Geoacoustic models for the Icelandic Basin

Geoacoustic models inferred from amplitude versus range data at 220 Hz are presented for three locations in the Icelandic Basin. The data were obtained using a deep-towed pulsed cw source and two receivers anchored near the bottom. For ranges out to 4800 m, the data were analyzed using an iteration of forward models technique in which the parabolic equation method and a Hankel transform method were used sequentially to compute acoustic fields for different bottom parameters until best fits to the data were obtained. The inferred geoacoustic models consist of a sediment layer containing a positive, linear sound-speed gradient overlying an isovelocity sub-bottom. The geoacoustic parameters include the layer thickness, the sound-speed gradient, and the sound-speed discontinuities at the water–bottom and sub-bottom interfaces. The density and attenuation of the structures are also determined. The geoacoustic models are substantiated by other types of measurements, which include piston coring, 3.5-kHz seismic profiling, and in situ sediment profiling. These additional measurements, combined with the 220-Hz results, yield a consistent picture of the geoacoustic nature of the three areas. Specifically, the highly coherent character of the 220-Hz data, the clearly visible sub-bottom layering in the 3.5-kHz records, and the low values of attenuation are related to the fine-grained nature of the sediment at two of the sites. At the third site, the presence of coarse sediment provides an explanation for the major incoherent component in the 220-Hz data, the poorly discernible layering in the 3.5-kHz records, and an anomalously high value of attenuation.