Transcript Slide 1
Geoelectricity Introduction: Electrical Principles Let Q1, Q2 be electrical charges separated by a distance r. There is a force between the two charges that goes like FK Q1Q2 r2 This is called Coulomb’s law, after Charles Augustin de Coulomb who first figured this out. Charles Augustin de Coulomb (1736 - 1806) Later, Ampere figured out what the units should be based on the flow of charge though parallel wires. We define a material property eo called the permittivity constant: F 1 Q1Q2 4eo r 2 which is approximately equal to 8.85419 x 10-12 C2N-1m-2 (C = Coulomb which is a unit of charge. One Coulomb is defined as the amount of charge that passes through a wire of 1 Ampere current flowing for 1 second). ANDRÉ-MARIE AMPÈRE ( 1775 - 1836 ) Note similarity to force of gravity. There are many analogues. We can define the electric field (similar to gravity acceleration field) as a force per unit charge: F Q2 E Q1 4e o r 2 units of E in this form are N Q-1. We think of a field as lines along which a charge Q1 would move if were attracted by the charge Q2. Also analogous to gravity, we define an electrical potential U and relate it to the field by a negative gradient: U E r And we define U as the work per unit charge required to bring an object from infinity to r: r U Edr r Q 4e r o 2 dr Q 4eo r Instead of absolute potentials we normally talk about potential differences which we call volts (V; after the Italian physicist Alessandro Volta). There is a famous relation between the voltage, current, and resistance in a wire called Ohm’s Law: V IR Georg Simon Ohm 1787-1854 However, resistance is not really an intensive material property (like, say, density) and so is not appropriate for application to rocks. We define instead the resistivity r as: The unit of r are Ohm-meters or W-m: We then write the 3D equivalent of Ohm’s law as V I r E rJ L A where we recognize E as the potential gradient (V/L) and J = I/A is called the current density. Note that we also define the conductivity s as 1/r. Units: R I V r s E Ohms Amperes or Amps Volts Ohm-meters mhos/meter or siemens/meter Volts/meter Electrical Conduction 1. Electronic or Ohmic: free electrons. A property of metals. Very efficient. Ranges over ~24 orders of magnitude Conductors r < 1 Ohm-meter Resistors/Insulators r > 1 Ohm-meter Semi-conductors r ~ 1 Ohm-meter; electrons only partially bound Good conductors: Ok conductors: Semiconductors: Insulators: metals, graphite sulphides, arsenides most oxides carbonates, phosphates, nitrates (most rocks) 2. Ionic or Electrolytic: Dissolved Ions in a fluid (water). Very efficient but more space problems with bigger elements moving around. Thus it is not as efficient as electronic Water is very important in this process, which makes electrical methods very good for addressing water related problems. We use the empirical Archie’s Law for a porous medium: r a rw m n S where is the porosity, S is the fraction of pores filled with water, rw is the resistivity of the water, and m, n, and a are material constants. Generally 0.5 < a < 2.5, 1.3 < m < 2.5, and n ~ 2. Often we just assume “2” for all of them. rw examples: Meteoric Rain Fresh Water (Seds) Sea Water (Ocean) 30-1000 Wm 1-100 Wm 0.2 Wm Material Resistivity (Ohm-meter) Air Infinite Pyrite 3 x 10-1 Galena 2 x 10-3 Quartz 4 x 1010 - 2 x 1014 Calcite 1 x 1012 - 1 x 1013 Rock Salt 30 - 1 x 1013 Mica 9 x 1012 - 1 x 1014 Granite 100 - 1 x 106 Gabbro 1 x 103 - 1 x 106 Basalt 10 - 1 x 107 Limestones 50 - 1 x 107 Sandstones 1 - 1 x 108 Shales 20 - 2 x 103 Dolomite 100 - 10,000 Sand 1 - 1,000 Clay 1 - 100 Ground Water 0.5 - 300 Sea Water 0.2 3. Dielectric: Caused by the relative displacement of protons and electrons within their orbital shells. Of no importance at low f (to DC) but is very important at high frequency AC. The net