RADIOWAVE RESISTIVITY MEASUREMENTS IN NORTHERN MAINE FOR IDENTIFYING BEDROCK TYPE

A preliminary ground resistivity survey using the VLF radiowave method was conducted in northern Maine near the site of the proposed Dickey-Lincoln dam and hydroelectric project. This survey was intended to establish if adequate resistivity contrasts exist between bedrock types in the area to warrant further bedrock distribution studies by airborne resistivity techniques. A more complete understanding of the geology of this area was considered necessary in an attempt to locate rock types suitable for construction purposes near the proposed construction sites. Field observations were made on all rock types commonly found in the area, including the dominant gray slates and the less common orthoquartzites and granodiorites. The granodiorites are considered most suitable for construction purposes. Results of this field study suggest that an airborne survey could differentiate between the granodiorites and the surrounding gray slates in areas of shallow overburden, although resistivity contrasts the slates and the orthoquartzite were probably not great enough to differentiate between these rock types. More than 70% of the resistivity values from the granodiorite sites exceeded 10,000 ohm-m; in contrast, approximately 8% of the slate values and none of the orthoquartzite values exceeded 10,000 ohm-m. A VLF resistivity profile was also obtained along a segment of the southern section of the proposed dam centerline. Resistivity values correlated well with subsurface information obtained during earlier Corps of Engineers drilling investigations. Major breaks in the resistivity data agreed with variations in ground conditions such as increases in till thickness over bedrock and the occurrence of silt and clay in the valley of the Saint John River. Resistivities range from 100 to 9000 ohm-m along the line. The lowest values were associated with the silts and clays and the highest with areas where bedrock is near the surface.

Media Info

  • Features: Figures; Tables;
  • Pagination: 11 p.
  • Serial:
    • Issue Number: 238

Subject/Index Terms

Filing Info

  • Accession Number: 00129384
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Mar 29 1976 12:00AM