FUNDAMENTAL OF ELECTROMAGNETIC METHODS IN EXPLORATION GEOPHYSICS

 Mr. Djedi S. Widharto
INTRODUCTION TO GEOPHYSICS
Geophysics is the study of the earth by quantitative physical methods, especially by seismic reflection and refraction, gravity, magnetic, electrical, electromagnetic, and radioactivity methods. Meanwhile exploration geophysics is 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.
There are many geophysical methods which each method can be used in specific condition. Geophysical method can be categorized as surface method and borehole method. Surface method consist of seismic methods, potential field methods, electrical method, electromagnetic method, and nuclear method. Borehole method consist of in-hole procedures, cross-hole procedures, surface to borehole procedures, and logging techniques.

MAGNETOTELLURIC
One of electromagnetics methods is magnetotelluric (MT). It is a geophysical method to estimate subsurface electrical property (resistivity or conductivity) distribution by measuring naturally time-varying EM fields. Parameter of this method is resistivity and conductivity. Source of MT signals comes from interaction of the Earth’s permanent magnetic field with particles from solar wind and with atmospheric lightning which induce electric currents in the subsurface.
There are impact of pore fluids and geologic processes on resistivity. Saline brine, clay alteration, dissolution, temperature, pressure, faulting, and shearing are factor which cause increasing of resistivity. Then, factor which cause decreasing of resistivity are hydrocarbones, carbonate cementation, and silicification.
This is relationship between electric (E) and magnetic (H) fields. For a homogeneous or layered (1-D) medium
                                      Ex= ZHy…………………………………………………….……………………. (1)
Which Z=scalar impedance. For a medium with 2-D symmetry
                                      Ex= ZxyHy………………………………………………….………………….. (2)
                                      Ey= ZyxHxZxy≠ Zyx …………………………….………………………..(3)
For a general 3-D medium
                                      Ex= ZxxHx+ ZxyHyE = ZH …………………………………….……….(4)
                                      Ey= ZyxHx+ ZyyHyZ= tensor impedance……………………(5)

MT has advatanges and disadvantages. The advantages are great depth of penetration, provides information in non-seismic areas, no transmitter required, light-weight equimpment, good production rate, can access almost anywhere, cause little impact on environment, has better resolution than gravity or magnetic, and well-developed 2-D/3-D interpretation procedures. The disadvantages are coupling with lateral conductors, irregular natural signal and industrial noise, resolution less than seismic, complex data processing, static shift of apparent resistivity curves sometimes significant, and inversion techniques rely on smooth models, tougher to interpret in complex areas.

In the aquisition of MT, there are mobile acquisition unit and remote reference acquisition unit. Mobile acquisition unit is used to find data. The place where data is collected must be deserted, so the noise can be minimalized. Remote reference acquisition unit is used to be referenced and it must motionless. Data processing include measurement of orthogonal EM field, extract impedance tensor, then we will get apparent resistivity and phase sounding curves (ohm.m) and (degree) vs frequency (Hz). Data presentation include pseudo-section, data modelling, and bostick transform.

Resume from Mr. Djedi S. Widharto ShortCourse (2/21/2014)


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