برآورد مشخصات آبخوان شمال غرب قم به روش SNMR

نویسندگان

1 دانشجوی کارشناسی ارشد ژئوفیزیک، گروه فیزیک زمین، مؤسسة ژئوفیزیک، دانشگاه تهران، ایران

2 دانشیار، گروه فیزیک زمین، مؤسسة ژئوفیزیک، دانشگاه تهران، ایران

چکیده

روش تشدید مغناطیسی هسته‌ای سطحی یا Surface Nuclear Magnetic Resonance یا به اختصارSNMR یکی از جدیدترین تکنولوژی‌های برآورد خصوصیات هیدرولیکی خاک‌ها، آبخوان‌ها و مخازن زیرسطحی است. این روش معادل روش تصویرسازی تشدید مغناطیسی در پزشکی (روش MRI) یا روش طیف‌‌ نگاری تشدید مغناطیسی در صنایع دارویی و شیمیایی است. درواقع این روش جدیدترین تکنولوژی ژئوفیزیکی ارائه شده در چند دهه اخیر است. روش‌‌های متداول ژئوفیزیکی نظیر لرزه‌ای، ژئوالکتریکی و الکترومغناطیسی، تنها پس از تفسیرهای پیچیده قادرند شاخص‌هایی از خصوصیات هیدرولیکی زیرسطحی به‌دست دهند. حال آنکه روش SNMR قادر است آب آزاد درون حفره‌ها را به طور مستقیم تشخیص دهد و دو پارامترمهم زیرسطحی، یعنی تخلخل و تراوایی را مستقیماً به‌دست آورد. برآورد مشخصات آبخوان در دهه‌های اخیر یک رویکرد چند جانبه یا Multidisciplinary داشته و اغلب داده‌های مختلفی از روش‌های هیدروژئولوژیکی و ژئوفیزیکی به طور همزمان در مدلسازی‌ها مورد استفاده قرار می‌گیرند. در این مطالعه نیز برای مدلسازی خصوصیات هیدرولیکی زیرسطحی از داده‌های SNMR همراه با داده‌های سونداژ مقاومت ویژه الکتریکی به طور همزمان استفاده شده است. به طور کلی جهت به‌دست آوردن مدل SNMR بهینه از زیرسطح، اطلاع از ساختار مقاومت ویژه زیر سطحی کمک شایانی به وارون سازی دقیق‌تر داده‌هایSNMR می‌کند. قابلیت‌ها و محدودیت‌های این روش، با یک مطالعه موردی اکتشاف آب در استان قم بررسی شده است. آبخوان مورد بررسی یک مخزن با تخلخل ثانویه‌ی حاصل از شکستگی می‌باشد که با ترکیب روش سونداژزنی الکتریکی و SNMR، برخی مشخصات هیدروژئولوژیکی و هندسه آن بدست آمده است. همچنین محل حفاری برای استحصال آب پیشنهاد شده است.

کلیدواژه‌ها


عنوان مقاله [English]

North Western Qom, Iran Aquifer Characterization br SNMR

نویسندگان [English]

  • Bahman Abbassi 1
  • Mohammad Kazem Hafizi 2
چکیده [English]

The objective of this research is to represent applicability of Surface Nuclear Magnetic Resonance (SNMR) in aquifer characterization, based on SNMR and Electrical Resistivity Tomography data. SNMR is the only geophysical method that directly detects water. Two important parameters, porosity and permeability are available through inversion of initial amplitude and decay time constant versus pulse moments. Usually, electrical methods are concurrently applied along with SNMR surveys, making it possible to easily manipulate SNMR inversion. A case study is introduced to explain the efficiency of SNMR for aquifer characterizations.
Introduction: Surface Nuclear Magnetic Resonance (SNMR) is a new geophysical method currently developed for shallow investigations of aquifers. Compared to the other geophysical methods, SNMR is a water selective method. Therefore, hydraulic properties of media are achievable through SNMR investigations. Relative to classical electrical methods, SNMR still costs much. In order to reduce the expense of the survey, it would be better to perform a sufficient electrical tomography in the region, and then few SNMR soundings for acquiring aquifer properties.
Basically, in SNMR the response of excited protons is measured as relaxation times in the water content (directly linked to saturated effective porosity) and pore size distribution (linked to permeability). Hydrogen nuclei have a magnetic moment and in an undisturbed manner press about the ambient geomagnetic field vector (Legchenko, et al., 2002). The resonance frequency ( ) is known as Larmor frequency and is proportional to the strength of the Earth’s magnetic field :
(1)
Where, is the gyromagnetic ratio for hydrogen nuclei. An SNMR measurement is made by disturbing protons with a secondary magnetic field ( ) transmitted at the resonant frequency ( ? ) that causes proton spinning vector ( ) to simultaneously tip away from its equilibrium and rotate about at the resonant frequency. In SNMR, is applied as an alternating magnetic field with a circular or square loop as transmitter at the surface. The loop is energized by a pulse of an alternating current :
(2)
The degree to which M is tipped is dependent on the transmitter pulse moment q, which is the product of the transmitter current and the duration of the pulse excitation ( ).The voltage induced in the receiver loop after transmitting a certain pulse moment q is fitted with a function of the form:
(3)
Where = initial signal amplitude; = the decay time constant; = Larmor frequency as equation 1; = phase shift between the signal and the excitation current. In 1D distribution of the subsurface, the initial amplitude is a function of q:
(4)
The water content distribution is related to the sounding curve . The magnetic induction field of excitation is defined in the kernel function , which is linked with the loop configuration. The excitation field can be varied by changing the pulse moment q. An increase of q leads to an SNMR response from deeper regions.
The inverse problem to obtain the one-dimensional water content distribution can be solved in different ways, which are discussed in several SNMR publications. The sounding curve is available after fitting and extrapolating the envelopes of the response signals at the several pulse moments. Commonly a non-linear least square algorithm estimates the signal parameters , and . The two parameters obtained as a result of geophysical inversion are free water content, which is porosity in a saturated aquifer ( ), determined from E0, and permeability revealed from inversion (K).
Discussion and Conclusion: The study area is located north-west of Qom, Iran. Thirteen Vertical Electrical Soundings in three profiles and three Magnetic Resonance Soundings are performed over recent river deposits of the area. The studies revealed a fractured aquifer containing a low permeable shallow reservoir with high porosity and a fractured deep permeable one with low porosities.
Fractured aquifers, due to bad sorting of their sediments, have usually low porosities and high permabilities. An electrical tomography also shows a distinct fractured pattern in a low porosity, high permeable zone, with low amount of resistivity, which is an indication of clay accumulation in the crush zone.
Joint application of SNMR and electrical resistivity methods are important in direct characterization of aquifer parameters including porosity, permeability and electrical resistivity. It is possible to study the relationship between electrical resistivities and porosity/permeability of the aquifer. In this study, qualitative interpretation shows an inverse relationship between electrical resistivity and porosity of the aquifer on the one side, and permeability on the other. Another suggestible approach, which is currently under study, is to explore the complex relationships in a quantitative way, such as using nature inspired algorithms, like Neural Network Estimations.

کلیدواژه‌ها [English]

  • Aquifer
  • Permeability
  • Porosity
  • Surface Nuclear Magnetic Resonance