, overburden thickness, layered inversions, EM-derived susceptibility and other advanced products. Horizontal coplanar coil pairs combined with a coaxial coil pair are excellent for interpreting conductors. RESOLVE is fully digital, offering lower noise and real-time signal processing as well as internal calibration coils for automatic phase and gain calibration in the air - out of ground effect - resulting in higher accuracy and reduced drift. RESOLVE offers the exploration professional horizontal and vertical resolution unparalleled in an airborne EM system. Multiple coplanar coils are exceptional for mapping horizontal layers. http://www.fugroairborne.com/services/airborne/em/resolve/index.shtml
Saltwater intrusion along the Baton Rouge Fault (Kuecher, 2004)
Modeling 3 Layer Earth
GEONICS EM 61