The IEEE Seventh Working Conference on Current Measurement Technology

Current and Wave Monitoring and Emerging Technologies

March 13-15 | Bahia Hotel | San Diego, CA, USA

  
     

Surface Currents Measured From a Sequence of Airborne Camera Images

J.P. Dugan

Status: Accepted

1725 Jeff Davis Hwy
Crystal Sq 2, Suite 703
Arlington , Virginia USA
22202

Phone: 703 413 0290
Email: dugan@arete-dc.com

Co-Authors:
C.C. Piotrowski
J.Z. Williams

Measurements of nearshore ocean currents are important for a number of reasons, including emergency storm response, military operations in very shallow water, understanding sediment transport, and planning/evaluation of littoral construction and dredging projects. Because direct measurements are difficult (and expensive) to make over extensive areas, particularly where the bottom is highly mobile, remote sensing techniques are of great interest. A digital camera system has been used to remotely measure surface currents from a small aircraft by measuring the Doppler shift of short- to medium-length gravity waves. Image sequences are collected by staring at an area about 2 km on a side for 1-2 minutes duration. Aircraft navigation data are used to map each image to a geodetic reference frame at the level of the mean ocean surface. The 3-D frequency-wavenumber spectrum of the radiance modulations is calculated in a grid of sub-regions and the gravity wave dispersion relation is identified in each. The 2-D velocity vector field, as well as water depths, are calculated by fitting the theoretical dispersion surface to the spectrum in each sub-region. Current retrievals in a complex, exposed tidal inlet channel and surrounding shoals are favorably compared with simultaneous in situ measurements. Values are within 10% in magnitude and 5 deg in direction for ADCPs on the bottom in the channel center and on a jet ski that was traversing the channel. Bars and shoals are clearly detected, although unrealistically large values of the current sometimes occur in locations where the waves clearly are breaking over the shoals. Unfortunately, these locations also are precisely where in situ data are unavailable. These errors are attributed to wave-current interaction and imaging processes that violate the model assumptions. A fix for this problem is hypothesized. The method shows considerable promise for broad surveys in areas having complex channels and shoals, particularly as maps of bathymetry, directional wave spectra, and ortho-images of the nearby shore can be retrieved with the same image time series used to retrieve the currents.

Submitted on October 09, 2002