Principals of Geophysics PDF

Geophysics developed from the disciplines of physics and geology and has no sharp boundaries that distinguish it from other. The use of physics to study the interior of the Earth, from land surface to the inner core is known as solid earth Geophysics. Solid Earth Geophysics can be subdivided into...

Geophysics developed from the disciplines of physics and geology and has no sharp boundaries that distinguish it from other. The use of physics to study the interior of the Earth, from land surface to the inner core is known as solid earth Geophysics. Solid Earth Geophysics can be subdivided into Global Geophysics or pure Geophysics and Applied Geophysics. Global Geophysics is the study of the whole or substantial parts of the planet. Geophysical methods may be applied to a wide range of investigations from studies of the entire earth to exploration of a localized region of the upper crust, such as plate tectonics, heat flow and paleo- magnetism. Applied Geophysics is the study of the Earth's crust and near surface to achieve an economic aim, or making and interpreting measurements of physical properties of the earth to determine subsurface conditions usually with an economic objectives ( e.g. discovery of fuel or mineral deposities). It Comprises the following subjects: 1- Determination of the thickness of the crust (which is important in hydrocarbon exploration. 2- Study of shallow structures for engineering site investigations. 3- Exploration for ground water and for minerals and other economic resources. 4- Trying to locate narrow mine shafts or other forms of buried cavities. 5- The mapping of archaeological remains. 6- Locating buried piper and cables. Example (Geophysics applications):  Exploration seismology used mainly in oil exploration, have been used in academic studies relating to the structure of the earth's crust and upper mantle.  Geophysical measurements within geographically restricted areas are used to determine the distributions of physical properties at depth that reflect the local subsurface geology.  An alternative method of geophysical investing subsurface geology is, of course, by drilling borehole, but these are expensive and provide information only at discrete locations. Geophysical surveying provide a relatively rapid and effective means of deriving distributed information on subsurface geology. Relation between Geology and Geophysics: Geology: It involves the study of the earth by direct observations on rocks either from surface exposures or from boreholes and the deduction of its structures, composition and historical evolution by analysis of such observations. GEOPHYSICS: It involves the study of the inaccessible earth by means of physical measurements, usually on or above the ground surface. It also includes interpretation of the measurements in terms of subsurface structures and phenomena. Geophysical studies are quantitative and tangible, whereas geological studies are qualitative and descriptive. Example (1) In exploration Geophysics for oil, the petroleum geologists extract quantitative information from geophysical data (such as seismic records, well logs,...). On the other hand, Geophysicists who are concerned with measurements of physical phenomena are incorporating more geology in order to increase the reliability of the conclusions. Example (2) The information gained about the sea floor spreading and plate tectonics is due to integrating geophysical and geological information. Every earth scientist, especially the geologist, should be familiar with the methods of geophysics. This familiarity should enable one to know: a – which of the geophysical methods can (or cannot) be of help in a given geological situation. b- The limitations of the geophysical methods. * The incorporation of the available geophysical information in interpretation of geophysical measurements is very important PHYSICAL PROPERTIES OF ROCKS The physical properties of rocks that are most commonly utilized in geophysical investigations are: - Density - Magnetic susceptibility - Elasticity - Electrical resistively or conductivity - Radioactivity - Thermal conductivity * These properties have been used to devise geophysical - Gravity method - Magnetic method - Seismic method - Electrical and electromagnetic methods - Radiometric method - Geothermal method 1- Rock Densities: Any Geologic condition that result in a horizontal variation in density will cause a horizontal variation in gravity or a gravity anomaly. It is therefore the significant parameter in gravity Exploration (i. e. the anomaly source is a local variation in density). * Two problems are faced in connection with this parameter (i. e. density). 1- The maximum density variation between different rocks and between rocks and minerals is approximately (2). This is a very small change compared to the range of magnetic susceptibility (≈105) and electrical conductivity (≈1010 ). 