Microwave probing of shallow water bottom topography in the Nantucket Shoals

Coordinated microwave and in situ measurements were performed during the 1982 Naval Research Laboratory Phelps Bank experiment (Nantucket Shoals). The measurements including airborne X band APD-10 synthetic aperture radar imagery and airborne X and shipboard L band coherent radars together with extensive hydrographic, oceanographic, and meteorological information are reported. The microwave measurements show large variations of backscatter power (over 20 dB at both L and X band) for winds less than 7 m/s. Generally, regions of strong backscatter cluster near large bottom slopes, suggesting their generation by the bottom topography. However, for winds greater than 7 m/s (7–12 m/s in this experiment) the variations of microwave backscatter power decrease to 2–3 dB for both frequency bands and become decorrelated from the bottom topography. An explanation for this may be that wind drift near the surface layer tends to mask the ocean surface features relating to the bottom topography when its magnitude is comparable to the rotary tide and the residual circulation due to tidal stress. A current gradient (front) seemingly not related to bottom topography also produced 20-dB variations in the power backscattered to the L band radar.

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