Methods and apparatus for investigating earth formations utilizing rotating electromagnetic fields

Abstract

Abstract IN ACCORDANCE WITH ILLUSTRATIVE EMBODIMENTS OF THE PRESENT INVENTION, METHODS AND APPARATUS FOR DETERMINING THE POSITIONING OF FORMATION BEDDING PLANES BY ROTATING ONE OR MORE ELECTROMAGNETIC FIELDS THROUGH FORMATIONS SURROUNDING A BOREHOLE ARE ILLUSTRATED. IN ONE EMBODUMENT, A SOLENOID-TYPE COIL IS ENERGIZED WITH CURRENT AND ROTATED AT A SUBSTANTIALLY CONSTANT VELOCITY ABOUT AN ADJUSTABLE AXIS. AN AMPLITUDE MODULATED SIGNAL WILL THEN BE PRODUCED IN THE COIL IF THERE IS A CONDUCTIVITY CONTRAST OR GRADIENT IN THE FORMATION PLANE WHICH DEFINES THE SWEEP OF THE ELECTROMAGNETIC FIELD. THIS AMPLITIDE MODULATED SIGNAL IS THEN DEMODULATED AND THE RESULTING MODULATION SIGNAL IS SYNCHRONOUSLY DETECTED BY THE SIGNAL WHICH CAUSES THE COIL ROTATION. THIS PRODUCES PHASE COMPONENTS WHICH ARE USED FOR BRINGING THE AXIS OF ROTATION OF THE COIL INTO A SUBSTANTIALLY PERPENDICULAR DIRECTION TO THE LOW GRADIENT FORMATION PLANE AND INDICATING THE POSITIONING OF THIS PLANE. IN ANOTHER EMBODIMENT, TWO ELECTROMAGNETIC FIELDS ARE ROTATED AROUND A BOREHOLE AT SPACED APART DEPTH LEVELS BY SEQUENTIALLY COUPLING A PLURALITY OF CIRCUMFERENTIALLY SPACED COIL PAIRS TO THE ADJOINING FORMATIONS. THE SIGNALS FROM EACH COIL OF THE COIL PAIRS ARE COMBINED IN SELECTED RELATIONSHIPS TO PRODUCE A COMPOSITE SIGNAL FOR EACH COIL PAIR. THESE COMPOSITE SIGNALS ARE THAN USED TO ADJUST THE RELATIONSHIP WITH WHICH THE INDIVIDUAL COIL SIGNALS ARE COMBINED UNTIL EACH COMPOSITE SIGNAL HAS SUBSTANTIALLY THE SAME AMPLITUDE THUS INDICATING THE POSITIONING OF THE LOW CONDUCTIVITY GRADIENT FORMATION PHASE.

Inventors

• 👤 Michel Marie Albert Gouilloud
• 👤 Julien M Loeb

Bibliographic Details