Reflection Surveys, Physics tutorial

Spread Lengths:

Distance from source to nearest geophone in shallow reflection survey is generally dictated by strength of source (and need to protect geophone) and may be as little as 2m when the hammer is being used. Even with explosives or heavy weight drops, minimum offsets of more than about 10m are unusual when observing shallow reflections. Reflection spread can be much shorter than refraction spread utilized to probe to similar depths, but with powerful sources and multi-channel recording, furthest geophone may be more than 100 m from source. Optimum spread length can be determined only by experiment, as most significant factors are arrival times of noise trains related with direct wave and any strong refracted waves. Field work should start with tests specifically designed to observe the arrivals, usually by using elongated spreads.

Arrays:

Ideally, reflected energy must arrive after near-surface waves (ground-roll and refractions) have passed but this may not be possible if depth of investigation is very small. In such cases, geophones may be connected in arrays to each recording channel. Reflected waves, that travel almost vertically, will reach all geophones in the array almost concurrently but direct waves will arrive at different times and generate signals which can interfere destructively.

Efficiency with which a wave is attenuated by the array is stated by relative effect (RE) compared to effect of same number of geophones placed together at array centre. Variation of RE with apparent wavelength (that for direct wave is equal to true wavelength), for linear array of five geophones evenly spaced on line directed towards shot point, is shown. Non-linear arrays generate more complex curves.

Simple arrays are preferred in field, as mistakes are easily made in setting out complicated ones. Range of frequencies over which attenuation of direct wave takes place is proportional to array length and it may be essential to overlap geophones in adjacent arrays. It would be unusual in shallow survey to use more than five geophones per array.

1168_Relative Effect of Array inline Geophones.jpg

The 100% level would be achieved with zero spacing between geophones. Apparent wavelength is equivalent to actual wavelength divided by sine of angle between wavefront and ground surface, and is infinite for wave rising vertically and equivalent to true wavelength for direct wave. Attenuation is concentrated between values of apparent wavelength divided by geophone spacing of approx 1.2 and 7. With 2 m spacing, a 500 m·s-1 wave would be attenuated at frequencies of between approx 35 and 200 Hz.

Shot Arrays:

Seismic cables for use with only 12 or 24 channels are not designed with arrays in mind, and non-standard connectors may have to be fabricated to link geophones to each other and to cable. It may be simpler to utilize arrays of shots instead.

Shot array using explosives generally involves simultaneous detonation of charges laid out in pattern resembling that of the conventional geophone array. If the impact source is utilized with enhancement instrument, same effect can be attained by adding together results achieved with impact at different points. This is the easiest way of reducing effects of surface waves when using hammer.

Common Mid-Point Shooting:

Enhancing signal-to-noise ratios by adding together numerous traces (stacking) is primary to deep reflection surveys. In shallow surveys, this method is usually utilized only to stack (improve) results attained with identical source and detector positions. If, though, data are recorded digitally, NMO corrections can be made (though not in field) to traces produced with different source-receiver combinations. The method usually utilized is to collect together the number of traces which have same mid-point between source and receiver (common midpoint or CMP traces), apply corrections and then stack. Number of traces collected together in CMP stack defines fold of coverage. Three traces forming a single synthetic zero-offset trace comprise 3-fold stack and are said to give 300% cover. Maximum fold obtainable, unless shot point and geophone line are moved together by fractions of geophone interval, is equivalent to half the number of data channels.

1356_CMP Schematic for 3-fold Cover.jpg

Figure shows successive geophone and source positions when the six-channel instrument is utilized to get 300% cover. Special cables and switching circuits are available for use in deep reflection surveys, but CMP fieldwork with instruments utilized for shallow surveys is extremely slow and laborious. Need to combine traces from some different shots make it hard to do CMP processing in field.

Shot points A, B, C and D are progressively one geophone group interval further to right. Shots A and D have no depth points in common.

148_Effect of Dip in CMP Shooting.jpg

Contrary to single-fold shooting, shot points also the geophone locations are different for different traces. Shot points and detector locations are equivalent and depth point on reflector moves up dip as offset increases. Move-out equation is most simply derived by noting that path from source to detector is equal in length to path SG' from source to detector image point and that geometric relationships between similar triangles imply equality of all lengths marked y. Pythagoras relationship can be applied to triangle SG' P, and times can be attained by dividing distances by V.

Thus, T0 = 2d/V and T = SG'/V

Geometry of a CMP shoot varies from that for single-fold coverage, and effect of dip is thus different. If interface dips at angle α, velocity deduced from CMP stack is equal to V/cos α and depth is equivalent to length of normal incidence ray from common mid-point to interface. Contrary to single fold gather the minimum time is related with normal incidence ray. Aim of stacking is to generate noise-reduced seismic trace which approximates to normal incidence trace, i.e. to trace which would have been generated had source and detector been coincident. Initials CMP replaced earlier acronym, CDP (common depth point) used for same method. Referring to depth points (reflection points) as common implies that all reflections in a gather have come from same point on subsurface interface that is true only for horizontal interfaces.

Depth Conversion:

Reflection events are recorded not in depth but in two-way time (TWT). Velocities are required to convert times in depths, but Dix velocities attained from NMO curves may be 10-20% in error, even for horizontal reflectors. Interpretations must be calibrated against borehole data wherever possible, and field crews must always be on lookout for opportunities to estimate vertical velocities directly.

Image is of small graben structure beneath unconformity. Position of true fault plane BB (indicated by the dashed line) can be estimated from the positions of the terminations of the sub-horizontal reflectors representing the sediment fill within the graben (although care must be exercised because many of the deeper sub-horizontal events are multiples). The event AA is the seismic image of BB. It is displaced because the techniques used to display the data assume that reflections are generated from points vertically beneath the surface points, whereas they are actually generated by normal-incidence rays that are inclined to the vertical if reflected from dipping interfaces. The reflections from the fault and the opposite side of the graben cross over near the lower symbol ''A'', forming a ''bow-tie''. Convex-upward reflections near point C are diffraction patterns generated by faulting.

Geometric Distortion:

Seismic reflection data are usually presented as sections prepared by playing out, next to each other and vertically down sheet of paper, traces from adjacent CMP gathers. Such sections are subject to geometric distortion. Artefacts like displaced reflectors, diffraction patterns and bow-ties, as affecting radar sections, also appear on seismic imagery. Refraction surveys are extensively utilized to study water table and, for engineering purposes, poorly consolidated layers near ground surface, and also in determining near-surface corrections for deep reflection traces. Travel times are generally only few tens of milliseconds and there is little separation between arrivals of different kinds of wave or of waves that which travelled by different paths. Only first arrivals that are always of P wave can be picked with any confidence.

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