ABSTRACT:A new mobile NMR scanner has been designed with a Halbach magnet for measurements of porosity and pore-size distributions of water-saturated cylindrical geological cores with diameters up to 60 mm. It is light (ϳ8 kg) and has a sufficiently homogeneous magnetic field in a large and accessible cylindrical volume. Practical aspects of the sensor construction are described. First experimental results obtained with the mobile Halbach sensor are reported in an attempt to measure porosity and pore-size distributions in geological drilled cores. Compared to the NMR-MOUSE, the Halbach tool is more sensitive, and porosities can be determined down to 3%. The scanner is a laboratory prototype of a future tool for measuring porosity and pore-size distributions of standardsize cores drilled from the ocean floor. The tool is particularly attractive for use on research vessels and on logging platforms.

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[1] ODP Leg 193 drilled into the PACMANUS hydrothermal field (Papua New Guinea), which is an active hydrothermal vent field associated with felsic magmatism in a convergent geodynamic setting. The PACMANUS hydrothermal field is part of the eastern Manus Basin and is located near the crest of Pual Ridge, a 500 to 700 meters high felsic neo-volcanic ridge. Two sites, Snowcap (Site 1188) and Roman Ruins (Site 1189), were drilled approximately 800 m apart. Although low core recovery limited the examination of the subsurface geology, geophysical downhole measurements provided continuous records of in-situ physical properties with depth and were used to reconstruct lithostratigraphic profiles. Downhole logging operations included standard wire line logging and Logging While Drilling (LWD) measurements. Electrical resistivity images of the borehole wall were examined to find characteristic textures and three different volcanic facies were distinguished: coherent dacite, volcaniclastic dacite and brecciated dacite. In addition, intervals with sulfide mineralization were identified based on characteristic responses of the photoelectric factor log. A comparison of the reconstructed profiles from both sites shows considerable differences in the proportions of facies and the thickness of individual layers. Thick units of coherent facies are predominant at Snowcap and indicate a proximal position of the site within the volcanic system of Pual Ridge. At Roman Ruins, thin individual layers with rapid changes in volcanic facies and a higher proportion of volcaniclastic and brecciated dacite suggest a medial position. These differences in volcanic facies show that the volcanic architecture at Pual Ridge is characterized by small-scale facies changes, emplacement of small-volume individual lava flows and complex geometries of individual emplacement units. Geophysical logging data suggest that subseafloor hydrothermal activity is focused to the area around one hole at Roman Ruins. Downhole logs from this hole show characteristic cyclic patterns in the electrical resistivity and photoelectric factor logs indicating that hydrothermal sulfide mineralization is concentrated at the tops of individual lava flows representing paleo-seafloor positions.Components: 9047 words, 15 figures, 2 tables.

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Estimating permeability from NMR well logs or mobile NMR core scanner data is an attractive method as the measurements can be performed directly in the formation or on fresh cores right after drilling. Furthermore, the method is fast and non-destructive. Compared to T 1 relaxation times, commonly measured T 2 distributions are influenced by external and internal magnetic field gradients. We performed two-dimensional T 1 and T 2 relaxation experiments on samples of Rhaetian sandstone, a rock with low porosity and small pore radii, using a mobile NMR core scanner which operates within a nearly homogeneous static magnetic field. Because small pore sizes are associated with high internal magnetic field gradients, standard methods from NMR logging in the oil industry cannot be applied for accurate permeability prediction. Therefore, a new model theory was developed, which describes the pore radius dependence of the surface relaxivity q 2 by both an analytical and a more practical empirical equation. Using corrected q 2 values, permeability can be predicted accurately from the logarithmic mean of the T 2 distribution and the physically based Kozeny-Carman equation. Additional core plug measurements of structural parameters such as porosity, permeability, specific inner surface area and pore radius distributions supported the NMR results.