Patent ID: 8098546

Claim:
A geophone ( 110 ) comprising: a cylindrical magnet ( 114 ) having a length-to-diameter ratio (l m /d m ) greater than 0.5 and less than 1.0; upper and lower pole piece caps ( 116 , 118 ) receiving the upper and lower ends of said magnet, respectively, said upper and lower pole piece caps ( 116 , 118 ) together defining an overall longitudinal pole-to-pole length (l pp ) and a parasitic air gap ( 125 ) therebetween having a longitudinal air gap length (l g ), said upper and lower pole piece caps ( 116 , 118 ) having a parasitic gap length to pole-to-pole length ratio (l g /l pp ) greater than 0.25; a tubular outer cylinder 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 an electrical coil ( 140 , 142 ) disposed between said magnet ( 114 ) and said housing ( 120 ) and movable in an axial direction with respect to said housing ( 120 ).