Patent ID: 8208347

Claim:
A geophone ( 110 ) comprising: a cylindrical magnet ( 114 ); upper and lower pole piece caps ( 116 , 118 ) receiving the upper and lower ends of said magnet ( 114 ), respectively; a tubular outer cylindrical housing ( 120 ), said magnet ( 114 ) and upper and lower pole piece caps ( 116 , 118 ) being coaxially received within and connected to said housing ( 120 ) and defining an annular region within said housing ( 120 ); a tubular bobbin ( 130 ) disposed in said annular region and movable in an axial direction with respect to said housing ( 120 ); upper and lower coils ( 140 , 142 ) wound about upper and lower longitudinal regions of said bobbin ( 130 ) that are adjacent to said upper and lower pole piece caps ( 116 , 118 ), respectively, said upper and lower coils ( 140 , 142 ) being electrically connected, whereby an electrical potential is induced in said upper and lower coils ( 140 , 142 ) by movement of bobbin ( 130 ) with respect to said housing ( 120 ); and a short-circuited coil ( 150 ) wound about said bobbin ( 130 ), said short-circuited coil ( 150 ) itself forming a closed electrical circuit.