30 main contents 31 spread lengths the distance from

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3.0 Main Contents 3.1 Spread lengths The distance from the source to the nearest geophone in a shallow reflection survey is usually dictated by the strength of the source (and the need to protect the geophone) and may be as little as 2 m when a hammer is being used. Even with explosives or heavy weight drops, minimum offsets of more than about 10 m are unusual when observing shallow reflections. A reflection spread can be much shorter than a refraction spread used to probe to similar depths, but with powerful sources and multi-channel recording, the furthest geophone may be more than 100 m from the source. The optimum spread length can be determined only by experiment, since the most important factors are the arrival times of the noise trains associated with the direct wave and any strong refracted waves. Field work should begin with tests specifically designed to examine these arrivals, generally by using elongated spreads. 3.2 Arrays Ideally, reflected energy should arrive after the near-surface waves (ground-roll and refractions) have passed but this may not be possible if the depth of investigation is very small. In such cases, geophones may be connected in arrays to each recording channel. Reflected waves, which travel almost vertically, will reach all the geophones in an array almost simultaneously but
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38 the direct waves will arrive at different times and produce signals that can interfere destructively. The efficiency with which a wave is attenuated by an array is defined by its relative effect (RE) compared to the effect of the same number of geophones placed together at the array centre. The variation of the RE with apparent wavelength (which for the direct wave is equal to the true wavelength), for a linear array of five geophones equally spaced on a line directed towards the shot point, is shown in Figure 2.5. Non-linear arrays produce more complex curves. Simple arrays are preferred in the field, since mistakes are easily made in setting out complicated ones. The range of frequencies over which attenuation of the direct wave occurs is proportional to array length and it may be necessary to overlap the geophones in adjacent arrays. It would be unusual in a shallow survey to use more than five geophones per array. Figure 2.5 Relative effect (RE) of an array of five equi-spaced in-line geophones. The 100% level would be attained with zero spacing between the geophones. The apparent wavelength is equal to the actual wavelength divided by the sine of the angle between the wavefront and the ground surface, and is infinite for a wave rising vertically and equal to the true wavelength for the direct wave. Attenuation is concentrated between values of apparent wavelength
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