پردازش و تفسیر دادهـهای رادار نفوذی به زمین (GPR)، به منظور تعیین ضخامت و توپوگرافی بستر یخچال علم کوه

نویسندگان

1 دانشجوی کارشناسی، گروه ژئوفیزیک، دانشکدۀ مهندسی معدن، نفت و ژئوفیزیک، دانشگاه صنعتی شاهرود، شاهرود، ایران

2 دانشیار بخش مهندس معدن، دانشگاه شاهرود

3 استادیار بخش مهندسی معدن، دانشگاه شاهرود

4 دکتری هواشناسی ماهواره، موسسه تحقیقات آب- پژوهشکده منابع آب تهران

چکیده

تعیین ضخامت یخچال‌های کوهستانی مانند علم‌کوه باتوجه به شرایط آب و هوایی بسیار سرد، ارتفاع زیاد یخچال، خطر سقوط بهمن و توپوگرافی زیاد کار بسیار دشواری است. در این پژوهش به‌دلیل شفاف بودن یخ برای امواج الکترومغناطیسی، غیرمخرب، سریع و دقیق بودن روش رادار نفوذی به زمین (GPR)، برای تعیین ضخامت و توپوگرافی بستر یخچال علم‌کوه از این روش استفاده شده است. برداشت داده‌های GPR با استفاده از آنتن 25 مگاهرتز به روش دور افت مشترک (Common offset mode) و فاصله‌ی بین فرستنده و گیرنده 6 متر انجام شده است. تقریباً کل یخچال‌های غربی منطقه مورد مطالعه (یخچال‌های علم‌چال، تخت‌چال و تخت سلیمان)، توسط روش GPR برداشت شده است. جهت اختصار فقط داده‌های سه پروفیل(a، b و c) واقع در یخچال علم‌چال، مورد پردازش و تفسیر قرار گرفته‌اند. بعد از پردازش‌های مناسب، بازتاب‌های بستر یخچال آشکارسازی شده و با قرار دادن سرعت مناسب موج الکترومغناطیسی در یخ (16/0 متر بر نانوثانیه)، ضخامت متغیر 50 تا 94 متر برای لایه یخ در زیر پروفیل‌های مورد بررسی تخمین زده شده است. نشانگرهای دامنه، فاز و فرکانس لحظه‌ای برای آشکارسازی و تفسیر بهتر لایه‌های زیر سطحی در مقاطع GPR، استخراج شده است. نشانگر‌ دامنه لحظه‌ای، بازتاب‌های بستر یخچال، قلوه سنگ‌های یخچالی و واریزه‌های داخل یخ را به‌خوبی تفکیک کرده‌ است. در نهایت در مقاطع عمقی پروفیل‌های مورد بررسی، مشاهده شد که توپوگرافی بستر یخچال کپی نسبتاً دقیقی از توپوگرافی سطح یخچال است.

کلیدواژه‌ها


عنوان مقاله [English]

Processing and interpretation of ground penetrating radar (GPR) data in order to determine thickness and basement topography of Alamkooh glacier

نویسندگان [English]

  • saeed parnow 1
  • Abolghasem Kamkar-Rouhani 2
  • Alireza Arab-Amiri 3
  • Neamat Karimi 4
1
2 School of Mining, Petroleum, and Geophysics
3 School of Mining, Petroleum, and Geophysics
4 Water Research Institute, Water Resources Research
چکیده [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.

کلیدواژه‌ها [English]

  • Alamkooh glacier
  • Ice thickness
  • Ground penetrating radar (GPR)
  • Topography of glacier basement
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