sistivity data is principally limited by uncer- tainty of electrode locations and the transmitter
current and appears to be generally within 5. IP repeatability is approximately 2 mrad, based
on numerous reciprocal repeat points in the survey.
4. Survey results
Ž .
Five-meter dipole data from Lines A 7.5 S Ž
. and B 75 S are presented as Figs. 3 and 4,
respectively. The former are representative of results over the simulated waste, while the latter
are as far removed from the waste as could be managed within the CTP. The results of Line B
Ž
. Fig. 4 serve to illustrate the background resis-
tivity and IP patterns over the CTP. Clearly there is a huge IP response from the
Ž simulated waste typical mining exploration ap-
. parent IP responses are 20 to 50 mrad . Note
that the ‘pants–leg’ pattern in Fig. 3, i.e., the two strong anomalous diagonals, is an artifact
of the dipole–dipole pseudosection plotting con-
Fig. 4. Background resistivity and IP response, Line B, 5-m dipoles, 1 Hz.
vention showing the apparent resistivity or phase lag in the centre of both dipoles. Regarding the
real location of the dipoles, it becomes clear that the responses originate within a single re-
gion near the middle of the line where one of the dipoles is placed. The negative apparent IP
effects measured on Line A are due to body polarization effects, common in high-contrast
circumstances involving 2- and 3-D bodies.
Relatively small IP effects are observed on the background line. Other measurements in the
area suggest that the intrinsic polarization effect of the clay, soil, and basalt is less than 5 mrad,
or somewhat less than the largest values ob- served on line B. Some residual IP effect is due
to lateral effects from the ‘large objects’ only one dipole length to the north of the line.
The resistivity data of Fig. 3 show a conduc- tive zone located near the centre of the line,
with apparent resistivities decreased to about one half to one third of background. This fea-
ture forms a viable target for methods based on conductivity contrasts. Another prominent con-
ductive feature on the eastern end of the back-
Ž .
ground line Fig. 4 is not due to any known waste and has no corresponding IP response.
This non-waste-related
resistivity response
shows an apparent resistivity far less than that of the simulated waste, and appears on the short
separations, indicating a shallow causative body. The feature was detected on some of the other
lines as well as by several of the other survey methods used in the EMID work. The unknown
conductive feature may be due to variations in the source, handling, or compaction of the cap
material, or to subsequent modifications such as spills during operations in the area. In any case,
it is clear that buried waste is not the only cause of conductive anomalies at INEL.
5. Forward modelling and inversion