2- It is not possible to measure density in situ. A density borehole logger has been used to a limited extent in oil exploration * It is necessary to make density measurements in the laboratory on small samples of outcrops or drill cores. In this case, the laboratory results do not necessarily give the true bulk density of the formation, since the samples may be weathered, fragmented or dehydrated. * Sedimentary rocks have lower densities than igneous and metamorphic rocks. Their densities depend on: composition, porosity and pore fluids, their age and depth below surface (i.e. the density increases with depth and time because the rock becomes compacted and consolidated). * Igneous rocks are denser than sedimentary rocks * basic igneous rocks have larger densities than acidic forms Porosity of minor significance in igneous and metamorphic rocks, unless they are highly fractured. Examples *Density of metamorphic rocks increases with the degree of metamorphism since this process tend to fill pore spaces and recrystallize the rock in a denser form. *The density of metamorphic rock increases as the acidity decreases. * Non – metallic minerals are of lower density than the average of rocks. *Metallic minerals are heavier than this average. Copper 8.7 silver 10.5 Galena 7.5 2- Magnetic susceptibility of rock and minerals (K) When a magnetizable body is subjected to an external magnetizing field (H), it acquires a magnetization that is lost when the applied field (H) is removed. * Such a magnetization ( ji ) is said to be induced by the applied magnetizing field (H).  ( Ji ) is parallel and proportional to the applied field (H) Ji = KH (K) is called the magnetic susceptibility. A substance is called Diamagnetic if the (K) is negative; it is called Paramagnetic and Ferromagnetic if (K) is positive. Magnetic susceptibility is the significant variable in magnetic, playing the same role as density in gravity. Sedimentary rocks have the lowest average susceptibility and basic igneous rocks have the highest. The susceptibility depends upon the amount of ferromagnetic minerals present (i.e. magnetite, ilmenite or pyrrhotite). 3- Elastic properties of rocks ( Elasticity): The seismic method utilizes the propagation of waves through the earth. This propagation depends upon the elastic properties of rocks. The size and shape of a solid body can be changed by applying forces (stresses) to the external surface of the body. These external forces (stresses) are opposed by internal forces (stain) which resist the changes in size and shape. As a result the body tends to return to its original condition when the external forces are removed. This property is called Elasticity, The theory of elasticity relates the forces which are applied to the external surface of a body to the resulting changes in size and shapes. The relations between the applied forces and the deformations are expressed in terms of Stress and Strain Stress is a measure of the forces (F) per unit area across a surface element (A) within the material. S=F/A when (F) is perpendicular to the area element, the stress is called Normal Stress Normal stress can be classified into Tensile stress if the force is directed away from the material or Compressive stress if the force is directed into the material.  * When (F) is tangential to the area element, the stress is a Shearing stress Strain is a measure of the relative deformation (expressed per unit length or per unit volume) of a body when it is subjected to a stress. * It is the change in size or shape. A change in shape with no change in volume is called a Shear strain or distortion A change in volume without change in shape is called a dilatation or contraction. Strains that are associated with relative change in length in the direction of stresses are called Normal strains. Elastic properties of materials (Elastic or elastic constants) The elastic properties of a material are described by certain elastic moduli or elastic constants which specify the relationships between different types of stress and strain. The hanging body with load on its bottom is responded to this load by a change of its length associated with change of its diameter, the new dimensions of the body will show increasing in its length and decreasing in its diameter with new Length L and new diameter d (Lo and do are the original length and diameter of the body). Two kinds of strain will resulting longitudinal and transverse. Remember that the strain is the response of the to stress Poisson measures strain resulting in both directions Long and Trans and divide each others with negative sign for transverse strain to indicate its shorten from original shape. possion's ratio is a constant number for each material Poisson measures this ratio for each material where the strain for materials are different. For example if we have concrete showing long strain value of 0.01 due to any stress this directly lead us to know that its transverse strain is 0.002 Example: A cylinder of a length (L) and diameter (D) when subjected to a tensile stress parallel to (L), the length will be elongated by (∆L) and the diameter will be shortened by (∆D). The opposite will occur if it is subjected to a compressional stress, the length will be shortened by (∆L) and the diameter will be increased by (∆D). In either case, σ = (∆D/D)/ (∆L/L) For most rock, the value of (σ) is about 0.25, for liquids the value of (σ) attains its maximum possible value of (0.5) as the liquids have no rigidity (μ = 0). The relations between the elastic moduli are given by the following formulas: E = 9 μ K / (μ + 3K) K= E/3 (1-2σ) μ = E/2 ( 1+σ) σ = (3K-2μ) / (6K+2 μ). 