Resistivity Surveying

1
Resistivity Surveying

• Galvanic
• uses fixed electrodes driven into surface
• requires direct electrical contact between
  electrodes and surface
• small electrode spacing to image very near to
  surface not practical
• impractical for work on ice surfaces
• Induction (e.g., EM-31)
• alternating primary EM field induces secondary
  emissions from subsurface
• does not require contact with surface
• arctic ice surveys performed on foot and in air
  7
• requires a calibration curve (Fig. 3) to match
  received signal with measured ice thickness 2

Figure 3. Calibration curve for induction
surveyFigure 6 from Kovacs, et al. The dots are
points where EM-31 readings were obtained with
borehole measurements of ice thickness. The solid
line is a regression curve matching the data.

• Capacitive
• dipole-dipole electrodes pulled across surface
• current flows into surface through capacitor
  effect
• not dependent on direct electrical contact
• depth of signal determined by spacing of the
  dipoles?n-spacing where n is a multiple of a
  single dipole length
• unusually small electrode spacings can be handled
  by OhmMapper data acquisition unit as well as
  RES2DINV software

Figure 4. James Inman pulling OhmMapper
capacitive resistivity system along 300-m test
line on Chukchi Sea ice just offshore of BASC
facility near Barrow, Alaska.
Figure 5. Lengthening the rope/isolating link
between Tx and Rx increases depth of signal
penetration. Image courtesy of Geometrics, Inc.