Ellen Livingston, Steve Wolf, Bruce Pasewark
Naval Research Laboratory, Washington, D.C.
The NRL Surf Noise Measurement Supplementation experiment is a supplement to the Wave and Surf Generated Ambient Noise Measurements experiment (DUCK94 Exp. #23) supported by the NRL Undersea Noise Program. This experiment is intended to measure aspects of surf zone noise generation and propagation during a time when concurrent measurements of the ambient environmental processes are being measured in the DUCK94 experiment. The Surf Supplement experiment will increase the duration, reliability and safety of the currently planned sonobuoy noise collection processes, and augment those measurements with directional noise measurements on an acoustic array, measurements from a controlled source for understanding the propagation characteristics of the waveguide, and measurements of air and wet sand propagating surf noise.
The original plan for the Wave and Surf Generated Ambient Noise Measurements Experiment calls for several deployments of four DIFAR sonobuoys at ranges from 1 to 10 km from shore. The deployments will be made before storms or high wave events and data will be transmitted for eight hours on standard sonobuoy RF frequencies (EFS Channels 1-99) in the 136-173 MHz band. Deployments will be made by leased commercial vessel in positions away from other sensors to avoid physical interference. We will augment these measurements as follows:
a. Five Arlington Research Laboratory, University of Texas (ARL:UT) bottom mounted DIFAR sonobuoys with five-day lifetimes will be deployed to complement the measurements from the 8-hour lifetime DIFAR sonobuoys as currently planned. One will be attached to the end of the FRF pier. The remaining four will be deployed at from 1.5 km to 6 km from shore, east of the minigrid survey area. The deployments will be made before the start of the experiment by leased commercial vessel. Once deployed, these sonobuoys will transmit data continuously for five days on standard sonobuoy RF frequencies (EFS Channels 1-99) in the 136-173 MHz band. As needed, the ARL:UT sonobuoy batteries will be recharged for further data collecting.
b. A 45-m or 63-m long horizontal bottom mounted acoustic array will be deployed at approximately 750 m longshore and 550 m cross-shore from the FRF baseline. The array will be deployed before the start of the experiment by divers and secured to four pipes. It will be cabled to the pier for data transfer. The array will provide wavenumber/angle-frequency analysis of the surf zone noise generation.
c. A J13 or J9 sound source will be deployed in the breaking wave surf zone to measure the characteristics of the propagation waveguide. This measurement is critical to understanding how surf zone noise is propagated offshore when the source point is in water whose depth is less than critical depth. The sound projector will be suspended through grates in the FRF pier at several cross-shore distances. It will be operated intermittantly during selected surf conditions for up to two hour time periods.
d. Surf noise propagating in the atmosphere will be measured with up to six shielded directional microphones cabled to recorders. These will be deployed in pairs on the beach as close to the longshore position of the array as possible. They will extend along the beach for at most 30 m. These devices will be set out for from two to six hours. This measurement is necessary to help understand the surf noise generation mechanism by spilling and plunging breaking waves.
e. The contribution of the surf activity to surface seismic noise will be determined by installing a number of vertical- and three-axis geophones in saturated sand near the water's edge. The geophones will be installed in the wet sand south of the FRF pier for up to two hours of data collection during selected surf conditions.
All data will be analyzed in conjunction with wave and surf measurements from other experiments to develop and improve theoretic nearshore acoustic noise models.
For a list of addresses of the investigators involved in Duck 94 click here.