3- Electrical properties of rocks : Several electrical properties of rock are significant in electrical prospecting which are: A- Natural electrical potential. B- Electrical conductivity (or the inverse, electrical resistivity). C- Dielectric constant. Electrical conductivity is the most important while the others are of minor significance.  B: Electrical conductivity (or the inverse, electrical resistivities) Electrical current is propagated in rocks and minerals depending on the electrical resistivities of these materials , The electrical resistance of a material is expressed in terms of its resistivity Example if the resistance between opposite faces of a conducting cylinder of length (L) and cross sectional area (A) is (R), the resistively (ρ) is given by: ρ=RA/L If "A" is in meters2 "L" in meter, "R" in ohm, "ρ" will be in ohm- meter. If these dimensions are in cm ,"ρ" will be ohm – centimetre, where: 1 Ωm - 100 Ω cm. 'R' is given in terms of the voltage (V) applied across the ends of the cylinder and the resultant current (I) flowing through it , by ohm's law: R= V/I "R" in ohm "v" in volt and "I" in ampere. The conductivity (σ ) is the reciprocal of resistivity: σ = I / ρ = L/RA= (I/A) / (V/L) = J/E mhos/m or mhos/ cm, where J = current density (ampere/m2), E= electric field (volt/m). Most mineral grains are insulators except metallic ores and clay minerals. Electric conduction in these mineral grains being through interstitial water in pores and fissures. The conductivity of a porous rock varies with the volume and arrangement of the pores and the conductivity and amount of contained water. Hard rocks are bad conductors of electricity, but conduction may take place along cracks and fissures. in porous sedimentary formations, the degree of saturation and the nature of the pore electrolytes govern the resistivity. 5 – Radioactivity of rocks: Radioactivity of rocks and minerals are attributed to traces of uranium, thorium and the isotope of potassium (K40) and their radioactive decay products. Among the earth's rocks, granites and shale show the largest radioactivity. In general, the radioactivity in sedimentary rocks and metamorphosed sediments is higher than that in igneous and other metamorphic types, with the exception of potassium – rich granites. 6- Thermal properties of rocks: It is a fact that the temperature increases with depth. Therefore, heat must be flowing upward in the earth. The amount of heat flow depends on the thermal conductivity of the rocks. The thermal conductivity is a measure of how easily heat flows through a material. GENERAL REVIEW OF GEOPHYSICAL METHODS The physical properties of rocks have been used to devise geophysical methods that are essential in the search for minerals, oil and gas and other geological and environmental problems. * These methods are: 1- Gravity method 2- Magnetic method 3 - Seismic method 4- Electrical method 5- Electromagnetic method 6- Radiometric method 7- Geothermal method Geophysical methods respond to the physical properties of the subsurface media (rocks, sediments, water, voids, etc.. ) and can be used Successfully when one region differs sufficiently from another in some physical property. These methods can be classified into two distinct types: 1-Passive methods: Which detect variations within the natural fields associated with the earth, like the gravitational and magnetic fields, such as gravity, magnetic, some electric and some electromagnetic methods, radioactive and geothermal methods. 2-Active methods: These artificially generated signals transmitted into the ground and then modify the received signals in ways that are characteristic of the materials through which they travel. Examples of these methods are seismic and some electrical methods.  GENERAL REVIEW OF GEOPHYSICAL METHODS Generally, natural field methods (passive methods) can provide information on earth properties to greater depths and are simpler to carry out than artificial source methods (active methods). Moreover, the artificial source methods are capable of producing a more detailed and better resolved picture of the subsurface geology. The various geophysical methods depend on different physical properties. For example: gravity methods are sensitive to density contrasts within the sub-surface geology and so are ideal for exploring for major sedimentary basins where there is a large density contrast between the lighter sediments and the denser underlying rocks. The basic geophysical methods are listed below with the physical properties to which they relate and their main uses. Geophysical methods and their main applications Applications :- 1-Hydrocarbon exploration (coal, gas, oil) 2-Regional geological studies (over areas of 100s of km2 ) 3- Exploration of mineral deposits. 