The instrument used to transform seismic energy into an electrical voltage; a seismometer, seis, detector, receiver, jug, bug, or pickup. Geophones ordinarily respond to only one component of the ground's displacement, velocity, or acceleration that is involved in the passage of a seismic wave. Three mutually orthogonal phones are used to record all three components.
Most land geophones are of the moving-coil type. A coil is suspended by springs in a magnetic field (the magnet may be integral with the case of the instrument). A seismic wave moves the case and the magnet, but the coil remains relatively stationary because of its inertia. The relative movement of a magnetic field with respect to the coil generates a voltage across the coil, the voltage being proportional to the relative velocity of the coil with respect to the magnet (when above the natural frequency of the geophone). Below the natural frequency, the output (for input of constant velocity of magnet motion) is proportional to frequency and hence to the acceleration involved in the seismic wave passage.
Geophone in Sheriff's Dictionary
- geophone array
- The use of areal, linear, or (occasionally) vertical patterns with more than one geophone per channel. Used to discriminate against events with certain apparent wavelengths. See Figure A-20 and array (seismic).
- geophone cable
- Insulated cable to which geophone groups are connected.
- geophone distance
- Usually group interval (q.v.), sometimes geophone interval or geophone offset (q.v.).
- geophone distortion
- Waveshape changes produced by nonlinear response of a geophone. Very small with modern geophones. See also distortion.
- geophone interval
- 1. The distance between adjacent geophones within a group.
- 2. Sometimes used for group interval, the separation between the centers of adjacent geophone groups.
- geophone offset
- The distance from the source point to a geophone or to the center of a geophone group.
Figure 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.
Figure 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. (Sheriff and Geldart, 1995, 249.)
- geophone pattern/array (seismic)
- 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).
- 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 (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.
- geophone planter
- A device or a person that positions geophones for receiving seismic signals; used especially for planting phones several feet deep in marsh.
- geophone station
- The location of the center of a geophone array, sometimes of an individual geophone.
- Evenden, B. S., Stone, D. R. and Anstey, N. A., 1971, Seismic prospecting instruments, 1. Gebru¨der-Borntraeger.
- Sheriff, R. E. and Geldart, L. P., 1995, Exploration Seismology, 2nd Ed., Cambridge Univ. Press.
- The Geophone: Its essential features and the related test procedures, Prakla report,~1973 File:Prakla geophone
- Compare hydrophone and streamer