Identification of Geomorphological Structures of Forest Areas Using Radar Images and Openness Analysis

The forest area of northern Iran is located in the Alborz structural zone. Because of active tectonics of the Alborz Zone, many morphotectonic structures have been developed in the forest area. Among these structures, we can mention the important faults that occurred in this region, such as the Caspian fault and the North Alborz fault, or numerous landslides. The study of geological and morphotectonic studies in forest areas is very difficult due to the presence of tree cover on the soil surface and the impediment to direct observation of the landform and the ground. Therefore, the use of conventional methods of geotechnical studies such as optical satellite image processing in these areas does not work. Compared to traditional methods, the use of radar data in ground-level studies is one of the relatively new approaches in the field of remote sensing science that takes advantage of more capabilities in this field. In addition to using radar data to determine the amount of displacement that occurred at the ground level due to various factors, this data can also be used to prepare a high-precision digital elevation model, a model that directly reflects bare surface properties and extracts very useful information, especially when the land is covered by forests and trees.

An openness technique expressing the degree of dominance or enclosure of a location on an irregular surface was developed by Yokoyama et al. (2002). This technique calculates an angular measure of the relationship between surface relief and horizontal distance. It uses the horizontal surface distance and elevation-related angle to compute the slope information of an irregular terrain surface at different positions, and the results can be used to identify the topographic features of the area. This method calculates the zenith and nadir angles at equally spaced locations in eight azimuth directions from the line of sight of the terrain. RRIM is a new 3D visualization approach proposed by Chiba et al. An RRIM is a multi-layered illumination-free image that can be used to simultaneously visualize topographic slopes, concavities, and convexities. The basic concept of an RRIM is to multiply three landform element layers: topographic slopes, positive openness, and negative openness. An RRIM is generated using an overlay of a red-colored slope map on the I-value map. The red color is used to describe the slope angle because it has been empirically demonstrated to provide the richest tone for human eyes. This overlay highlights the 3D topography on a single image, where the I-value performs an illumination role and the saturation of red describes the steepness of the topography.

By performing radar interferometry technique on ALOS/PALSAR images, a digital elevation model of 12 meters from the study area was prepared. The shaded relief maps obtained from different elevation models have been compared. Based on the results, the RRIM model of an area along the North Alborz Fault shows evidence of displacement caused by this fault. The fault wall of the North Alborz fault has been identified. The fault line is marked with yellow arrows. It shows a range of 30 km that has been displaced. In order to identify the landslides occurring in the study area, a 12 m digital elevation model and the RRIM method were used. Landslide areas with yellow arrows are shown. These landslides occur mostly in areas close to the main waterways and in areas with a higher slope. The location of the 3 areas identified by this method is on the landslide map of the country and is approved. The red dots are the landslide ranges and the blue circles are the landslide positions identified in this study that are completely in agreement with the landslides.

In forested areas, due to dense tree cover, the study of surface features and phenomena is limited. As a result, it is difficult to map topographically in these areas. But, radar images can be very helpful in such cases because they can capture data from under the cover of trees. The results of applying the interference method on the mentioned radar images led to the preparation of a digital elevation model of 12 meters above the ground in the study area. Since the common methods used in the display and analysis of geomorphology have shortcomings such as deformation of surface features, as a result of changes in the direction of lighting, in this study, the openness method and RRIM were used. These methods overcome the limitations of older methods and provide better capabilities. Field surveys conducted in the study area and adaptation of the identified landslides to the landslide location map of the country indicate the confirmation of the efficiency of the methods used in this study. Therefore, these methods can be used in similar areas.