[en] Posiva Oy carries out a programme for spent nuclear fuel disposal. Subsurface characterization work is underway at the ONKALO tunnel, in Olkiluoto. Detailed hydrological conductivity properties in an averagely fractured rock mass are studied in the HYDCO niche. Crosshole mise-a-la-masse survey was carried out between the two 23-24 m long horizontal drillholes, ONK-PP262 and ONK-PP274 that are 3 m apart from each other, and between the drillhole and the access tunnel. The field work was carried out in April- May 2011. The survey was aimed to define continuity of the fractures between the drillholes. Nine current earthing stations in drillhole ONK-PP262 and 13 stations in ONK-PP274 were used. Earthings were placed at fractures or conductive layers detected in core logging, assessed with geophysical logging and imaging, and with Posiva flow log measurement. Earthing used a 0.5 m long electrode assembly operated with a wireline. The potential profile was measured in the adjacent drillhole at 0.1 m spacing, using a similar electrode assembly and wireline. Potential was measured also on 130 m long profile along the tunnel wall from the earthings in drillhole ONK-PP262. The remote voltage reference and the current earthing were placed offset from the survey area. The equipment used in survey consists of ABEM Terrameter resistivity tool, as well as software, cable winch and downhole probes belonging to Posiva Flow Log tool. The high resistivity bedrock, and electrically fairly weakly conductive fractures residing in the rock mass provide rather small conductivity contrast. Nevertheless there are connections between several fractures or groups of fractures between the drillholes. Several fractures merge together in the electrical potential response. Interpretation defined six zones between the drillholes where fractures indicate continuity, and may serve as hydraulic interference pathway. One of the zones is extending also to the tunnel wall. Measurements and interpretation served as a tool in definition of the local geological model. Information of the fractures, their orientations and continuity is useful in designing and assessing the hydrological investigations in the drillholes. (orig.)