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Spatial variations in the geomagnetic field must be taken into account if secular variation master curves and directional magnetic dates are to be optimized. Two methods for relocating remanence vectors have been proposed and in this paper their relative accuracies are compared using a numerical model based on the present-day field. A method which converts archaeomagnetic directions via a virtual geomagnetic pole is shown t o be the more efficient transformation. For an 'archaeomagnetic region' the size of the British Isles, (900 km radius), the maximum error in relocating vectors to a central location is predicted to be of the order of 1.2".

S U M M A R Y This paper describes a method in which vertical resistivity sections are generated tomographically from measurements on a linear array of equally spaced electrodes inserted a t the ground surface. T h e array is multiplexed t o a resistivity meter which gathers one set of all possible independent apparent resistivity measurements and the geophysical section is then reconstructed by backprojecting these weighted data, along equipotentials, into t h e subsurface. T h e technique has been evaluated numerically a n d in field trials over shallow archaeological structures at Fountains Abbey.

Xu, B. and NOEL, M. 1993. On the completeness of data sets with multielectrode systems for electrical resistivity survey. Geophysical Prospecting 41,791-801. This paper describes how, using a surface linear array of equally spaced electrodes, potential data can be obtained for use in electrical resistivity imaging. The aim is to collect a complete data set which contains all linearly independent measurements of apparent resistivity on such an array using two-, three-or four-electrode configurations. From this primary data set, it is shown that any other value for apparent resistivity on the array can be synthesized through a process of superposition. Numerical tests show that such transformations are exact within the machine error for calculated data but that their use with real field data may lead to noise amplification.

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