def __init__(self, petro, mgr=None, **kwargs):
        """Initialize instance with manager and petrophysical relation."""
        petrofop = kwargs.pop('petrofop', None)

        if petrofop is None:
            fop = kwargs.pop('fop', None)

            if fop is None and mgr is not None:
                # Check! why I can't use mgr.fop
                # fop = mgr.fop
                fop = mgr.createForwardOperator()
                self.checkData = mgr.checkData
                self.checkError = mgr.checkError

            if fop is not None:
                if not isinstance(fop, pg.frameworks.PetroModelling):
                    petrofop = pg.frameworks.PetroModelling(fop, petro)

        if petrofop is None:
            print(mgr)
            print(fop)
            pg.critical('implement me')

        super().__init__(fop=petrofop, **kwargs)