magnetotelluric and transient electromagnetic ... - Orkustofnun

Report 21 413 Wameyoσ = 1 σ w + σ s(8)Fwhere σ = Bulk conductivity (S/m);σ w = Conductivity of water (S/m);σ s = Interface conductivity (S/m);F = Formation factor of the rock, given by ρ/ρ w (from Equation 3).The interface conductivity, σ s , is caused by fluid-matrix interaction and depends mostly on themagnitude of the internal surfaces (porosity) and of their nature (surface conditions). The main tworeasons for surface conductivity are the presence of clay minerals (alteration) and surface double-layerconduction.The alteration process and the resulting type of alteration minerals are dependent on the type ofprimary minerals, chemical composition of the geothermal fluid and temperature. The intensity of thealteration is furthermore dependent on the temperature, time and the texture of the host rocks.Alteration intensity is normally low for temperatures below 50-100°C. At temperatures lower than220°C, low-temperature zeolites and the clay mineral smectite are formed. Smectite has hydrated andloosely bound cations between the silica plates, making the mineral conductive and with a high cationexchange capacity (Árnason et al., 2000).In the temperature range from 220 to about 240-250°C, the low-temperature zeolites disappear and thesmectite is transformed into chlorite in a transition zone, the so-called mixed-layered clay zone, wheresmectite and chlorite coexist in a mixture. At about 250°C the smectite disappears and chlorite is thedominant mineral, marking the beginning of the chlorite zone, hence, highly resistivity, since chloriteminerals have cations that are fixed in a crystal lattice, making the mineral resistive. At still highertemperatures, about 260-270°C, epidote becomes abundant in the so-called chlorite-epidote zone andthe resistivity becomes even higher.In a typical geothermal system, one would expect to find a deep anomaly in electrical conductivityassociated with thermal excitation of conduction in the massive crystalline rock comprising thebasement. At shallower depths in the section, one would expect to find an anomaly in electricalconductivity associated with a reservoir filled with hot geothermal fluids.3. CENTRAL-LOOP TEM METHOD3.1 TheoryReceiverIn the Central-Loop TransientElectromagnetic method, a steadyReceiver coilcurrent is transmitted in a wire loop,laid on the ground at the area to beexamined (Figure 3). This currentis allowed to flow for a sufficientlylong time to allow turn-on transientsTransmitterin the ground to dissipate. Thissteady current is then abruptlyterminated in a controlled fashion(Figure 4). At the instant ofTransmitter loopFIGURE 3: Central-loop time domain TEM layouttransmitter turn-off, eddy currents reproduce the static magnetic field due to the transmitter loop butthen decay rapidly. The decaying primary magnetic field, in accordance with Faraday’s law, induceseddy currents immediately below the transmitter loop. As the initial near-surface eddy current decays,