INVESTIGATING OVERBANK DEPOSITS AT ELEPHANT BUTTE, NEW MEXICO USING GROUND-PENETRATING RADAR

Ground penetrating radar (GPR) has been proven effective in numerous cases, especially for non-intrusive surveying of shallow subsurface anomalies. Elephant Butte, located 20 miles northeast of Truth or Consequences, New Mexico, hosts the state’s largest reservoir: Constructed in 1915, the Elephant Butte Reservoir (EBR) is crucial to the Rio Grande Project (RGP), which aims to provide power and irrigation to south-central New Mexico, Texas, and northern Mexico. However, the reservoir experiences an Annual Storage Loss percentage of 1.2-2.0% due to significant sediment deposition, primarily resulting from drought conditions. The Rio Grande, coursing through the reservoir, is critical to the RGP.

The study area, inundated almost 60 percent of the year, is situated north of EBR on a peninsula-like landform near the Rio Grande. Since the dam’s construction, the area has experienced a range of inundation events due to fluctuating water levels. To understand the reservoir’s depositional history, six GPR survey lines were conducted using the 250 MHz GPR Noggin SmartCart—three lines perpendicular and three parallel to the Rio Grande. The data were analyzed using EKKO_Project™ Software with noise rejection filters, migration, and topography shifts, revealing stratification features such as hummocky bedding, erosional surfaces, and channel fill.

We determined from sediment grain samples, fine-grained sand, interbedded with coarse-grained sand and organic material, was the dominant sediment-type, suggesting the study area was experiencing high-energy depositional conditions. For ground-truth, trenches were excavated up to 60 cm to understand the shallow subsurface stratigraphy. The trenches and GPR data reveal that the first meter of the subsurface is characterized by hummocky bedding, indicating that the study area exhibits rapid sedimentation, consistent with high-energy deposition, primarily from annual monsoon flooding and snowmelt.

Additionally, satellite imagery indicates that the study area functions as a tie channel, or bi-directional flow channel, which explains the hummocky bedding stratification observed. To enhance our understanding of depositional processes and dynamics, a fence diagram was implemented to provide a detailed subsurface view. This study demonstrated GPR's capability in identifying and mapping geological layers effectively.