## 6 Discussion  
### Analysis of Reflection Wave Pattern Offset on the Top of Sediment Inside the Cavity  
In the forward simulation results of the five influencing factors, a consistent downward offset phenomenon is observed in the reflection peak position of the sediment's top surface in the cavity. From the propagation pathway perspective, seismic waves originate from ground blast sources and travel to underground salt cavities. These waves propagate through the cavity roof, into the brine in the cavity, and subsequently through the residue. Unlike reflections from the cavity roof, the wave reflected from the residue's top surface undergoes an additional brine penetration process. This results in a change in medium  
Figure 23: Congruent diagrams of waveform seismic record profiles and cavity models with different horizontal connection channel height. The blue dashed line is employed to enhance the visualization of the relative positions of the reflected waveforms originating from the horizontal channels across the various models. Figure 24: Relationship between horizontal channel height and seismic reflection offset at the bottom of the channel. along the path, leading to the downward offset feature of the reflected wave peak. Regarding the propagation properties of sound waves in various media, the velocity of seismic waves alters when they enter from one medium to another. This phenomenon, known as wave dispersion, can cause changes during propagation, particularly at interfaces between different media, where dispersion becomes more evident. The propagation speed of seismic waves is inversely proportional to the medium's density and directly proportional to its modulus (as described by Equations (1) and (2)). Furthermore, pores in the medium disrupt the continuity of wave propagation, reducing its speed [48]. From the seismic reflection properties perspective, amplitude, frequency, and continuity are closely related to the lithology above and below the interface. A larger spacing between adjacent reflection interfaces results in a longer time interval between the reflection waves generated at these interfaces. In the context of a brine cavity with residue at the bottom, the presence of three different media -strata, brine, and residue-complicates the seismic reflections. Particularly, the low density and modulus of the brine significantly slow down the propagation velocity of seismic waves from the residue surface. This slowing effect causes an increase in the time interval as the waves travel through the brine layer. As a result, it appears as if the distance between the residue surface and the cavity top surface is greater than it actually is, leading to a noticeable downward shift of the reflection wave peak on the seismic waveform. This \"beading\" phenomenon is common observation in the seismic analysis of carbonate karst reservoirs [49; 50; 51], where the complex geological structure and varying media properties contribute to intricate seismic reflections. \[V_{p}=\sqrt{\left(k+4/3\mu\right)/\rho} \tag{1}\]  
\[V_{s}=\sqrt{\mu/\rho} \tag{2}\]  
where \(V_{p}\) and \(V_{s}\) are the longitudinal wave and transverse wave velocities, respectively. \(k\) is the medium volume modulus. \(\mu\) is the medium shear modulus. \(\rho\) is the medium density. Reference [45] conducted a seismic forward simulation to investigate the reflection characteristics of horizontal salt cavities filled exclusively with brine, without any insoluble residues. The findings revealed a downward trend at the cavity's bottom, mirroring the reflection patterns observed at the top surface of the residue in this study. This confirms a phenomenon that when a low-density medium is present above the reflection interface, a downward offset in the seismic waveform's reflection characteristic is inevitable. This also provides direction for further research on the correlation between medium density and the magnitude of downward offset.