The CHURCH OPAL Exercise is one of a series of LRAPP Exercises specifically designed to acquire environmental acoustic data required for ASW program decisions. It will be conducted during September 1975 in a region of the Northeast Pacific Ocean in accordance with the CHURCH OPAL Exercise Plan (U), August 1975 (Secret). This Exercise includes the following ocean acoustic experiments related to the Moored Surveillance System (MSS), the Sound Surveillance System (SOSUS), and the Surveillance Towed Array Sonar System (SURTASS): (1) Noise Floor Characteristics, (2) ASEPS Evaluation - Inter-Array Coherent Processing, and (3) Horizontal Directionality of Ambient Noise - Towed Array Performance. The objective of the Noise Floor experiment is to establish the characteristics, persistence, and geographic extent of the low frequency ambient noise floor. The noise floor is a depth below which distant shipping noise is dramatically reduced, thereby providing the opportunity for short range acoustic sensors to attain a substantial performance gain.