@article{tseng:1998:nei,
    author = {Tseng, P.-H. and Lee, T.-C.},
     title = {Numerical evaluation of exponential integral:
              {T}heis well function approximation},
   journal = {Journal of Hydrology},
      year = {1998},
    volume = {205},
    number = {1-2},
     pages = {38-51},
       doi = {10.1016/S0022-1694(97)00134-0}}