SEISMIC REFLECTION

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Seismic reflection maps contrasts in seismic impedance, which is the product of seismic velocity (the speed at which seismic waves are transmitted by the soil or rock) and density. The “reflection coefficient” – the amount of energy reflected at a boundary – is directly proportional to the difference in seismic impedance. Seismic reflection is much more sensitive to subtle changes and is capable of imaging much finer detail than seismic refraction. However, data acquisition and processing are significantly more complex. Most practitioners of seismic reflection tend to specialize in the technique, and it is not commonly used in small-scale engineering applications where budgets tend to be limited. But where the budget permits and the geology is favorable, seismic reflection is by far the best tool for detailed mapping of stratigraphy and structure.

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COMMON APPLICATIONS

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• Mapping stratigraphy, structure, and hydrogeology

• Identification and mapping of faults

• Mapping depth to bedrock/bedrock topography

• Mapping depth to ground water

• Mapping thickness of landslides

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CONSIDERATIONS

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• In seismic reflection, the depth of penetration is less dependent on spread length than seismic refraction. In general, the length of the active spread should be roughly equal to the required depth of penetration.

• For most shallow seismic reflection, a hammer and plate is a good source if the ground surface is relatively hard. In softer ground, a downhole seisgun is an excellent source. For deeper work, small explosives like Kinepak are ideal. In urban areas, the mini-Sosie technique can be a good choice, especially when working close to a road. In general, accelerated weight drops are inferior for shallow work due to relatively low frequency content.

• Any Geometrics seismograph can be used for seismic reflection, but the Geode is ideal due to its distributed architecture, large channel capacity, and reflection-centric acquisition software tools. The number of channels you use will determine both your field production rate and data quality, each benefitting from more channels rather than less. In general, a minimum data redundancy of 24-fold is recommended, and for this we recommend 96-120 channels.

• Crew size depends greatly on the scale of the survey and the source used. Contact Geometrics to discuss your specific project.

• An experienced seismic processor is absolutely necessary to assure a quality end product.

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BENEFITS/LIMITATIONS

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• In order for seismic reflection to be applicable, there must exist relatively continuous reflecting horizons. This is not always the case. For instance, a fluvial environment tends to be a bad choice for reflection, while a lacustrine environment can be ideal.​

• Resolution is limited by the frequencies that can be recorded, with higher frequencies yielding higher resolution. As a general rule, high frequencies tend to attenuate faster and over shorter distances than low frequencies do, so starting with the highest frequency source available is a plus. However, source frequency tends to be inversely related to source strength, and source strength is what ultimately controls the depth of penetration. So the goal should be to use the smallest source you can that still gets you to the desired depth. Paradoxically, it is often easier to map deeper reflectors than shallow reflectors because of the difficulty of transmitting high frequencies. Successful reflection work in the upper 100m can be exceedingly difficult, but it is not impossible. Click here for a good comparison of sources for seismic reflection.

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DELIVERABLES

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The final product of a reflection survey is a time section such as that shown below.   This will typically include a geological interpretation overlaid on the seismic data. Other deliverables include data processing sequence and maps showing line locations.

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