## 4 Laboratory Data Experiment  
To demonstrate the applicability of the proposed method, a laboratory experiment was conducted using a sand tank model at Guilin University of Technology, as shown in Figure 10. The sandpit contained five abnormal bodies including two empty pipes on the left, cuboid empty abnormal bodies in the center, and two solid pipes on the right. The sandpit was divided into three levels, with dielectric constants of 3.0 and 2.4 for the first and second levels, respectively, while the bottom level represented the sandpit boundary. The schematic diagram of the laboratory model is shown in Figure 11. The parameters of abnormal are listed in Table 4. For data acquisition, we used a 900 MHz antenna of a GSSI SIR-4000 radar in point measurement mode; the sampling points per trace and the time window were set as 512 and 10 ns, respectively. The channel spacing was set to 0.004 m, and the length of the survey line was 2.5 m, and a total of 625 traces were collected, as shown by the B-scan in Figure 12.  
Figure 8: Forward recording after noise (residual record). Figure 9: (a) LSRTM-TV imaging result of noised record; (b) LSRTM-MTV imaging result of noised record. \begin{table}
\begin{tabular}{c c c} \hline \hline  & LSRTM-TV & LSRTM-MTV \\ \hline MSE & 0.0667 & 0.0572 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of MSE with noised record between LSRTM-TV and LSRTM-MTV. Figure 11: The schematic diagram of laboratory model. Figure 12: The original B-scan profile of laboratory data. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Number** & **Material** & **Size (cm)** & **Burial Depth (cm)** & **Dielectric Constant** \\ \hline
1 & Pipes (solid) & 5 (radius) & 22 & 4.0 \\
2 & Pipes (solid) & 10 (radius) & 50 & 4.0 \\
3 & Cuboid abnormal (empty) & 20 \(\times\) 10 & 15 & 1.0 \\
4 & Pipes (empty) & 5 (radius) & 20 & 1.0 \\
5 & Pipes (empty) & 10 (radius) & 50 & 1.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The dielectric constants of the laboratory model. Figure 10: Physical laboratory model. In the original B-scan profile, the reflected intensity was conjointly determined by two factors: the burial depth and the difference in relative permittivity between the anomalies and the background medium. Essentially, as the burial depth increases, the attenuation of electromagnetic wave energy becomes more pronounced, leading to a decrease in the energy of the reflected wave. Simultaneously, a higher contrast in relative permittivity between the anomalies and the background leads to a greater disparity in wave impedance, consequently amplifying the reflected energy. As observed in Figure 12, various nuances become apparent when considering these factors. Anomaly 1, despite its relatively shallow burial depth, had a minimal wave impedance difference, resulting in weaker reflected wave energy. Anomaly 2, buried deeper and with a smaller wave impedance difference, resulted in the weakest reflected wave energy. In stark contrast, anomalies 3 and 4, with shallower burial depths and greater wave impedance differences, exhibited the strongest energy in the reflected waves. Anomaly 5, although buried deeper, outperformed anomaly 2 in reflected wave energy owing to a more pronounced wave impedance difference. To accurately locate and depict the subsurface anomalies, the proposed LSRTM-MTV method, introduced in this paper, was utilized to process the collected data. Considering the stratification of the sandpits, a two-layer uniform initial model was established to optimize the imaging results and compare them with conventional LSRTM method, as shown in Figure 13. While both methods were able to generally identify the presence of five subsurface anomalies, the conventional LSRTM method exhibited significant background clutter. This clutter complicated the precise localization and morphological characterization of these anomalies, as shown in Figure 13a. In contrast, LSRTM-MTV remarkably mitigated such background clutter, leading to a far more accurate representation of the anomalies.