NMR logging in the earth's magnetic field
AssigneeBaker Hughes Incorporated
InventorGersh Zvi Taicher
Life of this patent
Estimating the term…
Abstract
An apparatus and a method for measurement of NMR relaxation parameters of earth formations uses a coil on a logging tool to substantially polarize nuclear spins within a volume of earth. The polarizing pulse then switched off adiabatically. The magnetization of the spins then precesses around earth's magnetic field at a Larmor frequency corresponding to the earth's field. A second coil on the logging tool is pulsed with a carrier signal having this frequency and modulated by a CPMG sequence to obtain spin echo signals. The spin echo signals may be received by the same coil or by a second coil. Alternatively, beam steering of two coils on the tool may be carried out to ensure a resultant field that has a significant component transverse to the earth's field for carrying out the spin echo measurements.
Claims30
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) pulsing a first coil on the borehole tool with a polarizing signal and aligning nuclear spins within said volume substantially parallel to a direction of a magnetic field produced thereby in said volume;
- (b) adiabatically discontinuing the polarizing signal and allowing said nuclear spins to reorient towards a direction parallel to a magnetic field of the earth;
- (c) pulsing a second coil on the borehole tool with a sequence of pulses for exciting nuclei within said volume;
- and
- (d) detecting nuclear magnetic resonance spin echo signals from said excited nuclei;
- (e) providing in response to said signals an output indicative of said parameter of interest of the volume of earth formation.
A method for determining a parameter of interest of a material in a region of investigation, the method comprising;
- (a) pulsing a first coil with a polarizing signal and aligning nuclear spins within said region of investigation substantially parallel to a direction of a magnetic field produced thereby in said region;
- (b) adiabatically discontinuing the polarizing signal and allowing said nuclear spins to reorient towards a direction parallel to a magnetic field of the earth;
- (c) pulsing a second coil with a sequence of pulses for exciting nuclei within said region;
- and
- (d) detecting nuclear magnetic resonance signals from said excited nuclei;
- (e) providing in response to the detected nuclear magnetic resonance signal an output indicative of said parameter of interest of a material in said region of investigation.
A borehole tool for determining a parameter of interest of a volume of earth formation surrounding the borehole, the tool comprising:
- (a) a first antenna for producing a static magnetic field in said volume for a duration selected for substantially polarizing nuclear spins within said volume and causing said polarized nuclear spins to precess about a direction of earth's magnetic field thereafter;
- (b) a second antenna for producing a pulsed magnetic field having a direction with a component transverse to said direction of earth's magnetic field, said second antenna pulsed with an AF signal modulated by a pulse sequence for forming spin echo signals in said volume.
An apparatus for determining a parameter of interest of a material in a region of investigation, the apparatus comprising:
- (a) a first antenna for producing a static magnetic field in said region of investigation for a duration selected for substantially polarizing nuclear spins within said region and causing said polarized nuclear spins to precess about said direction of earth's magnetic field thereafter;
- (b) a second antenna for producing a pulsed magnetic field having a direction with a component transverse to said direction of earth's magnetic field, said second antenna pulsed with an AF signal modulated by a pulse sequence for forming spin echo signals in said volume.
Dependencies are inferred from the claim text.