The method of least squares should be used to estimate m linearly independent unknown parameters from n observations (m less than n). The problem of estimating four hypocentral parameters (three spatial coordinates of the hypocentre and the time of earthquake occurrence) from P wave arrival times at n (greater than four) stations has the feature that the spatial parameters are linearly dependent on the temporal parameter. By using differences in P wave arrival times at pairs of stations In the array, the earthquake occurrence time can be entirely omitted from consideration, and the method of least squares can be rigorously applied to estimate the spatial coordinates of the hypocentre. The omission of occurrence time does not hamper such objectives of microearthquake surveys at engineering sites as identification of seismically active fauits near the sites.