def METHOD_NAME(self):
    """Projection matrix from x-faces to receiver locations.
    Returns
    -------
    scipy.sparse.csv_matrix
        The projection matrix from x-faces to receiver locations
    """
    if getattr(self, "_Qfx", None) is None:
        self._Qfx = self.prob.mesh.get_interpolation_matrix(
            self.receiver_locations, "Fx"
        )
    return self._Qfx