عنوان مقاله [English]
Determination of the thickness of a mountains glacier like Alamkooh is very difficult due to its extremely cold climate conditions and high elevation, risk of falling large snow segments or balls and coarse topography of the glacier. In this research, due to the transparency of ice for electromagnetic (EM) waves, and also, that ground penetrating radar (GPR) method is non-destructive, fast and accurate, this method has been used for determination of thickness and basement topography of Alamkooh glacier. The GPR data acquisition has been made by 25 MHZ transmitter antenna using common offset method, and transmitter and receiver spacing or separation of 6 meters. Almost all of the glaciers in the west of the study area (including Alamchal, Takhtechal and Takhtesoleiman) have been surveyed by the GPR method. In this research, only the GPR data from 6 survey lines of a, b and c in Alamchal glacier have been processed and interpreted. The processing step has been made using static correction, signal saturation correction filter, gain functions, f-k migration filter, sequential average filter and topography correction in order to obtain a clear picture from the subsurface layers. After applying these processing methods on the acquired data, the EM waves reflections from the interfaces of different layers including the reflections from the glacier basement have been detected, and by assigning a suitable EM wave velocity in the ice (0.16 m/ns), the varying thickness of 50-94 m for the ice layer laid under the survey lines has been estimated. In this research, the debris sediments inside the ice, trough or bowl-shaped structures in the ice and parabolic reflections indicating the presence of boulders inside the ice, have also been detected. Considering the similarity between GPR and reflection seismic methods, we can use seismic processing methods to process GPR data, GPR attributes comprising of instantaneous amplitude, phase and frequency attributes have been extracted from the GPR data for better detection and interpretation of the subsurface layers. The instantaneous amplitude attribute has well distinguished the EM waves reflections from the interfaces of different layers including the EM reflections from the glacier basement, boulders and sediment debris inside the ice. Furthermore GPR data are demonstrated as a two-dimensional (2-D) time section, in which vertical axis is the two-way time of electromagnetic wave, and the horizontal axis is the location of the middle point between the transmitter and receiver, or the distance of this point from the starting of the survey line. The topography of the ground surface generally causes a mess in the GPR data, but the data presentation and interpretation is based on the flatness of the ground surface. To remove the undesired mess in the data due to the topography and putting the events on their real locations, it is necessary to apply topography correction on the data. The results obtained from this research work indicate that if relatively coarse topography exists in the survey area, applying topography correction on the data is needed. Finally, after investigating GPR depth sections, it has been observed that the topography of the glacier basement is a relatively accurate copy of the topography of the glacier surface.