A seat belt retractor has a rotatable spool for retraction or payout of seat belt webbing wound thereon, depending upon the rotation direction of the spool. In an optical pickup enclosure, there is formed a groove recessed into a U shape, extended from the upper surface thereof opposed to an optical disc to the lower surface of the optical pickup enclosure. The voltage control loop compares the sensed output voltage with a predetermined reference voltage, and outputs a PWM control signal to one of the resonant converters so the output voltage of the one converter is equal to the predetermined reference voltage. The method comprising dividing regions around wells in the reservoir into a number of zonessections, and injecting or placing specific tracers with unique characteristics for each zonesection into the formation in these regions such that tracers are placed as integrated parts of the well completion or placed and immobilized in these regions through injection, squeeze or by other techniques.