Milton Garces, Claus Hetzer, and Mark Willis University of Hawaii, Manoa Source modeling of microbarom signals generated by nonlinear ocean surface wave interactions 2003 Infrasound Technology Workshop, San Diego, California

What are microbaroms, and why we care? Microbaroms signals, like microseisms, are believed to be created by the nonlinear interaction of ocean surface waves The microbarom peak near 0.2 Hz is right on the detection frequency band for 1 kt explosions IMS arrays with large apertures (> 1 km) were supposed to render microbaroms incoherent, but distinct coherent bursts may still be detected Microbaroms may be generated in open ocean or by reflections with coastline and islands, and are prominent on island stations Theoretical energy peak of microbarom radiation is near vertical, but this energy is lost Sufficient energy is radiated near the horizontal, where most microbarom arrivals are detected Study microbarom statistics at I59US and global distribution of microbarom signal levels

Microbaroms: 2002 N Swells, Aleutians Trade and S Swells

Microbaroms: Year 2002

Theory: Arendt and Fritz, 2000 Assumptions: Wave height small relative to wavelength of ocean wave Distance greater than acoustic wavelength Solution: For a prescribed surface wave displacement g(x,t) and vertical velocity u z (x,t), the acoustic pressure is:

Theory: Interfering plane waves,  2 =  1 = 0.2 Hz

Theory Consider a ocean surface wave displacement spectrum A, The radiated acoustic spectrum would be

Theory Vertical acoustic wavenumber and a function of the ocean wave and acoustic wavenumbers

Wave Watch 3 (WW3) WAVEWATCH III (Tolman 1997, 1999a) is a third generation wave model developed at NOAA/NCEP in the spirit of the WAM model (WAMDIG 1988, Komen et al. 1994). It is a further development of the model WAVEWATCH I, as developed at Delft University of Technology (Tolman 1989, 1991) and WAVEWATCH II, developed at NASA, Goddard Space Flight Center (e.g., Tolman 1992). WAVEWATCH III solves the spectral action density balance equation for wavenumber-direction spectra. The implicit assumption of these equations is that the medium (depth and current) as well as the wave field vary on time and space scales that are much larger than the corresponding scales of a single wave. Furthermore, the physics included in the model do not cover conditions where the waves are severely depth-limited. This implies that the model can generally by applied on spatial scales (grid increments) larger than 1 to 10 km, and outside the surf zone.

Wave Watch 3 (WW3) Surface winds and dominant period

Wave Watch 3 (WW3) Significant wave height

Wave Watch 3 (WW3) Significant wave height

Evaluating the Theoretical Model The peak source pressure occurs when k = -k’,  =  ’ The Wave Watch 3 model outputs the variance density, F, of the surface wave field as a function of frequency, f, and propagation direction, . The variance density can be integrated over angle and frequency to provide the total wave energy E, For preliminary amplitude estimates, we use Whitaker’s relationship

Microbaroms: January 21-28, 2003, 90s window, ½ second consistency, Hz

Microbaroms: January 21-28, 2003, Family Size

Microbaroms: February 22, 2003, 60s window, 1 second consistency, Hz

Sea State: February 22, 2003

Infrasonic Source: February 22, 2003 Geometric frequency steps (1.1*f) from 0.08 to 0.8 Hz, dynamic range of 90 db

REB Location: February 21-23, 2003, no seismic contributions

Need to add climatological specifications

Conclusions and future work All IMS infrasound arrays, and particularly those close to the ocean, are susceptible to microbaroms Microbaroms may be generated in the open ocean Obtained surface wave spectrum from Wave Watch 3 global model Developed an algorithm to evaluate a theoretical source pressure field induced by the open ocean surface wave field Used simple relationship to estimate global infrasonic field Need to add atmospheric specifications, attenuation losses, and direction of arrival information Need to incorporate better propagation algorithms to provide time dependent and frequency dependent estimates of the propagating infrasonic field Need to add reflections with coastline and islands – mesoscale problem, site dependent and not trivial Approach can be adapted to microseisms

Onwards