Applied Geophysics GPS 314 PDF

Applied Geophysics is concerned with investigating the earth crust and near surface to achieve a practical and economic aim. It involves determination of various properties of the Earth via the application of physical theories and experimental techniques. Geophysical Techniques Measure Physical Phe...

Applied Geophysics is concerned with investigating the earth crust and near surface to achieve a practical and economic aim. It involves determination of various properties of the Earth via the application of physical theories and experimental techniques. Geophysical Techniques Measure Physical Phenomena Such As: Gravity Magnetism Elastic waves Electricity Electromagnetic waves Which Are Sensitive to Sub-Surface Physical Properties Such As: Density Magnetic susceptibility Seismic wave velocity and density Resistivity Conductance/inductance/permittivity Geophysical survey can be carried out in all environments: (1) on land: ground survey (2) in air: airborne or aero-survey (3) at sea: marine survey (4) from space- satellite derived data GLACIO GEOPHYSICS GEOPHYSICAL METHODS GEOPHYSICAL METHODS Geophysical Methods Gravimeter: it measures the changes in gravity Magnetic Susceptibility Meter from the varying densities and distribution of masses inside the earth. Electrical Resistivity Meter Geophysical Methods Geophysical Methods  Geophysics and Mineral Exploration Mineral exploration is the process of finding ore or mineral deposits in commercially viable concentrations. A near accurate estimation on the volume of mineral deposits is very important because mineral exploration is a capital intensive operation. Mineral Prospecting is the physical search for minerals, fossils, precious metals or mineral. Minerals can be found throughout the world in the earth's crust but usually in such small amounts that they not worth extracting. A mineral deposit that contains enough minerals to be mined for profit is called an ore. Ores are rocks that contain concentrations of valuable minerals eg bauxite is a rock that contains minerals that are used to make aluminum.  Geophysics and Mineral Exploration To a geologist, a mineral is a naturally occurring solid, formed by geologic processes, that has a crystalline structure and a definable chemical composition. Almost all minerals are inorganic.  Geophysics and Mineral Exploration An ore is an occurrence of rock or sediment that contains sufficient minerals with economically important elements, typically metals, that can be economically extracted from the deposit. The ores are extracted from the earth through mining; they are then refined to extract the valuable element, or elements Geophysics Consists of: Measurements ('data acquisition'); Data processing; Interpretation Aims of Geophysical Measurements Are to Determine The Following: Determination of the lithology (Rock Type e.g. layering, Sands, gravel, clay. Detection of Rock Structures e.g fault, fold, inhomogenities and Discontinuities. Determination of rock Geometry e.g 2D or 3D. Detection of rock Fluid Content e.g water, gas, oil. Exploration of buried waste sites, pipes, tanks, contaminated soil. ANOMALIES: are departures from 'regular behaviour' caused by contrasts in physical properties from geological targets. It is a deviation from uniformity in physical properties. A portion of a geophysical survey, such as magnetic or gravitational, that is different in appearance from the survey in general and that is of exploration interest. Recommended Textbooks Environmental and Engineering Geophysics By Prem Sharma Introduction to Geophysical Prospecting By Milton Dobrin An Introduction to Applied and Environmental Geophysics (Paperback) by John M. Reynolds  Electrical Methods There are many electrical and electromagnetic methods used in geophysics. These methods are most often used where sharp changes in electrical resistivity (resistance in the ground) are expected - particularly if resistivity decreases with depth. The resistivity method involves the measurement of the ability of soil, rock and ground water to resist the flow of an electrical current. It is a function of the soil and rock matrix, percentage of fluid saturation and the conductivity of the pore fluids. The electrical resistivity method is used to map the subsurface electrical resistivity structure, which is interpreted by the geophysicist to determine geologic formations and/or physical properties of the geologic materials. The electrical resistivity of a geologic unit or target is measured in ohm-meters (Ω- m), and is a function of porosity, permeability, water saturation and the concentration of dissolved solids in pore fluids within the subsurface.  