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whether the electromagnetic fields are responding to subsurface rock bodies of effectively 1, 2, or 3 dimensions by computing skew and ‘tipper’, which are used to infer lateral variations in resistivity. If the measured resistivity response to the geology beneath an MT station truly is 1D or 2D, then the skew will be zero (if no noise in the data). Higher skews are an indication of the resistivity response to 3D structures. Te ‘tipper’ is calculated from the vertical component of the magnetic field and is a measure of the ‘tipping’ of the magnetic field out of the horizontal plane. For 1D structures, the tipper will equal zero. Te tipper responds primarily to vertical and sub-vertical structures. In marine MT, the electric and magnetic


fields are measured at seafloor receiver stations. Te fields are small, and vary in strength over hours, days and weeks. Geophysicists exploiting MT for greater depths have to take measurements for many hours at each station in order to get a good signal to ensure high-quality data. To record low frequencies, say 0.001 Hz, or 1 cycle per 1,000 seconds, we need to record for 16 min (1,000s) to get a single sample of data. To get enough samples for a decent statistical average of the data we need to record for several hours. Unfortunately, the MT method lacks the sensitivity required


to resolve the details of thin resistive structures such as hydrocarbon reservoirs, but it is sensitive to the bulk (large- scale) background resistivity structure, and so is a natural complement to the CSEM method in many situations.


Figure 1.65: Magnetotelluric signals from lightning strikes. The electrical discharges from thunderstorms radiate powerful electromagnetic fields that propagate in a wave guide between the Earth’s surface and the ionosphere, with a small part of the energy penetrating into the Earth. Figure from Martyn Unsworth’s course notes (GEOP424, University of Alberta).


MT data can be acquired as part of CSEM surveys when the


controlled source is inactive. Te low-frequency and thereby deep-sensing nature of MT surveying makes the technique useful for mapping regional geology.


1.4.4 Induced Polarisation Geoelectric Method


Figure 1.66: Below ~1 Hz, the MT source originates from the interaction of the solar wind with the Earth’s magnetic field, which, when strong, are known as geomagnetic storms. The interactions create small variations in the magnetic field (dH/dt), which induce electric currents in the Earth. Because of the large conductivity contrast between space and the ionosphere, EM waves are bent to vertical incidence. These vertically incident waves impinge on the surface of the Earth. Part of it is reflected back and the remaining part, which may have frequencies down to 0.00001 Hz, penetrates deep into the Earth. They induce currents in the Earth called telluric (earth) currents, which tend to flow in the more conductive rocks, in turn producing a secondary magnetic field.


While recording conventional resistivity measurements in 1920, Conrad Schlumberger noted that the potential difference between electrodes did not drop instantaneously to zero when the current was turned off. Instead, the potential difference dropped sharply at first, and then gradually decayed to zero after a given interval of time. Te study of the decaying potential difference as a function of time is the study of induced polarisation (IP). Te IP method is extensively used in


the search for disseminated metallic ores and, to a lesser extent, ground water and geothermal exploration. More recently, it has been applied to hydrocarbon exploration. Tere are two main mechanisms


through which the IP effect can be de- tected (Kearey et al., 2002; Sharma, 1986). By applying an external current, the


electrical conduction in most rocks (other than ores) is electrolytic by positive and negative ion transport through water in interconnecting pores. However, when metallic minerals such as pyrite or magnetite are present, blocking the pores, the ionic conduction is hindered by these mineral grains, in which the current flow becomes electronic. More specifically, negative ions reaching the mineral grain blockage will lose electrons and become neutral. Te electrons pass through the mineral grain to the other side, where they combine with positive ions on the


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