حذف نوفه‌های هارمونیکی با بسامد پایه متغیر سیگنال MRS با روش مبتنی برلوپ مرجع در حوزه‌ زمان

نوع مقاله : مقاله پژوهشی

نویسندگان

1 گروه فیزیک، دانشگاه آزاد اسلامی، واحد تهران مرکزی، تهران، ایران.

2 گروه فیزیک زمین، مؤسسه ژئوفیزیک، دانشگاه تهران، تهران، ایران.

چکیده

روش سونداژ تشدید مغناطیسی (Magnetic Resonance Sounding) که به اختصار MRS نامیده می‌شود، به‌عنوان یک روش ژئوفیزیکی سطحی اطلاعات مناسبی درباره پارامترهای هیدروژئوفیزیکی (مانند محتوای آب و هدایت هیدرولیکی) لایه‌های آبخوان ارائه می‌دهد. با وجود کارایی بسیار این روش، سیگنال ثبت‌شده با این شیوه، به‌شدت تحت‌تأثیر نوفه‌های الکترومغناطیسی از جمله نوفه‌های هارمونیکی‌و نوفه‌های اسپایکی قرار می‌گیرد. در این مقاله برای آن که بتوان برآورد درستی از پارامترهای سیگنال MRS به‌دست آورد، روشی برای حذف نوفه‌های هارمونیکی در حوزه زمان ارائه می‌شود. سیگنال‌های مصنوعی به‌منظور بررسی تأثیر پارامترهای گوناگون بهینه فیلتر، برای روش‌های حذف رویدادهای اسپایکی و هارمونیکی شرح داده شده و ارزیابی می‌شوند.
مبحث جدیدی که در این مقاله دنبال می‌شود بررسی کارایی الگوریتم پیشنهادی در مواقعی است، که سیگنال هارمونیکی دارای فرکانس پایه متغیر با زمان باشد. پیش از به‌کاربردن الگوریتم حذف نوفه‌های هارمونیکی، می‌بایست سیگنال‌های اسپایکی حذف و یا تا اندازه ممکن کاهش داده‌ شوند. بنابراین نخست، یک الگوریتم پردازشی آماری بر اساس روش SD-ROM برای حذف نوفه‌های اسپایکی ارائه می‌شود و پس از آن، روشی برای حذف نوفه‌های هارمونیکی، با فرض بسامد پایه ثابت و متغیر با زمان، بر اساس لوپ مرجع راه دور و با به‌کارگیری توابع انتقال ارائه می‌شود. نتایج عددی به‌کارگیری الگوریتم‌های پردازشی پیشنهادی در حوزه زمان نشان می‌دهد که از طریق کاربست روش‌های یاد‌شده، سیگنال‌های اسپایکی و هارمونیکی تا اندازه چشم‌گیری حذف می‌شوند و بنابراین به برآورد مناسبی از پارامترهای سیگنال MRS (دامنه اولیه، زمان آسایش، فاز و بسامد سیگنال) می‌انجامد.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Harmonic Noise Cancellation with Varing Fundametal frequency of MRS Signal Using Time-Domain Remote Reference Loop Method

نویسندگان [English]

  • Adnan Sharafi 1
  • Mohammad Kazem Hafizi 2
  • Reza Ghanati 2
  • Hamid Reza Shirvani Mahdavi 1
1 Department of of Physics, Islamic Azad University, Central Tehran Branch, Tehran, Iran.
2 Department of Earth Physics, Institute of Geophysics, University of Tehran, Tehran, Iran.
چکیده [English]

Magnetic Resonance Sounding (MRS), as a surface geophysical method, provides good information about the hydro-geophysical parameters (such as water content and hydraulic conductivity) of aquifers. The main advantage of the MRS method compared to other geophysical methods is that the surface measurement of the MRS signal responds directly to the presence of water below the surface. Despite the high efficiency of this method, the recorded signal is strongly affected by electromagnetic noises, including spike noises and harmonic noises. The first generations of MRS instruments were single-channel instruments. In single-channel instruments, both magnetic resonance excitation and signal recording are done with a single loop, and it is necessary to use various forms of filtering to eliminate noise, in particular powerline harmonics.
Then a new generation of multichannel MRS instruments with multiple is was built, so that the main loop is still used for magnetic resonance excitation and signal recording. In addition, a number of reference loops, physically displaced from the main loop, measure only noise. Parts of the noise recorded by the reference loops can correlate with the noise in the main loop. With proper signal processing, the noise in the reference coils can be filtered to obtain a replica of the noise in the main coil and when this replica is subtracted from the signal recorded in the main loop, hence, the MRS signal remains noiseless. One of the current challenges of MRS signal processing is the existence of harmonic noise with the variable fundamental frequency. If the harmonic signal from a specific source has a fundamental frequency that varies with time, most of the proposed algorithms will not perform well in eliminating harmonic noise. Therefore, a new topic that is followed in this paper is to evaluate the performance of the proposed algorithm in cases where the harmonic signal has a fundamental frequency that varies with time.
In this paper, in order to obtain an accurate estimate of the parameters of the magnetic resonance sounding signal, a method for eliminating spike and then harmonic noise in the time domain is presented. Synthetic signals are contaminated with different electromagnetic noises to investigate the effect of different optimal filter parameters for spike and harmonic event elimination methods. Spike noise has a detrimental effect on the performance of the harmonic noise elimination algorithm. Hence, spike signals must be deleted or adjusted before applying the harmonic noise elimination algorithm. First, a statistical processing algorithm based on the signal-dependent rank-order mean (SD-ROM) filter for eliminating spike noise is presented and after deleting spike noise, a method for eliminating harmonic noise is assumed based on the fixed and variable fundamental frequency with time using remote reference loop. Numerical results of applying the proposed processing algorithms in the time domain show that by applying the mentioned methods, a considerable amount of spike and harmonic signals are removed leading to a good estimate of the parameters of the magnetic resonance sounding signal (i.e., initial amplitude, relaxation time, phase and frequency of the signal).

کلیدواژه‌ها [English]

  • Harmonic noises
  • Magnetic resonance sounding
  • Spiky noises
  • Transfer function
  • variable fundamental frequency

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فیزیک زمین و فضا
دوره 49، شماره 2
تاریخ انتشار الکترونیکی: 8 شهریور 1402
شهریور 1402
صفحه 293-311

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