Applied Geophysics (Electric, Refraction) Course PDF

‫الجيوفيزياء التطبيقية‬ Applied Geophysics (Electric, Refraction) DR. MUHAIMEN AL_MUTAR, COLLEGE OF SCIENCE, UNIVERSITY OF BASRA, DEPARTMENT OF GEOLOGY  ELECTRICAL RESISTIVITY TECHNIQUES Geophysical resistivity techniques are based on the response of the earth to the fl...

‫الجيوفيزياء التطبيقية‬ Applied Geophysics (Electric, Refraction) DR. MUHAIMEN AL_MUTAR, COLLEGE OF SCIENCE, UNIVERSITY OF BASRA, DEPARTMENT OF GEOLOGY  ELECTRICAL RESISTIVITY TECHNIQUES Geophysical resistivity techniques are based on the response of the earth to the flow of electrical current. In these methods, an electrical current is passed through the ground and two potential electrodes allow us to record the resultant potential difference between them, giving us a way to measure the electrical impedance of the subsurface material. The apparent resistivity is then a function of the measured impedance (ratio of potential to current) and the geometry of the electrode array. Depending upon the survey geometry, the apparent resistivity data are plotted as 1-D soundings, 1- D profiles, or in 2-D cross-sections in order to look for anomalous regions.  In the shallow subsurface, the presence of water controls much of the conductivity variation. Measurement of resistivity (inverse of conductivity) is, in general, a measure of water saturation and connectivity of pore space. This is because water has a low resistivity and electric current will follow the path of least resistance. Increasing saturation, increasing salinity of the underground water, increasing porosity of rock (water-filled voids) and increasing number of fractures (water-filled) all tend to decrease measured resistivity. Increasing compaction of soils or rock units will expel water and effectively increase resistivity. Air, with naturally high resistivity, results in the opposite response compared to water when filling voids. Whereas the presence of water will reduce resistivity, the presence of air in voids should increase subsurface resistivity.  Resistivity measurements are associated with varying depths depending on the separation of the current and potential electrodes in the survey, and can be interpreted in terms of a lithologic and/or geohydrologic model of the subsurface. Data are termed apparent resistivity because the resistivity values measured are actually averages over the total current path length but are plotted at one depth point for each potential electrode pair. Two dimensional images of the subsurface apparent resistivity variation are called pseudosections. Data plotted in cross-section is a simplistic representation of actual, complex current flow paths. Computer modeling can help interpret geoelectric data in terms of more accurate earth models.  Geophysical methods are divided into two types : Active and Passive Passive methods (Natural Sources): Incorporate measurements of natural occurring fields or properties of the earth. Ex. SP, Magnetotelluric (MT), Telluric, Gravity, Magnetic. Active Methods (Induced Sources) : A signal is injected into the earth and then measure how the earth respond to the signal. Ex. DC. Resistivity, Seismic Refraction, IP, EM, Mise-A-LA-Masse, GPR. Electrical method Instruments 1. Terrameter Signal Averaging System - SAS4000 (Sweden) 2. Syscal (France). 3. Many equipment types were made in USA and other countries.  Electrical method Applications: 1. Calculate the number and depths of the underlying layers and their thicknesses. 2. Calculate the horizontal and vertical resistivities for the underneath layers. 3. Locate the subsurface archaeological remains. 4. Engineering purposes: detect the underlying cavities, voids, weak zones and the locations of dam leakage. 5. Water table determination and its flow direction, pollution of Soil and ground Water and to locate the contact zone between the marine and fresh water (intrusion of the saline marine water). 6. Petroleum Exploration: the most prominent applications of electrical techniques in petroleum Exploration are in well logging. Resistivity and SP are standard Logging techniques. 7. Mineral Exploration: Electrical methods interpretation is difficult below 1000 to 1500 ft. Electrical exploration methods are the dominant geophysical tools in Mineral Expl. Ohm’s Law Ohm’s Law describes the electrical properties of any medium. Ohm’s Law, V = I R, relates the voltage of a circuit to the product of the current and the resistance. This relationship holds for earth materials as well as simple circuits. Resistance( R), however, is not a material constant. Instead, resistivity is an intrinsic property of the medium describing the resistance of the medium to the flow of electric current. Resistivity ρ is defined as a unit change in resistance scaled by the ratio of a unit cross-sectional area and a unit length of the material through which the current is passing. Resistivity is measured in ohm-m or ohm-ft, and is the reciprocal of the conductivity of the material. Table 1 displays some typical resistivities.  