def write_crmod_volt(self, filename, mid):
        """Write the measurements to the output file in the volt.dat file
        format that can be read by CRTomo.

        Parameters
        ----------
        filename: string
            output filename
        mid: int or [int, int]
            measurement ids of magnitude and phase measurements. If only one ID
            is given, then the phase column is filled with zeros

        """
        ABMN = self._get_crmod_abmn()

        if isinstance(mid, (list, tuple)):
            mag_data = self.measurements[mid[0]]
            pha_data = self.measurements[mid[1]]
        else:
            mag_data = self.measurements[mid]
            pha_data = np.zeros(mag_data.shape)

        all_data = np.hstack((ABMN, mag_data[:, np.newaxis],
                              pha_data[:, np.newaxis]))

        with open(filename, 'wb') as fid:
            fid.write(bytes(
                '{0}\n'.format(ABMN.shape[0]),
                'utf-8',
            ))
            np.savetxt(fid, all_data, fmt='%i %i %f %f')