Neutron logging method and apparatus for determining a formation characteristic free of environmental effects

摘要

A method for determining porosity or other formation characteristic of a sub-surface geological formation traversed by a borehole is claimed. A neutron logging tool is passed through the borehole while irradiating the formation with neutrons. The neutron logging tool includes a neutron source and first and second detectors spaced from the source by different distances. Neutrons exiting the formation are detected with the neutron detectors and count rate signals are generated. In response to these signals, an indication of porosity, substantially independent of error due to tool standoff from said borehole wall, is produced. In addition, values of tool standoff are also generated. These standoff values are then filtered by a filter to reduce statistical variations and are used to generate improved indications of porosity.