Information on the presence of clay-rich layers in the inner-valley
alluvium is essential for quantifying the amount of water transmitted
between the Rio Grande and the Santa Fe Group aquifer system. This
report describes a study that used electromagnetic surveys to provide
this information. In the first phase of the study, electromagnetic
soundings were made using time-domain and frequency-domain electro-
magnetic methods. On the basis of these initial results, the time-
domain method was judged ineffective because of cultural noise in the
study area, so subsequent surveys were made using the frequency-domain
method. For the second phase of the study, 31 frequency-domain
electromagnetic surveys were conducted along the inner valley and
parallel to the Rio Grande in the Albuquerque area in the spring and
summer of 1997 to determine the presence of hydrologically significant
clay-rich layers buried in the inner-valley alluvium. For this report,
the 31 survey sections were combined into 10 composite sections for
ease of interpretation.
Terrain-conductivity data from the surveys were modeled
using interpretation software to produce geoelectric cross sections
along the survey lines. This modeling used lithologic logs from
two wells installed near the survey lines: the Bosque South and
Rio Bravo 5 wells. Because of cultural interference, location of
the wells and soundings, complex stratigraphy, and difficulty
interpreting lithology, such interpretation was inconclusive.
Instead, a decision process based on modeling results was developed
using vertical and horizontal dipole 40-meter intercoil spacing
terrain-conductivity values. Values larger than or equal to 20
millisiemens per meter were interpreted to contain a
hydrologically significant thickness of clay-rich sediment.
Thus, clay-rich sediment was interpreted to underlie seven
segments of the 10 composited survey lines, totaling at least
2,660 meters of the Rio Grande inner valley. The longest of these
clay-rich segments is a 940-meter reach between Bridge and Rio Bravo
Boulevards.
