Abstract: Soft soils, characterized by high water content, high compressibility and low shear strength, frequently trigger excessive settlement and stability problems. Consequently, sites underlain by soft ground usually require ground improvement, and verifying the effectiveness of such treatment is an essential step for project safety and for closing the construction loop. Drawing on a quality-control program for deep-mixing columns installed along a natural-gas pipeline in the Pearl River Delta, this paper first compares the advantages and limitations of common geophysical methods and then selects the microtremor (passive surface-wave) technique and ground-penetrating radar (GPR) for comprehensive field trials at two work sites. After data processing, the detection results of the two methods on cement-mixed columns in soft soil are analysed in detail. The trials show that, in soft, water-rich ground, GPR is strongly affected by the high water table and by conductive or heterogeneous inclusions; its effective penetration is limited to 3–4 m, which is insufficient to image the column heads or to verify the designed improvement depth. By contrast, the microtremor method exhibits a high signal-to-noise ratio and can reliably probe to about 30 m depth. By establishing an untreated reference zone within the same site and comparing the apparent shear-wave (Vs) velocities of identical soil layers, stratification and anomalous zones can be identified. The results demonstrate that the microtremor technique is feasible and efficient for large-area quality assessment of deep-mixing improvement, and can serve as guidance for similar projects in the future.