effect is to change the permittivity eo to e as: e eo where is the dielectric constant. Note that generally is a function of frequency; (f) ~ 1/f. Here are some typical values of : Water Sandstone Soil Basalt Gneiss 80 5-12 4-30 12 8.5 Note that in EM we define a Displacement field D as D = eE Maxwell’s Equations B E t D H J t Where J = sE, B = mH, and D = eE (and all are vectors). So in general the electric and magnetic fields are coupled. However, in the case of an isotropic, homogeneous medium they separate as: E 2E E ms em 2 t t 2 H H 2 H ms em 2 t t Note these are the same equation with different variables, and that they are a combination of the diffusion and wave equations. We’ll solve these in a bit when we talk about MT. 2 Electrical Methods There is an alphabet soup of electrical methods (SP, IP, MT, EM, Resistivity, GPR) which we will discuss in turn. Most are sensitive to resistivity/conductivity in some way, except for GPR (dielectric constant). As we saw before, natural materials vary in resistivity by several orders of magnitude. Self Potential (SP) Measure natural potential differences in the earth Sources: Electrokinetic or streaming potentials: moving ions. Electrochemical (Nernst and diffusion) diffusion: ions with different mobilities get separated Nernst -> same electrodes, different concentrations Mineralization -> different electrodes (materials) Ore bodies always give negative potentials. Measurement with porous pots. Signals range from few mv to 1 V. 200 mV is a strong signal. Self Potential Across a Fault Mise a la Mase Monitoring Fluid Flow The Earth’s electric field. The ground generally has negative charge, so the Earth’s E field points down into the earth. The atmosphere is generally positive, with ions produced by cosmic rays. These bombard the Earth, which neutralize the surface. However, the negative charge is replenished by lightning storms. Tellurics Natural electric currents in the earth. These are cause by decaying magnetic fields in the earth. They are like large swirls that follow the sun. Electromagnetic fields arise from time-varying currents in the ionosphere and tropical storms (lightning strikes). Fields propagate as plane-waves vertically into the Earth, inducing secondary currents. We measure a voltage difference, and figure that current density results in from a constriction or redirection of current. J E r V rL Note you can measure in perpendicular directions to get the areal direction of current and identify a resisitive body. Magnetotellurics Simultaneous measurement of the magnetic and electric fields in the Earth. Let’s solve Maxwell’s equation for the H field (it will be the same for the E field): 2 H H 2 H ms em 2 t t Let’s assume a monochromatic field: H(x,t) = H(x)eiwt Note this is like the separation of variables trick we did for heat conduction 2 H iwmsH emw 2 H The first term is called the conduction term, the second the displacement term The relative sizes of these terms (conductive to displacement) is s/ew. So, if conductivity is large and/or frequencies are small, then the first term dominates. If conductivity is small and/or frequencies are large, the displacement term is large. For rocks and natural field frequencies, the conductive term is about 8 orders of magnitude greater than the displacement term, so for this kind of observation we have 2 H iwmsH Which is the heat conductivity diffusion equation we solved before. We take the exact same steps and find H H o e az ei wt az where wms a 2 1/ 2 Note that for normal values of m in the Earth, the attenuation term becomes e az wms 1/ 2 f 3 exp z exp 2 10 z r 2 z is in meters, ris in Wm, and f is in Hz. The skin depth zs is when the field is Ho/e: z s 500 Examples r 10-4 Wm r f 102 Wm f 10-2 50 m 5 x 104 m 103 0.16 m 160 m Now, as an H field penetrates the