Em Geophysics Unibraw 21.02.2014

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  • Universitas BrawijayaFebruary 21, 2014

    FUNDAMENTAL OF ELECTROMAGNETIC METHODS IN EXPLORATION GEOPHYSICS

    Djedi S. WidartoSr. Geoscientist / Chief New Energy & Green TechnologyUpstream Technology CenterPT PERTAMINA

  • Education : BS in geology (ITB, 1985) M.Eng. in mineral resources engineering (Waseda University, 1991) Dr.Sci. in science, geology and mineralogy (Kyoto University, 1994)

    Working Experiences : March 1986 June 2008, Research Ctr for Geotechnology, LIPI, Bandung

    Last post: Principle Researcher in Applied Geophysics June 2008 present, Upstream Technology Center, PT Pertamina (Upstream), Jakarta

    Present Position: Senior geoscientist / Specialist in electromagnetic geophysicsChief of New Energy & Green Technology, Upstream Technology Center, PT Pertamina

    Award : 2006, Peneliti Utama Terbaik Indonesia, Riset Unggulan Terpadu, Kementerian RISTEK 2006 2004-2005, National Science Council Scholarship Award, National Central Univ, Taiwan 1995 2008, Japan Society for the Promotion of Science, Research Scientist at Japanese universities (Kyushu,

    Hokkaido, and Chiba Universities) 1997, TWAS/UNESCO Scholarship Award at the Flinders Univ of South Australia 1991-1992, SEG Scholarship Student (US) / ASIA 21 Scholarship Student (Japan)

    Professional Membership : 1990 present, Society of Exploration Geophysicists (SEG) 1989 present, European Society of Geoscientists and Geoengineers (EAGE) 1989 present, Society of Exploration Geophysicists Japan (SEGJ) 1986 present, Indonesian Association of Geophysicists (HAGI) 1986 present, Indonesian Association of Geologists (IAGI) 2007 present, Inter-association Working Group EMSEV (Electromagnetic Studies on Earthquakes and

    Volcanoes)

    Djedi S. Widarto

  • GEOPHYSICS:The study of the earth by quantitative physical methods, especially by seismic reflection and refraction, gravity, magnetic, electrical, electromagnetic, and radioactivity methods (Sheriff, 1999).

    EXPLORATION GEOPHYSICS / GEOPHYSICAL PROSPECTING / APPLIED GEOPHYSICS:Making and interpreting measurements of physical properties of the earth to determine subsurface conditions, usually with an economic objective, e.g., discovery of fuel or mineral deposits. Properties measured include seismic, gravity, magnetic, electric, and temperature (Sheriff, 1999).

    PETROLEUM/GEOTHERMAL GEOPHYSICS:Making and interpreting measurements of physical properties of the earth to determine subsurface conditions related to hydrocarbon/geothermal.

  • Geophysical Methods

    Surface Methods

    Seismic Methods : Seismic reflection methods Surface wave (refraction) methods Micro-earthquake

    Potential Field Methods : Gravity & magnetic

    Electrical Methods Resistivity methods Self-potential Mise a-la masse methods Induced polarization

    Electromagnetic Methods Magnetotelluric (natural + controlled-source) methods Time-domain electromagnetic methods Ground penetrating radar Very-low frequency methods Seismo-electric method

    Nuclear Methods Nuclear magnetic resonance (NMR) method

    Borehole Methods

    In-Hole Procedures

    Cross-Hole Procedures

    Surface to Borehole Procedures: Velocity surveys

    Vertical seismic profiling

    Logging Techniques: Electrical methods Acoustic logging Nuclear logging

    Flow logging Other methods of logging

  • Techniques applying physical laws (or theory) to the study of the solid Earth,

    Estimation of subsurface physical property distribution by measuring relevant parameters:

    Geophysical Methods

    Method Measured Rock Property

    SEISMICS Travel time & amplitude Elastic moduli (density & velocity)

    GRAVITY Variation in gravitational field Density

    MAGNETICS Variation in magnetic field Magnetic susceptibility

    ELECTRICAL / ELECTROMAGNETICS

    Specific resistivity Electrical conductivity

    GPR Travel time Dielectric constant

    NUCLEAR Variation in natural radioactivity Nuclear decay

  • Method and Survey Procedure

    Expected Anomaly

    Interpretation

    Aero- or ground magnetic (covers a large area)

    low anomaly Ql : can be associated with thermally altered zones

    Qt : geometry (?)