Note that, in Table 1, the resistivity ranges of different earth materials overlap. Thus, resistivity measurements cannot be directly related to the type of soil or rock in the subsurface without direct sampling or some other geophysical or geotechnical information. Porosity is the major controlling factor for changing resistivity because electricity flows in the near surface by the passage of ions through pore space in the subsurface materials. The porosity (amount of pore space), the permeability (connectivity of pores), the water (or other fluid) content of the pores, and the presence of salts all become contributing factors to changing resistivity. Because most minerals are insulators and rock composition tends to increase resistivity, it is easier to measure conductive anomalies than resistive ones in the subsurface. However, air, with a theoretical infinite resistivity, will produce large resistive anomalies when filling subsurface voids. MECHANISM OF ELECTRICAL CONDUCTION Mechanism of electrical conduction in Materials is the conduction of electricity through materials which can be accomplished by three means : 1. The flow of electrons Ex. In Metal 2. The flow of ions Ex. Salt water. 3. Polarization in which ions move only a short distance under the influence of an electric field and then stop. 1. Metals : Conduction by the flow of electrons depends upon the availability of free electrons. If there is a large number of free electrons available, then the material is called a metal, the number of free electrons in a metal is roughly equal to the number of atoms. The number of conduction electrons is proportional to a factor called (ε). n ≈ ε E/KT E αn T α 1/n ε : Dielectric constant K: Boltzman’s constant T: Absolute Temperature. E: Activation Energy.  Metals may be considered a special class of electron semi conductor for which E approaches zero. Among earth materials native gold and copper are true metals. Most sulfide ore minerals are electron semi conductors with such a low activation energy. The flow of ions, is best exemplified by conduction through water, especially water with appreciable salinity. So that there is an abundance of free ions. Most earth materials conduct electricity by the motion of ions contained in the water within the pore spaces. There are three exceptions: 1. The sulfide ores which are electron semi conductors. 2. Completely frozen rock or completely dry rock. 3. Rock with negligible pore spaces (Massive lgneous rooks like gabbro). It also include all rocks at depths greater than a few kilometers, where pore spaces have been closed by high pressure, thus studies involving conductivity of the deep crust and mantle require other mechanisms than ion flow through connate water. Classification of Materials according to Resistivities Values A) Materials which lack pore spaces will show high resistivity such as ⮚massive limestone ⮚most igneous and metamorphic (granite, basalt) B) Materials whose pore space lacks water will show high resistivity such as : ❑ dry sand and gravel ❑ Ice. C) Materials whose connate water is clean (free from salinity ) will show high resistivity such as : ❖ clean sand or gravel , even if water saturated. D) most other materials will show medium or low resistivity, especially if clay is present such as : clay soil weathered rock.  The presence of clay minerals tends to decrease the Resistivity because : 1. The clay minerals can combine with water. 2. The clay minerals can absorb cations in an exchangeable state on the surface. 3. The clay minerals tend to ionize and contribute to the supply of free ions. Factors which control the Resistivity 1. Geologic Age 2. Salinity. 3. Free-ion content of the connate water. 4. Interconnection of the pore spaces (Permeability). 5. Temperature. 6. Porosity. 7. Pressure 8. Depth  Field considerations for DC Resistivity 1. Good electrode contact with the earth - Wet electrode location. - Add Nacl solution or bentonite 1. Surveys should be conducted along a straight line whenever possible. 1. Try to stay away from cultural features whenever possible. ⮚ Power lines ⮚ Pipes ⮚ Ground metal fences ⮚ Pumps  Sources of Noise There are a number of sources of noise that can effect our measurements of voltage and current. 1- Electrode polarization. A metallic electrode like a copper or steel rod in contact with an electrolyte groundwater other than a saturated solution of one of its own salt will generate a measurable contact potential. For DC Resistivity, use non-polarizing electrodes. Copper and copper sulfate solutions are commonly used. 2- Telluric currents. Naturally existing current flow within the earth. By periodically reversing the current from the current electrodes or by employing a slowly varying AC current, the affects of telluric can be cancelled. 3- Presence of nearby conductors. (Pipes, fences) Act as electrical shorts in the system and current will flow along these structures rather than flowing through the earth. 4- High resistivity at the near surface. If the near surface has high resistivity, it is difficult to get current to flow more deeply within the earth. 