Processing seismic data

A method for determining a vertical component (formula I) for particle velocity from seismic data obtained at a receiver arranged within a water column, comprises determining the vertical component of particle velocity from the pressure obtained at the receiver using an operator accurate for seismic data obtained at a depth below the surface of the water column of up to at least 0.4 times the minimum wavelength of seismic energy. used to obtain the seismic data. The vertical component of particle velocity from the pressure obtained at the receiver can be determined using Equation (2). This equation is accurate and can be used for seismic data obtained at any depth below the surface of the water column. As an alternative, the equation may be truncated to reduce the necessary processing, although this may limit the maximum depth for obtaining data with which the truncated equation can be used.