    Schlumberger resistivity mapping and sounding (concentrated in the area between broad magnetic low and high)

    low anomaly Ql : can be associated with thermal fluids upflow and outflow zones

    Qt : shallow resistivity structure

    self-potential (across high and low resistivity areas)

    high or low anomaly

    Ql : ascending thermal fluid (and / or descending cold water)

    Qt : fluid flow (?)

    gravity (covers low and high magnetic areas)

    high or low anomaly

    Ql : existence of deep structure, i.e. intrusive body or caldera structures

    Qt : geometry of those above (the upper structure must be closely defined)

    Probable Sequence of Geophysical Exploration Methods Used to Investigate Young Volcanic Geothermal Prospect (revised from Sudarman, 1983)

  • Method and Survey Procedure

    Expected Anomaly

    Interpretation

    Thermal gradient and anomalous temperature (to figure out the cause of low resistivity layer)

    high anomaly

    Ql : uprising or horisontal thermal fluid movement, if depth to resevoiris relatively shallow

    Qt : defined the upper structure

    Magnetotelluric sounding low anomaly

    Ql : can be associated with thermal fluids upflow and outflow zones

    Qt : deeper resistivity structure

    micro-seismics (M< 3) high anomaly

    Ql : permeable zones, hydrothermal activity zone

    Qt : velocity distribution (?)

    Probable Sequence of Geophysical Exploration Methods Used to Investigate Young Volcanic Geothermal Prospect (revised from Sudarman, 1983)

  • Natural Source Magnetotelluric

  • MT is a geophysical method to estimate subsurface electrical property (resistivity or conductivity) distribution by measuring naturally time-varying EM fields,

    Dependence of electric and magnetic phenomena on the conductivity of the medium can be exploited to study the structure of solid Earth,

    Source of MT signals comes from interaction of the Earths permanent magnetic field with particles from solar wind and with atmospheric lightning which induce electric currents in the subsurface, thus

    no need for transmitter, simplifies the logistics

    random signals, low S/N (dead band ~1 Hz)

    What is Magnetotellurics (MT) ?

  • Methods to estimate subsurface electrical property (resistivity) distribution by measuring (naturally time-varying) EM fields over a range in frequencies :

    Magnetotellurics (MT, f < 10 Hz), Audio-frequency MT (AMT, f > 10 Hz), Controlled-Source Audio-Frequency MT (CSAMT),

    Transient EM / Time-Domain EM, Very Low Frequency EM (VLF-EM), LOTEM, ., etc.

    Ground Probing Radar (GPR)

    Airborne EM, Marine CSEM,etc.

    What is Magnetotellurics (MT) ?

  • transmitter generates

    time varying EM field

    induces Eddy currents

    in the conductor (Earth)

    generate secondary

    magnetic field

    electric and magnetic

    field are sensed at the

    receiver

    Electromagnetic Induction

  • Electrical Resistivities of Rocks

    Resistivity

    Res

    isti

    vity

    [O

    hm

    -m]

    Hyd

    rate

    s

    OIL

    SA

    ND

    S

    0.3 Wm

  • Interpreting subsurface resistivity: Impact of pore fluids and geologic processes on resistivity

    Saline brine

    Clay alteration

    Dissolution

    Temperature

    Pressure

    Faulting

    Shearing

    Hydrocarbons

    Carbonate cementation

    Silicification

    Metamorphism

    DecreaseIncrease

  • f > 1 Hz

    f < 1 Hz

    natural electromagnetic field104 10+4 Hz

  • Natural Electromagnetic Fields

    z

    x y

    Hx

    Hz

    Ex

    Hy

    Ey

    NaturalSignal

    ShortPeriod

    Longperiod

  • For a homogeneous or layered (1-D) medium

    Ex = Z Hy Z = scalar impedance

    For a medium with 2-D symmetry

    Ex = Zxy HyEy = Zyx Hx Zxy Zyx

    Z = vector impedance

    For a general 3-D medium

    Ex = Zxx Hx + Zxy Hy E = Z H

    Ey = Zyx Hx + Zyy Hy Z = tensor impedance

    Electric (E) and magnetic (H) fields relationship

  • Infinite distance of source sounding site

    plane wave assumption, time invariance of the source

    simplifies analysis of the governing equations

    Frequency domain and wide frequency bands

    intermediate to deep investigation depth

    Wide range of applications

    regional scale geological studies/tectonics

    mineral, geothermal and oil exploration

    Characteristics of MT Method

  • Characteristics of MT Method

    Resistivity contrast

    There must be a significant resistivity contrast within the depth of investigation for the method to be useful

    Contrast of 5:1 or greater

    Resolution depends on thickness and depth of unit being mapped:

    About 5~10% of depth, e.g. the top of a horizon at 10000 m can be mapped to +- 500 m

  • MT Advantages:

    Great depth of penetration

    Provides information in non-seismic or poor seismic areas

    No transmitter required

    Light-weight equipment - very portable

    Good production rate

    Can access almost anywhere

    Little impact on environment

    Better resolution than grav / mag

    Well-developed 2-D / 3-D interpretation procedures

    MT Disadvantages:

    Coupling with lateral conductors (e.g. sea)

    Irregular natural signal and industrial noise

    Resolution l