4- Engineering site investigation. 5- Hydrogeological investigation. 6- Detection of subsurface cavities. 7- Mapping of leachate and contaminant plumes. 8- Location and definition of buried metallic objects. 9-Archaeo-geophysics. 10- Forensic geophysics.  Several geophysical surveying methods can be used at sea ( marine geophysics ) or in the air (aero- geophysics )  Reconnaissance surveys are often carried out from the air because of the high speed of operation. In such cases the electrical or seismic methods are not applicable, since these require physical contact with the ground the direct input of energy. Geophysical methods are often used in combination. prospecting for oil usually includes gravity, magnetic and seismic surveying , The importance of such combination appears in the interpretation stage, ambiguity arising from the results of one survey method may be removed by consideration of results from a second survey method. Airborne versus ground geophysical methods: Airborne Ground reconnaissance work detailed investigations. Fast and inexpensive More expensive Several surveys can be done at once One survey at once More accuracy in some areas More detailed (sharp signal) The cost of an airborne electromagnetic survey, with magnetic and radiometric data included is likely to be 1/4 to 1/5 the cost of an equivalent ground EM survey  Geophysical Methods This just for quick review different geophysical methods we may give more focus on Gravity and seismic methods as they are of more interst in oil and gas exploration This just for quick review different geophysical methods we may give more focus on Gravity and seismic methods as they are of more interst in oil and gas exploration This just for quick review different geophysical methods we may give more focus on Gravity and seismic methods as they are of more interst in oil and gas exploration This just for quick review different geophysical methods we may give more focus on Gravity and seismic methods as they are of more interst in oil and gas exploration This just for quick review different geophysical methods we may give more focus on Gravity and seismic methods as they are of more interst in oil and gas exploration This just for quick review different geophysical methods we may give more focus on Gravity and seismic methods as they are of more interst in oil and gas exploration This just for quick review different geophysical methods we may give more focus on Gravity and seismic methods as they are of more interst in oil and gas exploration This just for quick review different geophysical methods we may give more focus on Gravity and seismic methods as they are of more interst in oil and gas exploration This just for quick review different geophysical methods we may give more focus on Gravity and seismic methods as they are of more interst in oil and gas exploration This just for quick review different geophysical methods we may give more focus on Gravity and seismic methods as they are of more interst in oil and gas exploration This just for quick review different geophysical methods we may give more focus on Gravity and seismic methods as they are of more interst in oil and gas exploration This just for quick review different geophysical methods we may give more focus on Gravity and seismic methods as they are of more interst in oil and gas exploration This just for quick review different geophysical methods we may give more focus on Gravity and seismic methods as they are of more interst in oil and gas exploration 1- Gravity method: It is mainly used for oil exploration. Sometimes in mineral and ground water prospecting. Gravity prospecting involves the measurement of variations in the gravitational field of the earth (i.e. minute variations in the pull of gravity from rock within the first few miles of the earth's surface). Different types of rock have different densities and the denser rocks have the greater gravitational attraction. If the higher–density formations are arched upward in a structural high, such as an anticline, the earth's gravitational field will be greater over the axis of the structure than along its flanks. A salt dome which is generally less dense than the rock into which it is intruded, can be detected from the low value of gravity recorded gravity recorded above it compared with that measured on either side. Anomalies in gravity which are sought in oil exploration may represent only one - millionth or even one - ten - millionth of the earth's total field. For this reason, gravity instruments are designed to measure variations in the force of