## 1 Introduction  
The composite electromagnetic (EM) scattering characteristics of targets above random rough surfaces have attracted a lot of attention in recent years due to the development of computational electromagnetic (CEM) for a variety of applications such as target recognition [1; 2; 3], environment detection [4], microwave remote sensing [5; 6; 7], spectra estimation [8; 9], and so on. The classical algorithms have been greatly enhanced by combining them with acceleration approaches. In [4], for example, the method of moments (MOM) is used with the sparse matrix canonical grid approach to speed up matrix column multiplications. In [10; 11; 12], the finite difference time domain approach (FDTD) is utilized with the perfectly matched layer absorbing boundary to compute the composite EM scattering from varied dimensions of rough surfaces. Ref. [13] employs the domain decomposition methodology in conjunction with the finite element method (FEM) to solve anisotropic EM scattering issues. With the focus of research shifting to electrically large targets and enormous rough surfaces, high-frequency approximation approaches with reasonable equivalent models are progressively appearing. For example, ref. [14] presented a hybrid method combining physical optics (PO) with the four-path model (FPM). Refs. [15; 16] modified the FPM to broaden the scope of application. However, because of the intrinsic limits of PO, these approaches may not be suitable for targets with cavities or dihedral structures. The shooting and bouncing rays (SBR) method, which combines the benefits of geometrical optics (GO) and PO, was presented in [17] to overcome these challenges. The SBR method is now extensively used and developed in many ways. To estimate the composite EM scattering from an electrically large and complex platform, ref. [18] used the SBR method hybridized with MOM. Ray tracing's temporal effectiveness is increased with OpenGL [19] and the Kd-tree structure [20]. The near-field EM scattering from the frequency domain [21] and time domain [22; 23] was extended to the SBR method's application scope. The composite EM scattering from multiple targets above the sea surface was studied using the SBR method and the two scale models (TSM) hybrid model in [24]. However, the processed rough surface in all of the literature mentioned above is either sea or ground surface, which only comprises one background surface type. The dihedral structures are only present in targets. In the actual radar detection situation, it is feasible for both different types of rough surfaces to coexist in a single radar beam. Ref. [25] implements a hybrid iterative solution to simulate composite rough surface EM scattering in a circular water zone encased within a sizable land rough surface. Ref. [26] calculates the EM scattering of an island's composite rough surface in deep water. Refs. [27; 28; 29] researched coastal environment composite scattering using various hybrid techniques. A type of environment with a natural semi-enclosed cavity structure exists, although little study has been done on it. The canyon/valley environment serves as a good example. Aircraft and missiles can evade radar detection by flying in a canyon or valley. At the same time, the defense side needs to research the composite scattering characteristics in such scenarios to improve the defense ability. Our study team proposed several methods for the issue. Ref. [30] studied the composite EM scattering characteristics from a target above a valley environment based on the SBR method and ray-density normalization concept. However, the valley, which is with an arched bottom, was not a typical type. In addition, the scattering from the rough surface was calculated coarsely. Traditional SBR method was used to calculate the properties of EM scattering in mountainous regions without taking targets into account in [31].