surface it will attenuate. Maxwell says that: D H J t Again, the dielectric term will be much smaller than the conductive term, so H J sE Assuming a simple H that is oriented in the y direction (Hy component), we evaluate: i H x 0 j y Hy k H y H y H y i k i J x sE x z z x z 0 From before H y az iwt az iwt Jx H oe e H oe e az (1i ) z z z Hoeiwt a(1 i)eaz (1i ) a(1 i) H J x 2aHei / 4 mws Hei / 4 1/ 2 Thus, the current (telluric) has a /4 phase shift relative to the initial H field. If mws is small, then H penetrates to great depth, and little J is produced. If mws is large,then H does not go to great depth, and big J is produced. The idea behind MT is to measure H and E simultaneously, and take the ratio of E in one direction to H in the perpendicular direction. From above 1/ 2 1/ 2 mws mw i / 4 i / 4 Ex H ye H ye s s so 1/ 2 Ey Ex mw H y s Hx We can make a “pseudo-section” of resistivity as follows: 1 Hy 1 Hy Ex s z s eff zeff so zeff 1/ 2 1 Hy 1 s eff 1 mw 1 Ex T Ex s eff E x s eff mw mw s eff mw H y 2m H y 1/ 2 where T is the period and w = 2/T. Thus Hy 1 Hy T Ex reff s eff E x E x 2m H y or r eff T Ex 2m H y 2 Assuming a typical value for m of 1.3 x 10-6 henrys/meter we can write: r eff Ex 0.2T Hy 2 1 zeff 5rT 2 where the units are E H T r z mV/km gammas seconds Wm km So the idea is to determine r as a function of frequency (for different E/H ratios) and then calculate the corresponding depth z. MT Recording Geometry MT Resistivity in subducting plate MT Cross section across a fault Resistivity An active technique. Pump current into the ground and measure spatial variation in voltage to get a resistivity map. Let’s consider what happens if we put an electrode into the ground, and start with an infinite space. We can think of it as a charge Q with associated electric fields and potentials. Everywhere around Q, as long as there are no sources or sinks (i.e. no other charges in the volume) then the potential U satisfies Laplace’s equation (in spherical coordinates): 2 U U 2 0 dr r dr 2 2U Note that 2 U 2 U 2 2r r 2 r r 2 dr dr dr dr r dr dr 2 U 2 U 2 U so Laplace’s equation is equvalent in this case to 2 U r 0 dr dr or U A 2 dr r A U r Note that integrative constants are zero because U and gradU -> 0 as r -> infinity. The current at any radius r is related to the current density by U A 2 I 4r J 4r s 4r s 2 4sA r r 2 2 Thus A and I 4s Ir U 4r 4rU r I If the electrode is at the surface of a half space instead of within an infinite space, then we repeat the above but use a hemisphere instead of a sphere and find Ir U 2r 2rU r I Now suppose we have two electrodes at the surface at points A and B, and we want to determine the potential at an arbitrary point C. If the distances to C are rAC and rBC, then U AC Ir 2rAC U BC Ir 2rBC we reverse the sign on UBC because current flows of electrode one (positive Q) and into electrode two (negative Q). The total potential at point C is then UC UAC UBC Ir Ir Ir 1 1 2rAC 2rBC 2 rAC rBC Similarly, the potential at another point D would be Ir 1 1 UD 2 rAD rDB And so the potential difference between points C and D is VCD given by VCD or Ir 1 1 1 1 UC U D 2 rAC rCB rAD rDB 2VCD 1 r I 1 1 1 1 rAC rCB rAD rDB 2VCD 1 r I 1 1 1 1 rAC rCB rAD rDB This is the fundamental resistivity equation. It is independent of any particular geometry, but there are some configurations which are more or less standard. Wenner: equal spacing between current and potential electrodes: 2VCD r I 2V 1 CD I 1 1 1 1 a 2a 2a a 2aV 1 CD I 1 1 2a 2a Schlumberger: Current electrodes are a distance 2L apart, potential electrodes are a distance 2l apart, the center of the potential electrodes is a distance x from the center of the current electrodes, and L >> l and L-x >> l (we are far from the ends). In this case rBC L x l rAC L x l rAD L x l rBD L x l 2VCD 1 r 1 1 1 I 1 L x l