5- Near- electrode Geology and Topography Rugged topography will act to concentrate current flow in valleys and disperse current flow on hills. 6- Electrical Anisotropy. Different resistivity if measured parallel to the bedding plane compared to perpendicular to it. 7- Instrumental Noise. 8- Cultural Feature.  ELECTRODE CONFIGURATIONS The value of the apparent resistivity depends on the geometry of the electrode array used (K factor) 1- Wenner Arrangement Named after wenner (1916). The four electrodes A , M , N , B are equally spaced along a straight line. The distance between adjacent electrode is called “a” spacing. So AM=MN=NB= ⅓ AB = a. Ρa= 2 π a V /I The wenner array is widely used in the western Hemisphere. This array is sensitive to horizontal variations.  2- Lee- Partitioning Array. This array is the same as the wenner array, except that an additional potential electrode O is placed at the center of the array between the Potential electrodes M and N. Measurements of the potential difference are made between O and M and between O and N. Ρa= 4 π a V /I This array has been used extensively in the past. 3- Schlumberger Arrangement. This array is the most widely used in the electrical prospecting. Four electrodes are placed along a straight line in the same order AMNB , but with AB ≥ 5 MN This array is less sensitive to lateral variations and faster to use as only the current electrodes are moved. 4- Dipole – Dipole Array. The use of the dipole-dipole arrays has become common since the 1950’s , Particularly in Russia. In a dipole-dipole, the distance between the current electrode A and B (current dipole) and the distance between the potential electrodes M and N (measuring dipole) are significantly smaller than the distance r , between the centers of the two dipoles.  SURVEY DESIGN Two categories of field techniques exist for conventional resistivity analysis of the subsurface. These techniques are vertical electric sounding (VES), and Horizontal Electrical Profiling (HEP). Vertical Electrical Sounding (VES) The object of VES is to deduce the variation of resistivity with depth below a given point on the ground surface and to correlate it with the available geological information in order to infer the depths and resistivities of the layers present. In VES, with wenner configuration, the array spacing “a” is increased by steps, keeping the midpoint fixed (a = 2 , 6, 18, 54…….). In VES, with schlumberger, The potential electrodes are moved only occasionally, and current electrode are systematically moved outwards in steps AB > 5 MN.  2- Horizontal Electrical profiling (HEP). The object of HEP is to detect lateral variations in the resistivity of the ground, such as lithological changes, near- surface faults……. In the wenner procedurec of HEP , the four electrodes with a definite array spacing “a” is moved as a whole in suitable steps, say 10-20 m. four electrodes are moving after each measurement. In the schlumberger method of HEP, the current electrodes remain fixed at a relatively large distance, for instance, a few hundred meters , and the potential electrode with a small constant separation (MN) are moved between A and B. Multiple Horizontal Interfaces For Three layers resistivities in two interface case , four possible curve types exist. Q – type ρ1> ρ2> ρ3 H – Type ρ1> ρ2< ρ3 K – Type ρ1< ρ2> ρ3 A – Type ρ1< ρ2< ρ3 In four- Layer geoelectric sections, There are 8 possible relations : ρ1> ρ2< ρ3< ρ4 HA Type ρ1> ρ2< ρ3> ρ4 HK Type ρ1< ρ2< ρ3< ρ4 AA Type ρ1< ρ2< ρ3> ρ4 AK Type ρ1< ρ2> ρ3< ρ4 KH Type ρ1< ρ2> ρ3> ρ4 KQ Type ρ1> ρ2> ρ3< ρ4 QH Type ρ1> ρ2> ρ3> ρ4 QQ Type  Another applications of Resistivity Techniques 1. Bedrock Depth Determination Both VES and CST are useful in determining bedrock depth. Bedrock usually more resistive than overburden. HEP profiling with Wenner array at 10 m spacing and 10 m station interval used to map bedrock highs. 2. Location of Permafrost Permafrost represents significant difficulty to construction projects due to excavation problems and thawing after construction. ❖Ice has high resistivity of 1-120 ohm-m 3. Landfill Mapping Resistivity increasingly used to investigate landfills: ⮚ Leachates often conductive due to dissolved salts ⮚ Landfills can be resistive or conductive, depends on contents  Disadvantages of Wenner Array 1. Interpretations are limited to simple, horizontally layered structures 2. For large current electrodes spacing, very sensitive voltmeters are required. Advantages of Resistivity Methods 1. Flexible 2. Relatively rapid. Field time increases with depth 3. Minimal field expenses other than personnel 4. Equipment is light and portable 5. Qualitative interpretation is straightforward 6. Respond to different material properties than do seismic and other methods, specifically to the water content and water salinity  Disadvantages of Resistivity Methods 1. Interpretations are ambiguous, consequently, independent geophysical and geological controls are necessary to discriminate between valid alternative interpretation of the resistivity data ( Principles of Suppression & Equivalence) 2. Interpretation is limited to simple structural configurations. 