abem Terraloc Pro 2 User Manual
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Contents
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ABEM Terraloc Pro 29412 Appendix D. Seismic MethodsThere are a variety of seismic methods used. The objective of the survey controls whichspecific method to use. This section will give you an overview of some commonly usedmethods. Please refer to the bibliography at the end of the manual. If a more detailedand thorough description of seismic methods is needed then Butler (2005) can berecommended as it is relatively new and has an extensive and updated listing ofreferences.In the refraction and reflection methods there is usually a division between shallow anddeep surveys.RefractionThe objective is to find out the arrival times of the head waves to map the depth to therefractors in which the waves travel. The refraction method is based on the assumptionthat the earth is made of layers of materials that increase in seismic velocity with eachsuccessively deeper layer. The key element is that an incident ray is critically refractedalong the boundaries between layers, before returning to the surface. From the firstarrival times it is possible to calculate the seismic velocity for each layer and the depthto the boundaries. The seismic velocity gives information about material properties, andwhat kind of material comprises each layer. Additionally, frequency analysis of therecorded signals can give more information about the material properties.The principles for seismic refraction techniques can be found in most geophysicaltextbooks. For a more detailed description of both theory and practice, see Sjögren(1984).Investigations performed with the refraction method can yield a variety of reliable datasuch as depth of various overburden layers, depth to bedrock, rock quality, soilcompositions and solidity, rip ability, excavatability, water tables and rock structure.The refraction seismic method can be used for a wide range of applications, for example:Underground Tunnels and their entrances, machinery halls, gas and oilstorage facilities, air raid sheltersFoundations Heavy industrial buildings, bridges, harbor quays andbreakwaters, dams, piling, airfieldsExcavations Harbor basins and entrances, pipelines, canals, roads, railwaysResource searches Gravel, sand and quarry sitesWater prospecting Groundwater table in the overburden, water bearing sections ofrockOre prospecting Mineralized weathered zones, buried channels with highmineral contentReflectionIn this method, the arrival time events are attributed to seismic waves that have beenreflected from interfaces where changes in acoustic impedance occur, and of waveshape changes. |
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