L x l L x l L x l If L-x >> l and L+x >> l, then 1 1 l l 1 l 1 1 L x 1 L x 1 2 L x L x L x L x Lxl and similarly for the other terms (substitute –l for l and –x for x). Plug all this in and eventually you get L2 x 2 2 VCD r 2Il L2 x 2 Dipole-Dipole In this case we imagine that both the current and potential electrodes are separated by a distance 2l and the distance between the inner current and potential electrodes is a multiple of this distance = 2l(n-1) where n >= 1 (when n = 1, they are together). In this case: rAC 2l (n 1) 2l 2ln rBC 2l(n 1) rAD 2l(n 1) 2l 2l 2l(n 1) rBD 2l (n 1) 2l 2ln 2V ln n 2 1 2VCD 1 CD r 1 1 1 I 1 I 2ln 2l(n 1) 2l(n 1) 2ln Making a resistivity pseudo section: Measure r at a given separation, mark a spot half way in between (d=l(n-1)) and plot this a a depth the same distance below the surface (i.e., depth = l(n-1). Resistivity Imaging around a Tunnel Resistivity Imaging in Limestone (Karst Lithology) Current Distribution Where is the current going, anyway? We can get an idea by examining the case of a homogeneous halfspace. Consider current electrodes a distance L apart. At a point P a distance r1 from the positive electrode and r2 from the negative electrode (and at a depth z): U Ir 1 1 Ex x x 2 r1 r2 we set r1 x 2 y 2 z 2 r2 L x y 2 z 2 2 1/ 2 3 / 2 1 2 2x x y 2 z 2 x 2 y 2 z 2 x r1 x 2 x x y z 2 2 2 3 / 2 1 L x 2 y 2 z 2 x r2 x L x 3 1 r x 3 r1 1/ 2 2L x L x 2 y 2 z 2 2 3 / 2 Hence I x L x Jx sE x 3 3 2 r1 r2 For illustration, let’s see what happens at the midpoint between the electrodes. x = L/2, L-x = L/2, so r1 r2 I Jx 2 L / 22 y 2 z 2 L/2 2 2 2 L / 2 y z IL 1 2 L / 22 y 2 z 2 3/ 2 3/ 2 2 2 2 3/ 2 L / 2 y z L/2 The current that flows across and element dydz is dIx = Jxdydz. Thus, the fraction of the total current I that flows between the surface and depth z is z Ix L dy dz I 2 0 L / 22 y 2 z 2 3 / 2 z Ix L dz 2 1 2 z tan 2 2 I 0 L / 2 z L This shows that half the current crosses above a depth z = L/2, and almost 90% above z = 3L. This gives you some idea on how current distribution depends on separation of electrodes. How about layers of resisitivity? It gets complicated fast. Let’s first consider two halfspaces separated by in interface. The upper halfspace has a resistivity r1 and the lower halfspace has resistivity r2. We have a current electrode in the upper halfspace. What is the potential at a point P in the upper halfspace and P’ in the lower halfspace? We define a “reflection coefficient” k and a transmission coefficient 1-k. If the point P is a distance r1 from C, then Ir1 1 k VP 4 r1 r2 where r2 is the “ray” distance to the interface and back to P from C, following the usual reflection law (equal angles of incidence and reflection). Note that r2 can be constructed by reflecting the normal from C across the interface and drawing a straight line to P. Similarly, the potential at P’ is Ir2 1 k VP' 4 r3 where r3 is the distance from C to P’. If we move the points P and P’ to the interface, (P=P’) then r1 = r2 = r3 and Ir1 1 4 r1 from which k Ir2 1 k r1 4 r1 r2 r1 k r2 r1 Note that –1 < k < 1. How about a layer over a half space? As in the case of the two halfspaces, we account for the bottom interface by summing potentials from the original electode (C1) and it’s mirror across the interface (C2). BUT now we have to mirror C2 across the other interface (surface) to produce C3, and mirror C3 across the lower interface to get C4 and so on ad infinitum. Hence: Ir1 1 VC1 4 r Ir1 k VC 2 4 r1 Ir1 k ka VC 3 4 r1 Ir1 k ka k VC 4 4 r2 Ir1 k ka k ka VC 5 4 r2 and so on, where the reflection at the surface depends on the resistivity of the air (ra) or r r1 ka a 1 r a r1 Because the resitivity of the air is very large. Ir1 1 2k 2k 2 VCi 2 r r r 1 2 i1 r1 r 2h hence 2 2 1/ 2 ; rm r 2mh 2 m Ir1 1 k V 2 2 2 2 r m1 r 2m h 2 1/ 2 Induced Polarization