Much signal processing is typically involved. In general, identifying a reflection event in a seismic record is more difficult than picking first arrivals for a refraction survey. Greater depths are possible, but the required array lengths mayĮxceed site dimensions, and the shot energy required to transmit seismicĪrrivals for the required distances may necessitate the use of large explosiveīy contrast, reflection surveys are not hampered by low velocity layers but they have difficulty imaging the top 50m of the earth because reflections from such shallow depth are difficult to distinguish from direct arrivals and sound waves travelling through the air. Seismic refraction is commonly limited to mapping layers to depths less thanģ0-50 meters. The depth to the layer of interest (for example the top of bedrock). Refraction requires geophone arrays with lengths of approximately 4 to 5 times Seismic refraction may yield incorrect results. clay) layers may overlie lower velocity (e.g. Signals in each layer increases with depth. Seismic refraction is most useful when the velocity of seismic (b) The common depth point is not achieved in theĬase of a dipping reflector. 1 (a) A set of raysįrom different shots to detectors are reflected off a common point on a 72 Common depth point (CDP) reflection profiling. Then this common reflection point lies midway between the source andįig. 71 A series of six shots and associated receivers that would have Strictly correct, as the bottom diagram illustrates.įig. Traces was referred to as a Common Depth Point (CDP) gather. The collection of seismic traces that correspond to a particular midpoint isĬalled a Common Midpoint (CMP) gather. The idea is illustrated on the upper figure. The various seismograms and stack these to get an enhanced signal to noise Ultimately the goal is to identify all the reflections due to that point on Specifically so that some subsurface points are sampled more than once. Multiple shots and receivers are used in seismic profiling The triangular central portion is ground roll caused by surface waves. Hyperbolae from numerous boundaries are observed, right. Theįirst arrivals are direct or critically refracted arrivals. The seismic traces all belong toĪ single source and hence this is referred to as a “Common Source Gather”. (b) Single-ended spreadĪ split spread seismic record is shown above. 70 Shot-detector configurations used in multichannel seismic reflection Split spreads are common in land surveys single-ended spreads areįig. Spread” in which case there is a central shot with receivers on both sides, orĪ “single-ended spread” in which the receivers are always on one side of the Survey geometry ¶ Multichannel data collection ¶įirst consider the source-receiver geometry.
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