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).

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.)