gravity from one place to another than the absolute force itself. The gravity method is useful wherever the formations of interest have densities which are appreciably different from those of surrounding formations. Gravity is an effective means of mapping sedimentary basins where the basement rocks have a higher density than the sediments. Gravity is also suitable for locating and mapping salt bodies because of the low density of salt compared with that of surrounding formations. Gravity can be used for direct detection of heavy minerals such as chromite 2- Magnetic method: Magnetic method is commonly used for locating concentrations of magnetic materials or determining depth to basement. Magnetic method deals with variations in the magnetic field of the earth which are related to changes of structures or magnetic susceptibility in certain near surface rocks. In mining exploration, magnetic methods are employed for direct location of ores containing magnetic minerals such as magnetite. Similarities with Gravity Geophysical exploration techniques that employ both gravity and magnetics are passive. By this, we simply mean that in these methods we measure a naturally occurring field of the earth. Identical physical and mathematical representations can be used to understand magnetic and gravitational forces. For example, the fundamental element used to define the gravitational force is the point mass. The fundamental magnetic element is called a magnetic monopole. The acquisition, reduction, and interpretation of gravity and magnetic observations are very similar. Differences with Gravity 1-The fundamental parameter that controls gravity variations of interest to us as exploration geophysicists is rock density. The densities of rocks and soils vary little from place to place near the surface of the earth. The highest densities we typically observe are about 3.0 gm/cm3, and the lowest densities are about 1.0 gm/cm3. The fundamental parameter controlling the magnetic field variations of interest to us, magnetic susceptibility, on the other hand, can vary as much as four to five orders of magnitude. This variation is not only present amongst different rock types, but wide variations in susceptibility also occur within a given rock type. Thus, it will be extremely difficult with magnetic prospecting to determine rock types on the basis of estimated susceptibilities. 2- Unlike the gravitational force, which is always attractive, the magnetic force can be either attractive or repulsive. Thus, mathematically, monopoles can assume either positive or negative values. 3- A properly reduced gravitational field is always generated by subsurface variations in rock density. A properly reduced magnetic field, however, can have as its origin at least two possible sources. It can be produced via an induced magnetization, or it can be produced via a remanent magnetization. It is difficult, however, to distinguish between these possible production mechanisms from the field observations alone. 4- Unlike the gravitational field, which does not change significantly with time, the magnetic field is highly time dependent. 3- Electrical methods: Electrical prospecting uses many techniques, based on different electrical properties of the earth's materials such as:  The resistively method is designed to give information about the electrical conductivity of the earth's rocks.  In resistivity method the current is driven through the ground using a pair of electrodes and the resulting distribution of the potential in the ground is mapped by using another pair of electrodes connected to a sensitive voltmeter.  The resistivity method has been used to map boundaries between layers having different conductivities.  It is used in groundwater studies to determine salinity.  The induced polarization (IP) makes use ionic exchanges on the surfaces of metallic grains (disseminated sulphides).  The self potential method is used to detect the presence of certain minerals which react with electrolytes in the earth to generate electrochemical potentials.  Electromagnetic methods detect anomalies in the inductive properties of the earth's subsurface rocks.  Electromagnetic methods are used to detect metallic ore bodies. Resistivity Induced polarization Self Potential Electromagnetic  4- Seismic methods: There are two main seismic methods, reflection and refraction: 1- seismic reflection method : This method is used to map the structure of subsurface formations by measuring the times required for a seismic wave, generated in the earth by a near surface exploration of dynamite, mechanical impact or vibration, to return to the surface after reflection from interface between formations having different physical properties. The reflections are recorded by detecting instruments which are called geophones responsive to ground motion. Variations in the reflection times from place to place on the surface indicate structural features in the strata below. Depths to reflecting can