Well logging techniques and apparatus are disclosed for determining formation resistivity at different radial depths of investigation and with greater accuracy than prior techniques, for determining the existence, locations and properties of beds and caves, and for determining changes in the size of...http://www.google.co.uk/patents/US4899112?utm_source=gb-gplus-sharePatent US4899112 - Well logging apparatus and method for determining formation resistivity at a shallow and a deep depth
