Dictionary:Array (seismic)

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1. A group of geophones or other seismic receivers connected to a single recording channel (geophone array) or a group of sources to be activated simultaneously (source array). The records from nearby sources when vertically stacked also effectively constitute a source array. Sometimes called a pattern (especially for a source array) or a patch (especially when the array is large).

FIG. A-20. Arrays used for geophone or source arrays. The array elements are shown by the open circles, triangles and numbers indicate the effective element locations and weightings in different directions; the inline direction is horizontal in each instance. (a) Inline; (b) perpendicular; (c) cross; (d) 3×3 diamond; (e) X-array; (f) rectangular array; (g) crow’s-foot array; (h) odd-arm star; (i) herring-bone array; (j) and windmill array.

2. The arrangement or pattern of a group of geophones or sources (Figure A-20). Arrays discriminate against events on the basis of their moveout or apparent wavelength; see directivity graph. For a uniform array (see Figure D-15) of n geophones separated by the distance d, the effective array length is nd and the first null response occurs when the apparent wavelength equals this. The half-width of the main lobe at 0.7 peak amplitude defines the pass wavelength. For a nonuniform array, the effective array length is the length of the uniform array that has the same pass wavelength. Compare spread.

FIG. D-15. Directivity graphs. (a) Polar plot showing the relative amplitude of a radiated wave (or the relative sensitivity to waves approaching a geophone array from different directions). The horizontal axis can be expressed in various ways. (b) Directivity of five inline geophones spaced 10 m apart. (c) Response of a tapered array of five geophones spaced 20 m apart and weighted 1:2:3:2:1; such weighting could be achieved with nine geophones distributed as the weighting. (d) Response of nine geophones equally spaced 5.5 m apart. The solid curves are for harmonic (steady-state) waves, the dashed curves for a transient with a bell-shaped spectrum peaked at 30 Hz and a width of 30 Hz. (From Sheriff and Geldart, 1995, 249.)