Patent ID: 6429654
Filing Date: 2002-08-06
Classification: G01N,G01V

Abstract:
A method for determining a parameter of interest of a volume of earth formation with a borehole tool conveyed in a borehole within the formation, the method comprising:(a) using a magnet assembly on the borehole tool for producing a static magnetic field having a field strength within predetermined limits in said volume of the formation and aligning nuclear spins within said volume parallel to a direction of the static field; b) producing a radio frequency (RF) magnetic field in said volume of the formation, said RF magnetic field having a direction substantially orthogonal to a direction of the static field, the RF field including a pulse sequence: Wâˆ’Tâˆ’tsâˆ’Râˆ’(tcpâˆ’echoâˆ’tcpâˆ’R)j wherein W is a wait period, T is a tipping pulse for tipping the nuclear spins at an angle substantially equal to ninety degrees to cause precession thereof, ts is a waiting time, tCP is the Carr-Purcell time, R is a refocusing pulse, j=1, 2, . . . J, and J is the number of echoes collected in a single sequence of pulses;(c) measuring with a receiver coil on the borehole tool nuclear magnetic resonance signals induced by the pulsed RE field in the formation; and (d) processing said measured signals to determine the parameter of interest; wherein the refocusing pulse has a duration selected to increase a signal to noise ratio (SNR) of said signals relative to a refocusing pulse having twice the duration of the tipping pulse; wherein processing said measured signals further comprises determining correction coefficients for correcting amplitudes of said spin echo signals to a theoretical value which would be obtained in a zero gradient static magnetic field.