In May 2025, Westward’s Geology Group conducted a successful drone-based, dual frequency bathymetry survey at Knife River’s active limestone quarry in Texas. Knife River required accurate data on the capacity of ponds near the plant. Westward utilized a drone equipped with a dual-frequency (50 kHz and 200 kHz) echo sounder to map the top of sediment and the underlying consolidated sediment in the ponds. A LIDAR survey was also flown to estimate the amount of freeboard available in the pond and get updated elevation data of the ponds.
Unlike standard bathymetry surveys, which typically just use a single 200 kHz frequency to detect the first reflective surface—often the top of soft sediment in dormant ponds—this approach provided another tool to read through suspended false echoes from obstructions like fish, suspended sediment, or vegetation, ensuring precise measurements.
Operators often seek to measure the thickness of soft, unconsolidated sediment at the bottom of ponds. In sand and gravel operations, this sediment may represent valuable, saleable material left behind. A dual-frequency echo sounder can help determine the thickness of this softer, unconsolidated sediment.
Using a DJI Matrice 350 drone, Westward deployed DJI’s L2 LIDAR system with an integrated RGB camera to fly the pond areas. The L2 has a data collection range of up to ~820 feet and is capable of covering up to ~600 acres in a max height flight when paired with the Matrice 350. Utilizing up to 5 returns, this LIDAR system is able to collect data at a rate of up to 1,200,000 pts/s while flying at a max speed of ~33.5 mph. Using 3 returns, it took approximately 10 minutes to fly the subject area and collect data at a 400 pts/m² resolution.
The L2 unit was then exchanged for the dual frequency echo sounder for the bathymetry survey. The goal of this survey was to estimate the depth to top of sediment and the depth to the consolidated sediment, hard bottom. The echo sounder was mounted to the bottom of the drone with a 2-meter cable. To collect accurate data, the sensor was pulled through the water at 0.7 meters per second while approximately 6 inches under the surface. The Flight was flown in a baseline grid pattern.
The results show a high volume of suspended sediment in the pond, as evident from the color and cloudiness of the water. During the flight, fines were being actively pumped into the pond at a high rate. Consequently, the 200 kHz sensor couldn’t detect the top of the unconsolidated material due to strong reflectance from the suspended sediment. However, the 50 kHz sensor penetrated the suspended sediment and identified the hard, consolidated pond bottom. In this pond, fines consolidate as they settle and are covered by additional fines, influenced by the high discharge velocity and material density.
As expected, the pond is deepest in the northeast corner at a depth of approximately 30 ft. The pond is part of an open loop system with water flowing from one pond to the next. Had the pond system been closed and dormant, we would have expected to see the top of the softer, unconsolidated sediment in addition to the hard, consolidated bottom.
The dual-frequency echo sounder is a great tool to effectively measure pond configuration and water depths. Mounting the sensor on a drone enhances safety compared to using a boat. Additionally, Westward can conduct bathymetry surveys in vegetated ponds using either sensor with Grasshopper Mode, which allows the drone to intermittently lower and raise the sensor to avoid obstacles like dense vegetation. These drone-based applications provide a highly effective and safe solution for all stakeholders involved in pond surveys. Many thanks to Knife River for partnering with us on this pilot study.