Calculate the time-average velocity.

        Parameters
        ----------
        depth: float
            Depth over which the average velocity is computed.

        Returns
        -------
        avg_vel: float
            Time averaged velocity.

def time_average_vel(self, depth):
        """Calculate the time-average velocity.

        Parameters
        ----------
        depth: float
            Depth over which the average velocity is computed.

        Returns
        -------
        avg_vel: float
            Time averaged velocity.
        """
        depths = [l.depth for l in self]
        # Final layer is infinite and is treated separately
        travel_times = [0] + [l.travel_time for l in self[:-1]]
        # If needed, add the final layer to the required depth
        if depths[-1] < depth:
            depths.append(depth)
            travel_times.append((depth - self[-1].depth) / self[-1].shear_vel)

        total_travel_times = np.cumsum(travel_times)
        # Interpolate the travel time to the depth of interest
        avg_shear_vel = depth / np.interp(depth, depths, total_travel_times)
        return avg_shear_vel