Electrical Methods Applications i. Shallow Ore prospecting (Massive Ore Deposit) ii. Geological Mapping e.g structures like dykes, faults and shear zones. iii. Groundwater and Engineering Investigation iv. Environmental Studies v. Hydrocarbon Exploration Applied current Methods: when a current is supplied by the geophysicist. Currents are either DC or low frequency waves. In the electrical resistivity method, the potential difference (voltage) is measured at various points;  In the induced polarization method, the rise and fall time of the electric potential are measured. The self-potential method uses currents generated by electro-chemical reactions (natural batteries) associated with many ore bodies. A battery acts as an energy supply, pushing electrons Consider the circuit: around the circuit battery A resistor resists the - + flow of current current meter A voltmeter measures R1 R2 i the potential difference between two points resistor V A current meter volt meter measures the current flow at a point What is the current in the circuit above? BASIC EQUATIONS: current= charge/sec past a point: I =dq/dt = coulombs/sec=amperes I = V/R, V = IR current density = current/cross sectional area: j=I/A resistance= potential /current = Ohms Ω =volts/ampere (Ohm's LAW) Resistance tells us the total drag on the current, but not the property of the material that is generating the drag. BASIC EQUATIONS: We need a measure of the resistance of a material. For a given pipe-shaped material we can define the resistivity as: resistivity = resistance x cross section area/ length:  = R A/L. , with units of m. R =  L / A, I = V/R , V/ I = R  L / A = V/I  = VA/IL, A/L.V/I, A/L.R (Unit is m) We can expect different geologic materials to have greatly different resistivities..  FIELD CONSIDERATION FOR ELECTRICAL RESISTIVITY METHOD i. Poor electrical Contact with the ground Wet electrode location Add NaCl Solution or Bentonite ii. Survey should be conducted along a straight line iii. Avoid cultural features and Noise Power lines, Pipelines, Ground metal fences, rail track iv. Coupling between wires and reels FACTORS CONTROLLING RESISTIVITY OF EARTH MATERIALS (Rocks) i. Porosity of formation ii. Degree of fluid saturation iii. Degree of salinity of saturating fluid iv. Lithology of the formation v. Temperature vi. Degree of compaction (Pressure) vii. Fractures viii.Depth ix. Interconnection of the pore spaces (Permeability) Resistivity of Materials S/N MATERIAL VALUE RESISTIVITY TYPICAL 1. Igneous & Metamorphic 10*2- 10*8 10*4 Rocks 2. Sedimentary Rocks 10- 10*8 10*3 3. Unconsolidated 10*-1- 10*4 10*3 4. Groundwater 1-10 5 5. Pure water 10*3 Electrode configurations/ Arrays The electrode patterns used in resistivity surveying almost always are the Wenner, Schlumberger, dipole- dipole, pole-dipole and pole-pole. In conducting an expanding-spread Wenner survey, we move all electrodes along a straight line after every reading so the spacing between electrodes remains equal and takes on certain prescribed values Electrode configurations/ Arrays In Wenner array four electrodes are arranged collinearly and the separations between adjacent electrodes are equal. The apparent resistivity of the medium measured with this array is given by: Electrode configurations/ Arrays The Schlumberger array is arranged with two current electrodes on the outside of the array, set apart by a distance at least five times the spacing between the two interior potential electrodes. The potential difference measurement is believed to lie at the mid span of the interior potential electrodes, at a depth approximately one half of the length between the exterior current electrodes. The Schlumberger Array is preferred for VES applications due to the strong vertical resolution and ease of setup in the field Electrode configurations/ Arrays The Schlumberger array The apparent resistivity measurement for the Schlumberger Array can be represented by equation given above. In the equation, the spread length or distance between current electrodes is L, and the length between the potential electrodes is expressed by the variable l. With respect to L and l, the apparent resistivity measurement is valid as long as the spread length, L, does not exceed 5 times the potential electrode spacing, l Electrode configurations/ Arrays Assignment Derive the apparent resistivity of Wenner and Schlumberger electrode configurations. Electrode configurations/ Arrays The Pole- dipole array Electrical Resistivity (ER) Techniques in ER Vertical Electrical Sounding (VES) Constant Separation Traversing (CST) The vertical electrical sounding (or drilling) deduces the variation of resistivity with depth below a point on the ground surface and the procedure is based on the fact that the current penetrates deeper with increasing separation of the current electrodes (Wenner and Schlumberger arrays by Reynolds, 1997). CST method is used to detect lateral variation in the resistivity of the ground. The two most common arrays for electrical resistivity surveying involving profiling mode are the Wenner and dipole- dipole arrays (Reynolds, 1997). Constant Separation Traversing Data Acquisition  ER Data Processing/Interpretation Resistivity values obtained on the field and calculated using the geometric factor can be interpreted quantitatively using the curve types depending on the contrast in resistivity values Using the multiple horizontal interface, for three layers of resistivity in two interface case, four possible curve types exist. They include the following ρ1> ρ2> ρ3 Q – type ρ1> ρ2< ρ3 H – Type ρ1< ρ2> ρ3 K – Type ρ1< ρ2< ρ3 A – Type  ER Data Processing/Interpretation Three layer geo-electric section curve types (H, A, K and Q)  ER Data Processing/Interpretation VES curve  ER Data Processing/Interpretation For 1D VES, inversion is carried out using geophysical inversion softwares (Winresist, Res1D etc). In CST, forward modeling is used to calculate the apparent resistivity values of 2-D resistivity data using DIPPRO software or Res2Dinv a popular inversion program (Geotomo) Resistivities and thicknesses of each layer can be derived from the apparent resistivity curve clearly. The apparent resistivity curve can be interpreted by different resistivity models TYPES OF NOISE Coherent (Cultural): systematic noise that can be filtered e.g. power