Applied Geophysics Lecture Notes PDF

APPLIED GEOPHYSICS (SETP 4223) LECTURER: TS. DR. CHONG AIK SHYE ROOM: N 01 (244) APPLIED GEOPHYSICS (SETP 4223) CHAPTER 1 INTRODUCTION INTRODUCTION • • Definition: Investigating the Earth, using the methods and techniques of Physics. Two types: a. Pure/general Geophysics: Study of the Earth by...

APPLIED GEOPHYSICS (SETP 4223) LECTURER: TS. DR. CHONG AIK SHYE ROOM: N 01 (244) APPLIED GEOPHYSICS (SETP 4223) CHAPTER 1 INTRODUCTION INTRODUCTION • • Definition: Investigating the Earth, using the methods and techniques of Physics. Two types: a. Pure/general Geophysics: Study of the Earth by using methods of physics. Deduce the physical properties of the Earth and its internal constitution from the physical phenomena associated with it, such as geomagnetic field. INTRODUCTION b. Applied/exploration Geophysics: • To investigate specific, relatively small-scale and shallow features which are presumed to exist within the Earth’s crust. • Uses surface methods to measure the physical properties of the subsurface Earth, in order to detect or infer the presence and position of ore minerals, hydrocarbons, geothermal reservoirs, groundwater reservoirs, and other geological structures. Application of Applied Geophysics • The techniques are used in the fields of: – Engineering geophysics • Study on soil mechanics (ground properties) • Provide development plans – Environmental geophysics • Study on disposal waste site • Monitoring waste deposits/location of contamination – Exploration geophysics • Search for economic deposits (include mineral, ground water, oil and gas). • Monitor fluid flow Methods in Applied Geophysics • Seismic methods, reflection and refraction seismology: main method in petroleum exploration. • Gravity method: petroleum exploration • Electrical surveying: mineral exploration • Magnetic surveying: mineral & petroleum exploration. • Electromagnetic surveying: environmental geophysics • Borehole geophysics: well logging Active and Passive • Passive: Measure naturally occurring phenomena – Gravity field – Magnetic field – Seismic arrivals - earthquakes • Active: Transmit a signal into the subsurface and record what come back – Seismic arrival – explosions – Electrical current – Electromagnetic waves Methods of Geophysical Survey and Usage Reflection Seismic Method • Most extensively used – based on application. • Uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismic source of energy. • By noting the time (travel time) it takes for a reflection to arrive at a receiver, it is possible to estimate the depth of the feature that generated the reflection. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Reflection Seismic Method • Depth of penetration up to 20,000 ft • Can detects lithology and gas directly. • Reflection seismic section images are quite similar to geologic cross-sections. Reflection Seismic Method • Depth of penetration up to 20,000 ft • Can detects lithology and gas directly. • Reflection seismic section images are quite similar to geologic cross-sections. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Refraction Seismic Method • Utilised refracted waves to map subsurface structure. • To record seismic signal at large distance from source point as compared to depth. • Was much used before 1930 for oil prospecting but has now been replaced by the reflection method. • In recent years, however, has found increasing use in civil engineering projects for bedrock investigation. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Gravity Method • Measurements of the gravitational field at a series of different locations over an area of interest. • The objective is to associate variations with differences in the distribution of densities and hence rock types (dense and compacted rocks will give higher gravitational attraction as compared to porous rocks). • Usually used in preliminary survey. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Magnetic Method • Measurement of the local geomagnetic field produced by the variations in the intensity of magnetization in rock formations. • Sedimentary rock has less magnetic as compared to igneous and metamorphic rocks. • To map subsurface structure of basement or search magnetic minerals such as magnetite. • Usually used in preliminary survey. • . QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Electric & Electromagnetic Methods • Electrical method – to get information on formation or rock body that has electrical conductivity. • Several types: Induce Polarization (IP) – when electronically conducting minerals (or metals) or clay minerals (due to ion exchange) are present in the ground. • Self Potential (SP) – mesuaring the natural potential differences which generally exist between any two points on the ground, due to electrochemical actions in rocks, eg. sulfite and graphite ores. • Electromagnet – measuring natural or induced electromagnetic field. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Electrical Resistivity Electrical Resistivity QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Methods in Applied Geophysics Method  Seismic  Gravity  Magnet  Electric: • Resistivity • IP • SP  Electromagnetic Parameter measured Physical properties  Travel times of seismic  Density and elastic waves moduli  Variations in the strength  Density of the gravitational field  Variation in the strength  Magnetic susceptibility of the geomagnetic field • Earth resistivity • Polarization voltage • Electrical potential  Response to electromagnetic radiation • Electrical conductivity • Electrical capacitance • Electrical conductivity  Electrical conductivity & inductance Application of Seismic Survey Application • Exploration for metallic minerals • Exploration for nonmetallic mineral • Exploration for petroleum & coal • Ground water • Engineering site study Suitable method S, G, M, (EM) M, EM, E, SP & IP S, (E), (G) E, EM, S, (G) E, S, EM, (G), (M) QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.