3. TRANSVERSE CURRENT MONITORING
Using horizontal arrival angles due to ray bending by the current as a way to measure the flow has been proposed by Farmer and Di Iorio [9]. MHRT can also be applied to the monitoring of current in the ocean by measuring the modal phase difference with the interferometer. Assume that we have a transverse ( y-component) current velocity, vy = vy (x,z). We set the source at the original point of the horizontal plan (x,y), the interferometer with aperture L is located at its end points at (R, +L/2) and (R, -L/2), and the angle between the two acoustic paths are +θ (see Fig. 4). In this case, the modal-phase difference of the interferometer with aperture L is given by
As we can see above, the retrieval from the modal-phase difference is directly proportional to the current velocity in the depth interval embraced by this mode and averaged across the strait. This is the information that is useful for the heat exchange estimation in the area of the strait. Numerical simulation has been done for the monitoring of the Fram Strait current [10].
4. TOMOGRAPHIC INVERSION OF THE FRONT PARAMETERS
The Polar front, resulting from the convergence of the warm Atlantic and the cold Arctic water masses along the line of contact, is known to have the highest contrast between properties of the divided water masses in Nordic seas. An overview of the data available in the Barents Sea has been given by Aleksandrv [11]. The SSP data as measured in May, 1985 are listed in Table 1. The width of the front is about L0 = 2 km, and a linear interpolation of the SSP with in the front has been used in our numerical simulation. The overview of the geometry of the acoustic propagation is shown in Fig. 5. Within the adiabatic mode context, the modal phase is given by
