A close interval survey that is used to establish a baseline for determining how much CP is needed for a structure is ________.

The correct answer is a native survey because this type of survey is specifically designed to measure the natural potential of a buried pipeline or structure before any cathodic protection (CP) is applied. The baseline established through a native survey is essential for determining how much CP is necessary to effectively mitigate corrosion.

By assessing the natural potential, a native survey provides critical data regarding the existing conditions and any electrochemical activity that might be occurring. This allows engineers and corrosion control specialists to develop an effective cathodic protection plan.

In contrast, other surveys like depolarized or critical surveys have different purposes or methodologies. A depolarized survey measures the potential change after an external electrical current is applied, while a critical survey evaluates the performance of cathodic protection systems under specific conditions. Both serve important roles in corrosion control but do not establish the initial baseline for CP requirements in the same way that a native survey does.