This project is the preliminary engineering survey work for the relocation project of a certain company. Due to the presence of a small reservoir built decades ago on the site, the soil layer at the bottom of the reservoir is silty soil, which is classified as soft soil. Soft soil has the characteristics of high natural moisture content, large natural void ratio, low shear strength, high compression coefficient, small permeability coefficient, low foundation bearing capacity, large foundation deformation, and long deformation stabilization time. During the site construction, the soft soil needs to undergo certain treatment before it can be used. In order to investigate the thickness of the soft soil layer at the bottom of the reservoir and provide a reference basis for engineering design, this low-altitude frequency-domain airborne electromagnetic survey work has been carried out.

The frequency-domain airborne electromagnetic scanner (NAFEM) has a line spacing of 20 meters. The measurement device is equipped with GPS. The GNSS module using network RTK for calculation is employed to record the position information of the measurement points. The speed of the line work is approximately 2 meters per second. The detection data and GNSS are recorded at a frequency of 1 Hz. Point extraction is performed every second, and apparent resistivity is calculated for cross-sectional comparison.

During this low-altitude frequency-domain electromagnetic survey at the reservoir, a total of 7 flight paths were conducted, with a total length of 880 meters.

The surface strata exposed within the working area are of the fourth-order artificial accumulation (Q4ml+pd), consisting of miscellaneous fill soil, plain soil, and cultivated soil; the bottom is the Zhenanxiang Formation of the Diandian Series, with dolomite as the rock type, and local dissolution is developed.

Based on the previous ground high-density direct current electrical survey conducted around the reservoir, the apparent resistivity of the fourth-order strata is 10 to 270 Ω·m. The higher the moisture content, the lower the resistance value; the resistivity of the reservoir bottom and underwater soft soil is lower than that of the ordinary fourth-order strata due to higher moisture content. The Diandian Series of the Diandian Formation of the dolomite is divided into two layers: the upper strong weathering and fragmentation zone has a resistivity of 100 to 800 Ω·m, and the lower medium weathering and intact dolomite has an average resistivity of 2156 Ω·m. The overall rock layer resistivity is much higher than that of the fourth-order strata.

The low-altitude frequency-domain electromagnetic survey conducted in this project achieved stable flight height. Therefore, a simplified apparent conductivity formula was used to complete system calibration and calculate apparent resistivity. Combined with the drilling data from the reservoir project, an appropriate depth correction coefficient was selected to conduct depth-apparent resistivity mapping, thereby completing the interpretation and inference of underground geological bodies.

Silt and silty soil within the reservoir are classified as soft soil. This survey was conducted during the dry season, and the water area of the reservoir contracted. However, the soil beneath the hardened ground within the reservoir still contains groundwater. As shown in the apparent resistivity contour map of the F4 flight route, even if there is no water on the reservoir surface, low-resistance layers are still developed in the deep part; the soil beneath the hardened layer in this area has the same physical properties as soft soil and is uniformly classified as soft soil.

Soft soil is mostly found at the bottom of the reservoir and has a high water content. Based on the regional engineering geological characteristics, a 100 Ω·m threshold was set as the dividing line: areas with apparent resistivity lower than 100 Ω·m within the reservoir bottom and its surrounding areas are classified as soft soil layers, while the high-resistance areas at the bottom of the profile are defined as strongly weathered dolomite; the overlying fourth-order clay layer is above the dolomite. According to the drilling exposure results, the upper part of the soft soil layer is a silt layer, and the lower part is saturated soft cohesive soil.

Using an excavator, the thickness of the silt layer at the dry area near the water body was revealed to be approximately 2 meters. Using 128Hz PPM real component detection data, through function fitting, the thickness of the silt layer within and adjacent to the water body was calculated and contour mapping was performed, as shown in the figure.

During the construction period, this water body was drained, and then the excavator was used on-site to verify the thickness of the silt layer. The calculated thickness of the silt layer was roughly in line with the actual situation.