Dual-frequency drone bathymetry in a turbid quarry pond

The challenge

Knife River needed accurate depth and sediment maps of a pond at its Texas limestone quarry to keep operations running efficiently. Westward Geology Group was contracted to do the survey without putting boats on the water.

The site made that hard:

• the pond was receiving discharge, so the water was turbid and full of suspended fines;
• steep banks ruled out launching a boat;
• submerged obstacles and active pumping added safety risks;
• vegetation along the margins limited access from the ground.

The solution

Westward used a two-phase workflow on a DJI Matrice 350 RTK. First, a DJI Zenmuse L2 LiDAR flight of about 10 minutes captured the pond geometry and surrounding elevation at roughly 400 points per m². Second, the drone carried SPH Engineering's bathymetry kit with a dual-frequency echo sounder working at 200 kHz and 50 kHz, controlled and logged by SkyHub.

The mission was planned in UgCS as an automated grid, with a six-inch (15 cm) tow depth, RTK positioning and line spacing set for the pond size. Steep banks were marked as no-go areas. During the flight the drone used terrain following and the operators stayed on shore.

The LiDAR point cloud was turned into a DEM, and the two sonar frequencies were processed into a depth grid. The team checked that the datasets lined up.

The results

• The 200 kHz signal mostly reflected off the suspended sediment. The 50 kHz signal passed through it and found the consolidated bottom.
• The final depth model, built from the 50 kHz data, showed a maximum depth of about 30 ft in the north-eastern part of the pond.
• The whole job took under an hour, compared with a multi-hour boat survey.
• No one entered the water, which removed the drowning risk.

Deliverables were a bathymetric map and an interpretation of the sediment layers. SPH Engineering presents the project as an example of mapping sediment in active, sediment-laden industrial ponds where single-frequency or boat methods struggle.

Where this applies in India

Many working ponds in India are turbid, steep-sided or unsafe for boats. The same two-phase approach suits:

• quarry and mine pit lakes that receive pumped discharge;
• settling ponds and sumps at aggregate, stone-crushing and mining sites;
• ash ponds and industrial water storage with high suspended solids;
• reservoirs where the true bottom is hidden under soft sediment.

Tell us the pond size, expected depth and water conditions, and we will specify the kit. SPH Engineering's bathymetry training is also available through HobiTech.

The equipment, available from HobiTech

Drone	DJI Matrice 350 RTK
Echo sounder	SPH Engineering bathymetry kit with a dual-frequency echo sounder (200 kHz and 50 kHz). The source does not name the model; our EchoLogger ECT D052S works at 50/200 kHz.
Onboard computer	SkyHub by SPH Engineering with the SkyHub echo sounder software
Flight planning	UgCS - Drone Flight Planning Software
LiDAR	DJI Zenmuse L2 was used for the elevation flight in this project. Ask us for current compatible payloads.

Case study by SPH Engineering, published October 2025. This page summarises it in our own words; the survey was carried out by Westward Geology Group for Knife River, not by HobiTech. Read the original case study on SPH Engineering ↗