21 - ERTH2&0&/1&...

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Unformatted text preview: ERTH2&0&/1& Exploration geophysics 1 EXPLORATION GEOPHYSICS Instructor: Claire Samson@ PhBDB@ PBEngB Photo: CB Samson ERTH2&0&/1& Exploration geophysics 2 Lecture obFectives To understand the physical concepts behind the seismic and gravity geophysical methods To understand how seismic and gravity data are acKuired and processed to lead information about subsurface physical properties ERTH2&0&/1& Exploration geophysics 3 Contents Exploration geophysics Exploration seismology Seismic wave propagation Seismic field data acKuisition Seismic data processing Gravity Physical basis Gravity field data acKuisition Gravity data processing Applications ERTH2&0&/1& Exploration geophysics & Exploration geophysics ERTH2&0&/1& Exploration geophysics 5 Exploration geophysics Making and interpreting measurements of physical properties of the Earth to determine subsurface conditions@ usually with an economic or environmental obFectiveU Sheriff@ RBEB 1VW&B Encyclopedic dictionary of exploration geophysicsB ERTH2&0&/1& Exploration geophysics X Exploration geophysics Examples of geophysical methods: Seismics methods Reflection seismics Refraction seismics Gravity Electromagnetic (EM) methods Magnetic methods Radiometric methods@ etcB ERTH2&0&/1& Exploration geophysics [ Exploration seismology ERTH2&0&/1& Exploration geophysics W Exploration seismology Mainly based on the reflection and refraction of primary (P) waves on boundaries between different media in the subsurface P\waves: Are easy to generate Travel fastest Propagate in liKuids and solids ERTH2&0&/1& Exploration geophysics V Exploration seismology Controlled approach: speciali]ed eKuipment is used to generate seismic waves at pre\set times Local scale Applications: By far, the most widely used method for oil and gas exploration Scientific crustal studies ERTH2&0&/1& Exploration geophysics 10 Seismic wave propagation ERTH2&0&/1& Exploration geophysics 11 Physical properties Seismic impedance Z [ kg/m 2 s]: ^ = v ! v [m/s] : P\wave seismic velocity ! [kg/m 3 ] : density In general@ harder the rock@ higher ^ ERTH2&0&/1& Exploration geophysics 12 Energy partitioning at interfaces Conservation of energy across the interface between two different media Relative proportion of energy reflected and transmitted is determined by: Impedance contrast ^ = ! B Angle of incidence of seismic ray ERTH2&0&/1& Exploration geophysics 13 Ray geometry (Medium 1) Z 1 = ! 1 v 1 (Medium 2) Z 2 = ! 2 v 2 Z 2 > Z 1 Transmitted P " 2 > " 1 ERTH2&0&/1& Exploration geophysics 1& Ray geometry Law of reflection Angle of reflection " 1 = Angle of incidence " 1 Snells law (law of refraction) (sin " 1 ) / v 1 = (sin " 2 ) / v 2 Dhen v 2 >v 1 @ transmitted ray is bent away from normal Note: angles measured with respect to the normal ERTH2&0&/1& Exploration geophysics 15 Ray geometry Critical refraction angle...
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21 - ERTH2&0&/1&...

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