Determining distance and direction from a first borehole to a second borehole including generating a time-varying magnetic field by axially reciprocating magnets of opposed polarity, at a first location in a second borehole, and detectable in the region of the first borehole. First and second sensors positioned at an observation point in the first borehole measure the amplitude and relative phase of components of the magnetic field. From these measurements, the direction, relative to the sensors, from an observation point to the first location is determined. The distance between these points is determined by measuring amplitude variations with depth of the magnetic field at observation points in the first borehole and computing theoretical variations in the amplitude for different assumed distances between the observation points and the location of the magnetic field source which are compared with theoretical variations to determine the distance between the first and second boreholes.