line. They have uniform polarity, definite frequency and are repetitive. Stacking will amplifies signal of interest as well as the coherent noise. Incoherent (Natural): random noise that can be stacked e.g. wind. They have non-uniform polarity, varying frequency and are not repetitive. NOISE REMOVAL Through filtering. A Filter is a system that discriminates against some of the information entering it. The discrimination is usually on the basis of frequency, although other bases such as wavelength or amplitude may be used. Coherent noise are usually removed by filtering Through stacking. Stacking involves combining several signals in order to improve the signal to noise ratio. Stacking of signals with incoherent noise will cancel out the noise. Types of Filter Band pass filter-allows a band of frequency to pass High pass (low-cut): allows high frequency components of a signal to pass Low-pass (high-cut): allows low frequency components of a signal to pass Notch filters sharply reject a very narrow band of frequencies. e.g 50-60 Hz ER Data Processing/Interpretation VESA VES1 133.3 VES2 147.3 A' 118.4 1356.5 354.9 5 152.5 10 1640.8 DEPTH(m) 15 1740.3 LEGEND 20 TOPSOIL 145.7 25 SANDY CLAY 30 141.4 SAND 35 145.7 RESISTIVITY(Ohm-m) Geoelectric section along profile AA ER Data Processing/Interpretation Quantitative Interpretation Limitations of Geophysical Methods Methods require contrast in physical properties Non-uniqueness: The existence of more than one solution regardless of the precision of observations. Resolution is determined by the wavelength of the signal Noise prevents recovery of low amplitude signal e.g. wind, traffic, water pumps, power lines  Induced Polarization Just as the Earth can behave like a battery, it can also behave like a capacitor; in the sense that if you send a current into the ground it charges up and when switched off, a current continues to flow until the charge dissipates, just like a condenser in an electrical circuit. The IP uses the capacitive action of the subsurface to locate zones of interest e.g. where clay/co nductive mineral is disseminated within their host rocks. IP is a current-stimulated electrical phenomenon observed as a delayed voltage response in earth materials. IP phenomena are of electrochemical origin, and depend mainly on the surface characteristics of the pore structure.  Induced Polarization (IP) What is collected? The chargeability in mv/v How is it measured? IP is often measured in conjunction with other electrical properties (spontaneous potential, single point resistance, and electrical resistivity) using a square AC waveform. The IP signal is measured at various time windows during the decay of the current from the on-time to the off-time. The chargeability is defined as the ratio of Electrode array for IP survey (b) this value to the primary on-time voltage transmitter & receiver responses (ΔV/V).  IP SURVEY The resistivity & IP methods involve the measurements of an impedance, with subsequent interpretation in terms of the subsurface electrical properties, and in turn, the subsurface geology. An impedance is the ratio of the response,i.e., output, to the excitation, i.e., input. In both resistivity and IP methods, the input is a current injected into the ground b/w current electrodes & the output is a voltage from the potential electrodes. In the frequency domain impedance measurements, the input current is a sine wave with frequency f, the output also is a sine wave function.  Induced Polarization (IP) Impedance can be also measured in the time domain, in which case the current is turned on and off periodical. The output is the voltage measured at various time when the transmitter current is off. Note that the input is again periodic, because measurements must be made for each of the several periods and then added together or stacked, to eliminate noise. Transmitted & received waveforms in the frequency domains  Induced Polarization (IP) Switching off the current leads to voltage decays Switching on the current make the voltage build-up In practice IP is measured as a changing voltage with time or frequency. The time and frequency IP methods are fundamentally similar and only differ in the way of viewing and measuring waveforms. The inducing current waveforms of both methods are not the same. In IP frequency mode, measurements are made while current is on whereas in the time Transmitted & received waveforms mode, the inducing current is off. in the time domains  Induced Polarization (IP) Logarithmic voltage amplitudes, varying with both time and frequency are shown:  Sources of IP Effects Membrane or ionic/electrolytic polarization: It occurs in rocks that do not contain metallic minerals. Membrane polarization occurs when pore space narrows to within several boundary layer thicknesses. The charges cannot flow easily, so they accumulate when an electric field is applied. The result is a net charge dipole that adds to any other voltages measured at the surface. The other form of membrane polarization occurs where clay particles partially block ionic solution paths. Upon application of an electrical potential, positive charge carriers pass easily while negative carriers accumulate. An excess of both cations and anions arises at an end of the membrane while a deficiency exists at the other extreme. Membrane polarization Electrode polarization  Application of Electrode polarization It varies directly with the mineral concentration, but because it is a surface phenomenon, it should be larger when the mineral is disseminated than when it is massive. The fact that disseminated mineralization gives good IP response is a most attractive feature, because other electrical methods do not work very work in these circumstances.  