Chapter 5 Electrical Methods PDF

CHAPTER 5 Electrical Methods Electrical Methods - Introduction • Electrical methods employ a variety of measurements of the effects of electrical current flow within the Earth. • The phenomena that can be measured include current flow, electrical potential (voltages), and electromagnetic fields....

CHAPTER 5 Electrical Methods Electrical Methods - Introduction • Electrical methods employ a variety of measurements of the effects of electrical current flow within the Earth. • The phenomena that can be measured include current flow, electrical potential (voltages), and electromagnetic fields. • A summary of the electrical methods is given below:  Resistivity Method - Measurement of electrical potential caused by current (DC) introduced into the ground as a means of studying earth resistivity. Factors that affect the measured potential, and thus can be mapped using this method, include the presence and quality of pore fluids and clays. Electrical Method  Induced Polarization (IP) - This is an active method that is commonly done in conjunction with DC Resistivity. • It employs measurements of the transient (shortterm) variations in potential as the current is initially applied or removed from the ground. • It has been observed that when a current is applied to the ground, the ground behaves much like a capacitor, storing some of the applied current as a charge that is dissipated upon removal of the current. Electrical Method • IP is commonly used to detect concentrations of clay and electrically conductive metallic mineral grains.  Self Potential (SP) - This is a passive method that employs measurements of naturally occurring electrical potentials commonly associated with the weathering of sulfide ore bodies. • The only equipment needed for conducting an SP survey is a high-impedence voltmeter and some means of making good electrical contact with the ground. Electrical Method  Electromagnetic (EM) - This is an active method that employs measurements of a time-varying magnetic field generated by induction through current flow within the earth. • In this technique, a time-varying magnetic field is generated at the surface of the earth that produces a time-varying electrical current in the earth through induction. Electrical Method • EM is used for locating conductive base-metal deposits, for locating buried pipes and cables, and for near-surface geophysical mapping.  Magnetotelluric (MT) - This is a passive method that employs measurements of naturally occurring electrical currents, or telluric currents, generated by magnetic induction of electrical currents in the ionosphere. • This method can be used to determine electrical properties of materials at relatively great depths (down to and including the mantle) inside the Earth. Electrical Method • In this MT technique, a time variation in electrical potential is measured at a base station and at survey stations. • Differences in the recorded signal are used to estimate subsurface distribution of electrical resistivity. Theory of Electricity • In 1827, Georg Ohm defined an empirical relationship between the current flowing through a wire and the voltage potential required to drive that current. • or I = (1/R) V • Ohm (1827) found that the current, I, was proportional to the voltage, V, for a broad class of materials. Theory of Electricity • The constant of proportionality is called the resistance , R, of the material and has the units of voltage (volts) over current (amperes), or ohms. • The geometrically-independent quantity that is used is called resistivity and is usually indicated by the Greek symbol ρ. RA • Resistivity, ρ = L • The units are, ohm-m (ohm-meters). Resistivity of Earth Materials • Although some native metals and graphite conduct electricity, most rock-forming minerals are electrical insulators. • Measured resistivities in Earth materials are primarily controlled by the movement of charged ions in pore fluids. Although water itself is not a good conductor of electricity, ground water generally contains dissolved compounds that greatly enhance its ability to conduct electricity. • Hence, porosity and fluid saturation tend to dominate electrical resistivity measurements. • In addition to pores, fractures within crystalline rock can lead to low resistivities if they are filled with fluids. Resistivity of Earth Materials • The resistivities of various earth materials are shown below. Material Resistivity (Ohm-meter) Rock salt 30 - 1 x 1013 Granite 100 - 1 x 106 Limestones 50 - 1 x 107 Sandstones 1 - 1 x 108 Shales 20 - 2 x 103 Dolomite 100 - 10,000 Clay Ground Water 1 - 100 0.5 - 300 Current Density and Equipotentials • Current flows (the red lines) out from the electrode (the green square) radially along straight lines. • If we measure the voltage drop imposed by the resistivity of the medium from a distance very far from the current electrode to various places in the media, the voltage drops would be constant along circular lines centered at the electrode. • These lines are referred to as equipotentials. Current Density and Equipotentials • The current density is defined as the amount of current passing through a unit area of an equipotential surface. Current density has the units of Amperes per meter squared. • Electric potential at distance away from current source on surface given as V(r)=ρI/2πr. Current Flow From Two Closely Spaced Electrodes • The current distribution and equipotentials produced within a homogeneous earth become more complicated. Resistivity Equipment • Current Source - A source of DC current is required. • Ammeter - A simple ammeter (a device for measuring electrical current) can be used. • Voltmeter - A simple voltmeter can also be used. The voltmeter must also be capable of measuring voltages from a few millivolts to a few volts. ABEM Terrameter Resistivity Equipment • Electrodes - To avoid problems associated with electrode potentials, sophisticated electrodes known as porous pots can be used. For DC resistivity surveys, the most commonly used electrodes are aluminum, copper, or steel rods about two feet in length. • Cables - To connect the electrodes to the various electrical components, cables must be employed. • These cables are typically insulated wires with stranded, copper-cored conductors. Resistivity Soundings • When doing resistivity sounding surveys, one of two survey types is most commonly used. For both of these survey types, electrodes are distributed along a line, centered about a midpoint that is considered the location of the sounding. • The simplest in terms of the geometry of electrode placement is referred to as a Wenner survey. The most time effective in terms of field work is referred to as a Schlumberger survey. Resistivity Soundings • For a Wenner survey, the apparent resistivity computed from measurements of voltage, ∆V, and current, i, is given by the relatively simple equation shown below. Resistivity Soundings • For a Schlumberger survey, the two current electrodes (green) and the two potential electrodes (red) are still placed in line with one another and centered on some location, but the potential and current electrodes are not placed equidistant from one another. Resistivity Soundings • The current electrodes are at equal distances from the center of the sounding, s. The potential electrodes are also at equal distances from the center of the sounding, but this distance, a/2, is much less than the distance s. • Most of the interpretational software available assumes that the potential electrode spacing is negligible compared to the current electrode spacing. • In practice, this is usually interpreted to mean that a must be less than 2s/5. Electrode Spacings and Apparent Resistivity Plots • Consider performing a Schlumberger sounding over the geologic model shown below. Advantages and Disadvantages of Wenner and Schlumberger Arrays • Schlumberger – advantages: • Need to move the two current electrodes only for most readings. This can significantly decrease the time required to acquire a sounding. • Because the potential electrodes remain in fixed locations, the effects of near-surface lateral variations in resistivity are reduced. Advantages and Disadvantages of Wenner and Schlumberger Arrays • Disadvantages: • Because the potential electrode spacing is small compared to the current electrode spacing, for large current electrode spacings, very sensitive voltmeters are required. • In general, interpretations based on DC soundings will be limited to simple, horizontally layered structures. Advantages and Disadvantages of Wenner and Schlumberger Arrays • Wenner – Advantages: • Potential electrode spacing increases as current electrode spacing increases. Less sensitive voltmeters are required. • Disadvantages: • All four electrodes, two current and two potential, must be moved to acquire each reading. • Because all electrodes are moved for each reading, this method can be more susceptible to near-surface, lateral variations in resistivity. • These near-surface lateral variations could potentially be misinterpreted in terms of depth variations in resistivity. Advantages and Disadvantages of Wenner and Schlumberger Arrays • In general, interpretations based on DC soundings will be limited to simple, horizontally layered structures.