The "gold" standard for permeability is to make measurements on core samples and to determine permeability with the methods outlined in API RP 40. All other techniques are calibrated back to core measurements. However, because core measurements sample such a minute part of the reservoir, we must rely on techniques that can be applied in a widespread fashion across the reservoir. These methods rely on measurements on sidewall samples, correlation to wireline logging responses, interpretation of nuclear magnetic resonance (NMR) logs, wireline formation tester pressure responses, and drillstem tests. This technique is valid for slightly to unconsolidated sandstone rock types.