Factors affecting IP IP is higher for disseminated than massive clay and metallic particles. It depends on the clay and metallic minerals in the rock.  it increases if water in the ground has a low conductivity.  it increases with decreasing porosity It varies with the amount of water in the ground It depends on the current intensity and the current frequency  IP Measurement Measurements of IP may be carried out either in the time or the frequency domain.  In both cases, the voltage is measured as a function either of time or frequency. Time-Domain measurements  The study of the decaying p.d as a function of time is considered as time domain.  In TD, the geophysicist access portions of the subsurface where current flow is maintained for a short period after the applied current is turned off.  (a) The simplest way to measure IP effects with time-domain equipment is to compare the residual voltage V(t) or overvoltage existing at a time t after the current is cut-off with the steady voltage Vc during the current-flow interval or observed voltage.  Chargeability M = V(t)/ Vc (mV/ V or %)  Factors affecting IP IP is higher for disseminated than massive clay and metallic particles. It depends on the concentration of clay and metallic minerals in the rock.  it increases if water in the ground has a low conductivity.  it increases with decreasing porosity It varies with the amount of water in the ground It depends on the current intensity and the current frequency  IP Measurements Apparent chargeability according to Siegel (1959) is defined as : Frequency effect. One measures the apparent resistivity at two or more alternating current frequencies. The FE is usually defined as: ρdc is apparent resistivity measured at DC or at very low AC frequency (0.05 – 0.5Hz) ρa cis the apparent resistivity measured at very high AC frequency (0.1 – 10Hz)  IP Measurements Also, the Percent frequency effect (PFE) is given by: Metal factor (MF). Since the IP effect varies with effective resistivity of the host rock, that is, the type of electrolyte, temperature, pore size, and so forth. The MF parameter corrects for this variation. It is a modification of FE and it is given by: Unit of MF is the same as conductivity [1/ohm-m or or Siemen/m ]  IP Measurements  In a frequency domain IP study, the apparent resistivity values are measured at two frequencies. of 0.1Hz and 5Hz are 120Ωm and 65Ωm respectively. Calculate the (i) Percentage Frequency Effect and (ii) Metal Factor for the IP effects.  In an IP surveys, measurements were made by sending dc pulses for a duration of 10s into the ground. The voltage remaining at time of 2s after the switch-off is 5 V and 12 V is the volatge that existed when the current was flowing. Estimate the polarizability in percent.  IP Measurements  Starting from the equation of frequency effect (FE) and assuming that the chargeability is given by  Show that In theory, because both frequency and time measurements represent the same phenomenon, their results ought to be the same; practically the conversion of time domain to frequency domain and vice versa is quite difficult  IP Measurements  In theory, because both frequency and time measurements represent the same phenomenon, their results ought to be the same; practically the conversion of time domain to frequency domain and vice versa is quite difficult  IP Measurements Multi-electrode & roll-along system as used for resistivity surveys is also valid in IP survey to reduce time of operation and to provide more data coverage.  IP Measurements Multi-electrode & roll-along system as used for resistivity surveys is also valid in IP survey to reduce time of operation and to provide more data coverage. Electrode arrays for resistivity and IP Survey  IP Measurements Multi-electrode & roll-along system as used for resistivity surveys is also valid in IP survey to reduce time of operation and to provide more data coverage. Electrode configuration and movement along survey line  IP Applications Search for disseminated ores, clay minerals, pollution, and groundwater. Over the past two decades IP measurements have improved, and new applications in the environmental field have emerged. Exploration of metalliferous mineral deposits Mapping electrochemical reactions for pollutants in the ground Exploration for groundwater (sensitive to clay in aquifers). Detection of disseminated mineral (difficult with resistivity) Disadvantages of IP It is expensive and slow than resistivity Electrochemical phenomena are still not well understood  IP Interpretation Plotting methods IP results are usually displayed in profiles of Chargeability, Percent frequency effect, phase and so forth , plotted against location. Pseudodepth plots from the results of using a particular electrode array is then obtained by contouring of a tent shape with 45 degree slope as shown. The results are plotted at the midpoint of the spread. In case of the double dipole or the pole-dipole, the midpoint of either the current or potential pair is taken as the station location. Data plot and Contouring  IP Interpretation Plotting methods IP results are usually displayed in profiles of Chargeability, Percent frequency effect, phase and so forth , plotted against location. Pseudosections of a) resistivity b) chargeabilty