Induced Polarization (IP) is the transient storage of voltage in the ground. We can produce it by turning the voltage in a resistivity array on and off. The origin of induced electrical polarization is complex and is not well understood. This is primarily because several physio-chemical phenomena and conditions are likely responsible for its occurrence. When a metal electrode is immersed in a solution of ions of a certain concentration and valence, a potential difference is established between the metal and the solution sides of the interface. This difference in potential is an explicit function of the ion concentration, valence, etc. When an external voltage is applied across the interface, a current is caused to flow, and the potential drop across the interface changes from its initial value. The change in interface voltage is called the "overvoltage" or "polarization" potential of the electrode. Overvoltages are due to an accumulation of ions on the electrolyte side of the interface waiting to be discharged. The time constant of buildup and decay is typically several tenths of a second. Overvoltage is therefore established whenever current is caused to flow across an interface between ionic and electronic conduction. In normal rocks, the current that flows under the action of an applied EMF does so by ionic conduction in the electrolyte in the pores of the rock. There are, however, certain minerals that have a measure of electronic conduction (almost all the metallic sulfides - except sphalerite - such as pyrite, graphite, some coals, magnetite, pyrolusite, native metals, some arsenides, and other minerals with a metallic lustre). The most important sources of nonmetallic IP in rocks are certain types of clay minerals (Vacquier 1957, Seigel 1970). These effects are believed to be related to electrodialysis of the clay particles. This is only one type of phenomenon that can cause "ion-sorting" or "membrane effects." For example, the figure below shows a cationselective membrane zone in which the mobility of the cation is increased relative to that of the anion, causing ionic concentration gradients and therefore polarization. In time-domain IP, several indices have been used to define the polarizability of the medium. Seigel (1959) defined "chargeability" (in seconds) as the ratio of the area under the decay curve (in millivolt-seconds, mV-s) to the potential difference (in mV) measured before switching the current off. Komarov, et al., (1966) defined "polarizability" as the ratio of the potential difference after a given time from switching the current off to the potential difference before switching the current off. Polarizability is expressed as a percentage. Chargeabilty M Frequency Effect 1 M Vo t2 V(t)dt t1 r f rF FE rF Metal Factor MF As F s f A rF r f A = 2 x 105 1 1 r r f F A r r f F IP Example - Mapping soil and groundwater contamination. Cahyna, Mazac, and Vendhodova (1990) used IP to determine the slag-type material containing cyanide complexes that have contaminated groundwater in Czechoslovakia. The figure shows contours of ηa (percent) obtained from a 10-m grid of profiles. The largest IP anomaly (ηa = 2.44%) directly adjoined the area of the outcrop of the contaminant (labeled A). The hatched region exhibits polarizability over 1.5% and probably represents the maximum concentration of the contaminant. The region exhibiting polarizability of less than 0.75% was interpreted as ground free of any slagtype contaminant. Ground Penetrating Radar (GPR) Ground-penetrating radar (GPR) uses a high-frequency (80 to 1,500 MHz) EM pulse transmitted from a radar antenna to probe the earth. The transmitted radar pulses are reflected from various interfaces within the ground, and this return is detected by the radar receiver. Remember from Maxwell: 2 E E 2 E ms em 