@misc{indiciaef269e5f40175,
  title = {A portable vacuum hammer seismic source for use in tunnel environments},
  author = {R. D. Carroll and J. E. Magner},
  year = {1993},
  doi = {10.3133/ofr93187},
  url = {https://pubs.usgs.gov/publication/ofr93187},
  note = {Source identifier: ofr93187}
}