This is an acronym for a well log used in the oil and gas industry. You might have encountered this acronym
HALC in your log evaluation software or any (ASCII) log files provided by the logging company.
APS High Resolution Near/Array Corrected Limestone Porosity is the offical name for this log. The usage of this log is well established. This log can be used to evaluate the porosity derived from a neutron logging measurement, assuming that the rock matrix material is calcite.
So basically this is a measure of the porosity as measured with an neutron logging device. In general this log therefore displays units of the apparent porosity. This is a measure of the initial estimate of porosity, normally from logs, not corrected for the (often large) effects of clays, gas, etc. Such estimates should only be used for qualitative interpretation of clean, gas-free intervals.
In general this log thus reflects units of the volume fraction. Better said, the ratio of the volumes occupied by two components, or by one component to the total volume of the system.This log usually has the unit
'Porosity'. This is the pore volume per unit gross volume. Porosity is determined from measurements on cores or interpreted from logs.
Data for this log is produced by a tool that uses a neutron logging technique and can be used to investigate the porosity of the formation. Similar logs are listed below: