def METHOD_NAME(self, locs):
    """Compute displacement field at locations.
    """
    (npts, dim) = locs.shape
    disp = numpy.zeros((1, npts, 2), dtype=numpy.float64)
    mask = numpy.logical_and(locs[:, 0] > -2.0e+3, locs[:,0] < 0.0)
    disp[0,:, 1] = +2.0 * mask
    return disp