General background of nuclear magnetic resonance (NMR) well logging is set forth, for example, in U.S. Pat. No. 5,023,551. Briefly, in NMR operation the spins of nuclei align themselves along an externally applied static magnetic field. This equilibrium situation can be disturbed by a pulse of an oscillating magnetic field (e.g. an RF pulse), which tips the spins away from the static field direction. After tipping, two things occur simultaneously. First, the spins precess around the static field at the Larmor frequency, given by .omega..sub.0 =.gamma.B.sub.0, where B.sub.0 is the strength of the static field and .gamma. is the gyromagnetic ratio. Second, the spins return to the equilibrium direction according to a decay time T1, the spin lattice relaxation time. For hydrogen nuclei, .gamma./2.pi.=4258 Hz/Gauss, so, for example, for a static field of 235 Gauss, the frequency of precession would be 1 MHz. Also associated with the spin of molecular nuclei is a second relaxation, T2, called the spin-spin relaxation time. At the end of a ninety degree tipping pulse, all the spins are pointed in a common direction perpendicular to the static field, and they all precess at the Larmor frequency. However, because of small inhomogeneities in the static field due to imperfect instrumentation or microscopic material heterogeneities, each nuclear spin precesses at a slightly different rate. T2 is a time constant of this "dephasing".
A widely used technique for acquiring NMR data both in the laboratory and in well logging, uses an RF pulse sequence known as the CPMG (Carr-Purcell-Meiboom-Gill) sequence. As is well known, after a wait time that precedes each pulse sequence, a ninety degree pulse causes the spins to start precessing. Then a one hundred eighty degree pulse is applied to keep the spins in the measurement plane, but to cause the spins which are dephasing in the transverse plane to reverse direction and to refocus. By repeatedly reversing the spins using one hundred eighty degree pulses, a series of "spin echoes" appear, and the train of echoes is measured and processed.
Further Background, set forth in the referenced copending parent application Ser. No. 08/936,892, is summarized as follows: The static field may be naturally generated, as in the case for the earth's magnetic field B.sub.E. The NML.TM. nuclear logging tool of Schlumberger measures the free precession of proton nuclear magnetic moments in the earth's magnetic field. See, for example, U.S. Pat. No. 4,035,718. The tool has at least one multi-turn coil wound on a core of non-magnetic material. The coil is coupled to the electronic circuitry of the tool and cooperatively arranged for periodically applying a strong DC polarizing magnetic field, B.sub.P, to the formation in order to align proton spins approximately perpendicular to the earth's field, B.sub.E. The characteristic time constant for the exponential buildup of this spin polarization is the spin-lattice relaxation time, T.sub.1. At the end of polarization, the field is rapidly terminated. Since the spins are unable to follow this sudden change, they are left aligned perpendicular to B.sub.E and therefore precess about the earth's field at the Larmor frequency f.sub.L =.gamma.B.sub.E. The Larmor frequency in the earth's field varies from approximately 1300 to 2600 Hz, depending on location. The spin precession induces in the coil a sinusoidal signal of frequency f.sub.L whose amplitude is proportional to the number of protons present in the formation. Additives in the borehole fluid are required to prevent the borehole fluid signal from dominating the formation signal. The tool determines the volume of free fluid in the formation.
A further nuclear magnetic resonance approach employs a locally generated static magnetic field, B.sub.o, which may be produced by one or more permanent magnets, and RF antennas to excite and detect nuclear magnetic resonance to determine porosity, free fluid ratio, and permeability of a formation. See, for example, U.S. Pat. Nos. 4,717,878 and 5,055,787.
Nuclear magnetic resonance has proven useful in medical applications to perform noninvasive examinations of the interior organs and structures of an organism. See P. Mansfield, Pulsed Magnetic Resonance: NMR, ESR, And Optics, 317-345 (D. M. S. Baugguley ed., Cleardon Press, Oxford, 1992). The desire for faster imaging led to the development of commercial and laboratory NMR imaging systems in the medical field which use various gradient-echo techniques consisting of radio frequency pulses, .alpha., in combination with switched magnetic field gradients to generate an image. See Stewart C. Bushong, Magnetic Resonance Imaging: Physical And Biological Principles, 279-286, (2d edition, 1996). Known techniques such as fast low angle shot (FLASH) and fast imaging with steady state precession (FISP) require an RF excitation pulse, .alpha., of approximately 90.degree. while other techniques vary the flip angle between 30.degree. and 70.degree. to maximize magnetic resonance strength.
As pointed out in the referenced copending Application, the tools and techniques developed in the prior art have various drawbacks that limit their utility in practical applications. These limitations include a shallow depth of investigation and restrictions on the shape and size of the region of investigation.