The guard tool behaves similarly to one elongated current electrode from which current flows radially in all directions to a distant current-return electrode. In practice, the current-emitting electrode is separated into three parts by insulation so that the center part, which is made short, can be treated as a discrete electrode without alteration of the current-flow pattern. The current from the center [[electrode]], which serves as both a current and measure electrode, flows in a thin horizontal layer at an angle of 90° to the tool because of its central location in the configuration.

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Through the method of focusing described above, this arrangement provides good resolution of thin beds and permits the use of the tool in boreholes filled with saline muds.

The log usually is presented with one [[resistivity]] [[curve]] and a [[gamma ray|gamma-ray]] curve and/or [[SP]] curve (recorded from an electrode not physically a part of the guard electrode). Compare [[laterolog]].