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1.
Gravity Geophysical Method
The gravity field on the surface of the Earth is not uniformly the same everywhere. It varies with the distribution of the mass materials below. This lateral change can be measured and interpreted in terms of likely causative geology. A Gravity survey is an indirect (surface) means of calculating the density property of subsurface materials. The higher the gravity values, the denser the rock beneath. The purpose of gravity surveys is to aid in the detection and delineation of subsurface geological features. Gravity meters measure the differences in gravitational attraction between points of measurement. Nongeological effects are removed from the measured values to produce gravity anomalies that are caused by rocks having anomalous density in comparison with the density of the surrounding rocks. 2. Magnetics methods The earths magnetic field induces a secondary magnetic field in ferrous materials. While all materials exhibit this susceptibility to a certain extent, iron and steel materials generally produce an effect that is easily measurable. Geologic materials with ferrous minerals (usually magnetite) are good targets. Man-made iron and steel items such as drums and tanks are often sought using measurements of the magnetic field. 3. Electromagnetic methods Electromagnetic methods (EM) measure electrical currents generated in the subsurface resulting from the induction of time-varying magnetic fields at the ground surface. The EM method measures the bulk conductivity of subsurface material beneath and between the instruments transmitter and receiver coils. The readings are commonly expressed in the conductivity units of milli-ohms/meter (m-ohms/m) or milliseimens/meter (mS/m). EM Surveys are used for locating subsurface zones of highly-fractured bedrock, buried steel drums and tanks, plumes of ground-water contamination, and clay-rich horizons. 4. Seismic methods techniques used in geophysical exploration, based on the study of the propagation characteristics of elastic (seismic) waves in the earths crust; such techniques are used to investigate the crusts geological structure. Seismic exploration makes use of reflected and refracted waves and the piezoelectric effect.
The principal seismic exploration techniques are the reflection method and the refraction method; both make use of the difference in the elastic properties and density of rocks. In the reflection method a seismic wave excited by an explosion or by mechanical action propagates in all directions from the source and reaches certain reflecting interfaces in succession. At each interface a reflected wave arises and returns to the earths surface, where it is registered by instruments. The reflection method makes it possible to study geological structure at depths of 0.10.2 to 710 km and to determine the depth of seismic boundaries with a precision to 12 percent. The method detects small angular inconsistencies, pinches, and sectors where facies change. The reflection method is the most precise and detailed method of studying sedimentary strata and is used primarily in the search for petroleum and natural gas; it also is used in studying certain ore deposits and in making regional geological studies. The refraction method is based on the observation of waves refracted in a layer with an increased propagation velocity of seismic waves. After the initial refraction, the waves cover a significant part of their path in the layer, are refracted a second time, and then return to the earths surface. 5. Radar Geophysical Methods GPR uses high-frequency (usually polarized) radio waves, usually in the range 10 MHz to 2.6 GHz. A GPR transmitter emits electromagnetic energy into the ground. When the energy encounters a buried object or a boundary between materials having different permittivities, it may be reflected or refracted or scattered back to the surface. A receiving antenna can then record the variations in the return signal. The principles involved are similar to seismology, except GPR methods implement electromagnetic energy rather than acoustic energy, and energy may be reflected at boundaries where subsurface electrical properties change rather than subsurface mechanical properties as is the case with seismic energy. 6. Electrical Geophysical Methods The electrical geophysical methods are used to determine the electrical resistivity of the earth's subsurface. Thus, electrical methods are employed for those applications in which a knowledge of resistivity or the resistivity distribution will solve or shed light on the problem at hand. The resolution, depth, and areal extent of investigation are functions of the particular electrical method employed. Once resistivity data have been acquired, the resistivity distribution of the subsurface can be interpreted in terms of soil characteristics and/or rock type and geological structure. Resistivity data are usually integrated with other geophysical results and with surface and subsurface geological data to arrive at an interpretation.
2D Electrical Resistivity Imaging For The Investigation of The Subsurface Structures at The Proposed Site For Kauridan Estate at Ibagwa - Nike, Southeastern Nigeria
