(19)

(11)

EP 0 494 976 A1

(12)

(43) Date of publication: 22.07.1992 Bulletin 1992/30 (21) Application number: 90915791.0 (22) Date of filing: 09.10.1990

(51) International Patent Classification (IPC):  G01V 8/ 14( . ) G01V 8/ 22( . ) (86) International application number: PCT/GB1990/001549 (87) International publication number: WO 1991/006018 (02.05.1991 Gazette 1991/10)

(84) Designated Contracting States: AT BE CH DE DK ES FR GB GR IT LI LU NL SE (30) Priority: 13.10.1989 GB 19890023141 (71) Applicant: TRETT, John Marlow Buckinghamshire SL7 2JU (GB)	(72) Inventor: TRETT, John Marlow Buckinghamshire SL7 2JU (GB) (74) Representative: Enskat, Michael Antony Frank  Saunders & Dolleymore, 9, Rickmansworth Road Watford, Hertfordshire WD1 7HE Watford, Hertfordshire WD1 7HE (GB)