Studies Of Grain Size Variations In Fluvial Sediments Of The Rio Grande Using Magnetic Techniques

Abstract

Characterizing the heterogeneity of fluvial sediments in the Rio Grande Valley near El Paso, Texas is important to understanding shallow groundwater flow, groundwater/river exchange and the build-up of soil salinity. We have found the magnetic technique an effective way to rapidly evaluate grain size changes within the river soils. This is in contrast to electrical techniques (resistivity, conductivity) that are strongly affected by seasonal variations in soil moisture and salinity. Medium to coarse-grained sands of the ancestral Rio Grande contain significant (up to 10% by weight) amounts of magnetite, and thus appear as magnetic highs. We have conducted magnetic surveys at four study areas within the Mesilla Valley northwest of El Paso. The sites have a range of sizes (10’s to 100’s of meters in dimension) and geologic complexity (homogeneous channel sands to complex interfingered crevasse splay and flood plain deposits). Borehole grain size analysis, as well as surface geophysical studies (e.g. conductivity, resistivity, seismic, GPR) were conducted at the sites. Our results indicate we can detect small crevasse splay (~2-3 m wide) channels buried at depths of 1-2 m, as well as larger channels (~20 m) at depths of ~20-30 m, through simple analysis of magnetic contour maps and magnetic gradients. We believe the technique has the potential for rapidly evaluating the location of channels within the Rio Grande valley, for corroborating surface soil maps and determining potential contaminant flow pathways.