2 t t At high frequencies, the second (wave) term dominates, so 2 E E em 2 t 2 This is just like seismic waves, only in this case the reflection coefficient and wavespeed depend on dielectric constant. Reflecting interfaces may be soil horizons, the groundwater surface, soil/rock interfaces, man-made objects, or any other interface possessing a contrast in dielectric properties. The dielectric properties of materials correlate with many of the mechanical and geologic parameters of materials. How it works: The radar signal is imparted to the ground by an antenna that is in close proximity to the ground. The reflected signals can be detected by the transmitting antenna or by a second, separate receiving antenna. As the antenna (or antenna pair) is moved along the surface, the graphic recorder displays results in a cross-section record or radar image of the earth. Spatial considerations: As GPR has short wavelengths in most earth materials, resolution of interfaces and discrete objects is very good. However, the attenuation of the signals in earth materials is high, and depths of penetration seldom exceed 10 m. Water and clay soils increase the attenuation, decreasing penetration. The objective of GPR surveys is to map near-surface interfaces. For many surveys, the location of objects such as tanks or pipes in the subsurface is the objective. Dielectric properties of materials are not measured directly. The method is most useful for detecting changes in the geometry of subsurface interfaces. Signal GPR waveform Unlike seismic waves, most of the GPR signal is perpendicular to the antenna (straight down) Typical Example of a GPR field trace. GPR on a small scale. Using GPR to locate graves, trenches, and sinkholes Electromagnetic Induction The way this works is you create your own magnetic field by passing current through a wire, usually a current loop. The magnetic field penetrates the ground, produces a current that induces a secondary field in an object in the ground. The receiving loop detects a combination of the original signal field and that produced by an object in the ground. Electromagnetic Induction: Quantitative Bits We start by generating our own magnetic field B. Use the diffusion part of Maxwell’s equation: B B ms t 2 Recall from the MT discussion H H o e az ei wt az where wms a 2 1/ 2 Some definitions: IC IR IT R L VC current in conductor current in receiver current in transmitter resistance of conductor inductance of conductor voltage of conductor Inductance produces a voltage proportional to the rate of change of current. Thus: dI VC I C R L C I C ( R iwL) dt The current in one entity produces a voltage in another through Mutual inductance. Let MTC MTR MCR Mutual inductance between transmitter and conductor Mutual inductance between transmitter and receiver Mutual inductance between conductor and receiver VC M TC dI T iwM TC I T dt Primary voltage in the receiver (due to the transmitter) dIT VP M TR iwM TR I T dt Secondary voltage in the receiver (due to the conductor) dIC VS M CR iwM CR I C dt From above VC M TC So dI T iwM TC IT I C ( R iwL) dt IC iwM TC iwM TC 2 ( R iwL) 2 2 IT ( R iwL) ( R w L ) The secondary and primary voltages at the receiver are therefore related as VS iwM CR I C M CR I C M CR iwM TC ( R iwL) VP iwM TR IT M TR IT M TR ( R 2 w 2 L2 ) M TC M CR iwR w 2 L2 M TR ( R 2 w 2 L2 ) Where VS M TC M CR VP M TR R 2 wL w 2 L2 i 2 L R R M TC M CR 2 i 2 2 2 M L 1 w L TR R 2 1 2 R wL R is called the response parameter. The real part of the quotient is called the in phase component, the imaginary part is out of phase and is called the quadrature component. VS M TC M CR 2 i M TC M CR i i Ae VP M TR L 1 2 M TR L 1 2 1 1 R tan tan wL 1 By observing both the amplitude and phase of the recorded EM field, we can estimate R and L. A variety of EM examples