| 摘要 |
The present invention relates generally to an apparatus and method for measu ringnuclear magnetic resonance properties of an earth fo rmation traversed by a borehole by generating gradient-echoes. The measurement can be made while drilling or us ing a wireline tool. The apparatus applies a static magnetic field, B a, in a volu me of the formation which polarizes the nuclei of hydrogenous connate fluids within the formatio n. The apparatus applies a second magnetic field, B b, in a volume of the formation . The magnetic fields B a and B b are substantially orthogonal in the volume of the formati on. A change in the polarity of the magnetic field, B b, reverses the direction of precessio n of the nuclei thereby generating a train of gradient-echoes. Each gradient-echo signal is transformed into the frequency domain and the signal frequency is mapped to a radial position in the volume of the formation in order to generate an image of the formation. Various pro perties of the formation, such as the effective diffusion coefficient of the formation flui d, longitudinal relaxation time, spin-spin relaxation time, and porosity can be estimated fr om the train of gradient-echoes.
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