be determined from the times using seismic velocity information. With reflection method one can locate and map such features as anticlines, faults, salt domes and reefs. Many of these are associated with the accumulation of oil and gas. 2- seismic refraction method : In refraction method, the detecting instruments recorded the arrival times of the seismic waves when refracted from the surface of discontinuity. These times give information on the velocities and depths of the subsurface formations along which they propagate. Refraction method makes it possible to cover a given area in a shorter time and more economically than with the reflection method. When shooting and sound waves created, suppose there are two layers with two velocities V1 and V2 Waves will spread through first layer as concentric circles with velocity V1 (first layer speed) velocity V1 is first layer velocity Until waves reaches V2 is second layer layer velocity velocity to boundary between two layers it will incident angle I is incident r is refracted angle change its direction and refracted according to Snell's law If V2>V1; then as i increase, r increase faster, this means refracted angle will be bigger than incident angle. This will continue happen till waves incidence at certain angle its refracted angle reach 90 degree which we call it critical angle Refraction will happen when waves incidence angle less than critical angle , with increasing incidence angle refraction angle will increase till critical angle at which refraction angel become 90, after this reflection will happen. Refracted waves at critical angle which will move at two layers boundary will move also into bottom layer but with lower V2 and then will be reflected and reach to geophones Type of waves detected by geophones: 1- Direct wave which propagate from source and moves through top layer and not reach to boundary between layers 2- critical waves (head waves) which will first moves at boundary and then reflected to geophones. 3- reflected waves which have an incidence angle bigger than critical angle and will fully reflected without any refraction. Our challenge here is to calculate both V1 and V2 along with layer depth Radioactive Method : This method is used to detect radioactive minerals such as uranium and thorium. Well logging method: This involves probing the earth with instruments which give continues readings recorded at the surface as they are lowered into boreholes. The rock properties which are covered by well logging techniques are electrical resistivity, self potential, gamma ray , density, magnetic susceptibility and acoustic velocity. Well logging is one of the most widely used of all geophysical techniques  Gamma Ray (GR): natural gamma ray emission from the formation  Density: formation density as measured by gamma ray Compton scattering via a radioactive source and gamma ray detectors. This may also include a photoelectric effect (Pe) measurement.  Neutron porosity: formation porosity derived from the hydrogen index (HI) as measured by the gamma rays emitted when injected thermal or epithermal neutrons from a source in the string are captured in the formation  Sonic: the transit time of compressional sound waves in the formation  Resistivity: the formation resistivity for multiple depths of investigation as measured by an induction-type wave resistivity tool  Natural gamma ray spectroscopy : This tool works on the same principal as the gamma ray, although it separates the gamma ray counts into three energy windows to determine the relative contributions arising from (1) uranium, (2) potassium, and (3) thorium in the formation. As described later in the book, these data may be used to determine the relative proportions of certain minerals in the formation.  Spontaneous potential (SP): This tool measures the potential difference naturally occurring when mud filtrate of a certain salinity invades the formation containing water of a different salinity. It may be used to estimate the extent of invasion and in some cases the formation water salinity.  Caliper: This tool measures the geometry of the hole using either two or four arms. It returns the diameter seen by the tool over either the major or both the major and minor axes. GEOPHYSICAL ANOMALIES It is the local variation in a measured parameter, relative to some normal background variation is attributed to a localized subsurface zone of distinctive physical property and possible geological importance. A local variation of this type is known as a geophysical anomaly. Example: The Earth's gravitational field after the application of certain corrections would everywhere be constant if the subsurface were of uniform density. Any lateral density variation associated with a change of subsurface geology results in a local deviation in the gravitational field This local deviation from the otherwise constant gravitational field is referred to as a gravity anomaly. It may be positive (high anomaly) or negative (low anomaly).