3. Topography and the effects of near surface resistivity variations can mask the effects of deeper variations. 4. The depth of penetration of the method is limited by the maximum electrical power that can be introduced into the ground and by the practical difficulties of laying out long length of cable. The practical depth limit of most surveys is about 1 Km. 5. Accuracy of depth determination is substantially lower than with seismic methods or with drilling.  Refraction method applications Refraction Instruments Terraloc Mark-6 Terraloc Mark-9 Geophone is essentially only type of sensor used on land. A geophone comprises a coil suspended from springs inside a magnet. When the ground vibrates in response to a passing seismic wave, the coil moves inside the magnet, producing a voltage, and thus a current, in the coil by induction. Geophones Refraction applications: 1. Used as a reconnaissance tool where no information available about subsurface geology. 2. Geotechnical and engineering applications to determine depth to interface. 3. Detection of fracture zones in connection to ground water prospecting. 4. Hazardous waste and disposal program. 5. Underlying soil foundation. 6. Elastic modulus of the underneath layers. 7. Ground water table. Refraction Theory In particular, seismic refraction surveys can provide rapid and a valuable technique to control and solve many problems underneath the ground surface. It based on the measurement of the travel time of seismic waves refracted at the interfaces between subsurface layers of different velocity. Seismic energy (P and S waves) are generated by a source located on the surface (hammer, weight drop or small explosive charge), propagate through the soil and rock, and are recorded by geophones at known distances from the source.  Seismic refraction is mainly depends on the Snell's law. Since the critical angle is equal 90 degree and then the critically refracted waves pass the interface between two media have velocities V1 and V2. When the seismic wave front encounters an interface where seismic velocity drastically (‫حاد‬/‫)عنيف‬ increases, a portion of the wave critically refracts at the interface, traveling laterally along higher velocity layers. Due to a compressional stresses existed along the interface boundary, a portion of the wave front returns to the surface as totally reflected waves.  A series of seismic receivers, geophones (right) are laid out along the survey line at regular intervals and receive the reflected wave energy. Seismic Refraction Data Acquisition: Time-Distance curve: First arrivals of the travel-time versus distance graphs are then plotted and velocities can be calculated for the refractor layers by the means of slope= 1/V. 1. For horizontal two-layer case: 2. For horizontal three-layer case: 3. For the dipping layer case: Seismic velocities of the rock materials: Possible range of seismic wave velocities in a typical rocks, water and air Material P wave Velocity (m/s) S wave Velocity (m/s) Air 332 Water 1400-1500 Petroleum 1300-1400 Steel 6100 3500 Concrete 3600 2000 Granite 5500-5900 2800-3000 Basalt 6400 3200 Sandstone 1400-4300 700-2800 Limestone 5900-6100 2800-3000 Sand (Unsaturated) 200-1000 80-400 Sand (Saturated) 800-2200 320-880 Clay 1000-2500 400-1000 Glacial Till (Saturated) 1500-2500 600-1000  Seismic velocities vary with mineral content, lithology, porosity, pore fluid saturation, pore pressure, and to some extent temperature. From this table of velocities, we can noticed some important points: 1. Travel time of waves depend on media (greatest in igneous, i.e. consolidated rocks, and least in unconsolidated rocks). In Igneous and Metamorphic Rocks, with minimal porosity, seismic velocity increases with increasing mafic mineral content. In Sedimentary Rocks, the effects of porosity and grain cementation are more important, and seismic velocity relationships are complex. 2. Seismic velocity increases markedly from unsaturated to saturated zone. 3. The acoustic velocity of a medium saturated with water is greatly increased in comparison with velocities in the vadose zone. Thus, the refraction method is applicable in determining the depth to the water table in unconsolidated sediments. First Break Picking The onset of the first seismic wave, the first break, on each seismogram is identified and its arrival time picked. Advantages: 1. Provides data to depths of 100 feet or more. 2. Resolves up to 2 or 3 layers. 3. Provides a 2D cross-section of P-wave velocity. 4. The source of seismic energy can be as simple as 8-pound sledge hammer. Limitations: 1. The survey line length (source to farthest geophone) may be 4 to 5 times the desired depth of investigation. 2. Sensitive to acoustic noise and vibrations. 3. Seismic velocity of layers must increase with depth (will not resolve low velocity layers below high velocity layers). 4. Will not detect thin layers. 5. Deep measurements may require explosives as an energy source.