تخمین عمق، مکان و هندسه بی‌هنجاری‌های مغناطیسی به روش عددموج محلی بهبودیافته

نویسندگان

1 موسسه ژئوفیزیک دانشگاه تهران

2 مؤسسه ژئوفیزیک دانشگاه تهران، دانشیار

چکیده

لازمه تحلیل صحیح داده‌های مغناطیسی، یک تخمین عمق واقعی از منبع بی‌هنجاری مغناطیسی به‌منظور تعیین نقاط حفاری و رسیدن به هدف مغناطیسی می‌باشد. روش عدد موج محلی بهبودیافته بر پایه معادلات سیگنال تحلیلی شکل گرفته است و می‌تواند مکان افقی و عمق بی‌هنجاری را بدون نیاز به آگاهی از هندسه و خودپذیری مغناطیسی منبع مشخص کند. پس از مشخص شدن این کمیت‌ها تقریبی از ضریب ساختار نیز قابل تخمین است. در این تحقیق، کد این روش برای اعمال بر روی بی‌هنجاری‌های دوبعدی در محیط متلب نوشته شده و توانایی آن بر روی داده‌های مصنوعی بدون نوفه و همراه نوفه آزمایش شده است. در قسمت داده-های مصنوعی از دایکی با شیب 45 درجه و مغناطیدگی یک آمپر بر متر ، زاویه انحراف 10 درجه و زاویه میل 64 درجه استفاده شده است. روش عددموج محلی بهبودیافته بر روی این دایک، بدون حضور نوفه و همچنین آلوده به نوفه با دامنه‌های مختلف، اعمال شده است. در ادامه این روش بر روی داده‌های میدانی منطقه گل‌بلاغی واقع در شهرستان زنجان اعمال شده و جواب‌های آن با جواب‌های به دست آمده از نرم‌افزار مدل‌ویژن مقایسه گردیده است. برای این بررسی از یک پروفیل به طول 525 متر با فواصل نمونه‌برداری یک متر استفاده شده است. پارامترهای توده بی‌هنجاری که به‌وسیله این روش به‌دست آمده با نتایج حاصل از نرم‌افزار مدل‌ویژن مطابقت دارد. روش عدد موج محلی و کد متلب نوشته شده، می‌تواند ابزار توانمندی برای بررسی بی‌هنجاری-های دوبعدی باشد.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

Estimation of depth, location and structure index of magnetic anomalies by enhanced local wavenumber method

نویسندگان [English]

  • Ramin Ghasemiannia 1
  • Behrooz Oskooi 2
1
2 Institute of Geophysics, Associate Professor
چکیده [English]

A reliable analysis of magnetic data is the correct estimation of the causative sources to plan for drilling to achieve the targets. This paper presents enhanced local wave number (ELW) method for interpretation of the magnetic data. ELW method has been proposed during the previous decades and is based on analytic signal to estimate the location and depth of the anomalies without having any knowledge about the geometry and magnetic susceptibility of the source. Final equation in this technique, is based on the depth and position and is independent of the structural index. The solution of normal this equation is obtained by assigning ELW kx and kz (the local wave number with respect to x and z) for different values of x and heights of continuation, z within a window centred with the peak of the analytic signal amplitude. A problem of over determined unknown parameters can be solved through a standard technique, using the least squares approach, therefore, the Golub algorithm is used to solve a set of linear equations. The ELW technique requires computation of horizontal and vertical derivatives of the first and second orders. Due to this characteristic, any high frequency noise present in the data gets substantially enhanced, masking the response from a target. To restrict the high frequency response, a window function is designed on the basis of the maximum frequency computed from Agrawal and Lal (1972). After finding these quantities the method can approximate the structure index. Although, an appropriate Matlab code for the method is introduced and tested on two dimensional synthetic data before and after adding noises. There is a peak in the curves of analytic signal and kx of ELW and also a turning point in the curve of kz of ELW witch shows the position of anomaly. Existence of these features shows that final responses of ELW method are correct. Synthetic data produced from a dyke like body with dip, magnetization, declination, inclination, depth and thickness are 45º, 1( ), 90º, 64º, 10m and 15m respectively. The ELW method has had reasonable responses for noises with different amplitudes up to 20nT and for noises with amplitude more than 20nT, ELW method looses its efficiency. Then, the method is tested by applying on the real data of Golbelaghi area in Zanjan, and ok compared with the results obtained from Model vision software. To do this a 525m profile is used. At the end, the depth and structure index are obtained about 4m and 0.8, respectively, using ELW method and the depth is estimated about 4.4m using model vision software. It is worthy to note that the depth of anomaly has been reported 4.5m by drilling. The parameters obtained from the introduced method for the anomalies show that the enhanced local wavenumber method and its introduced Matlab code can be a powerful tool in the studies of local anomalies. Because this method is automatic and quick, it can be used for large data sets like vast area or airborne data. This method is used on airborne data of Damghan region in another paper.

کلیدواژه‌ها [English]

  • Analytic signal
  • Enhanced Local Wavenumber
  • Golbelaghi region
  • Zanjan
Agarwal, B. N. P. and Lal, T., 1972a, A generalized method of computing second derivative of gravity field, Geophysical Prospecting, 20, 385-394.
Agarwal, B. N. P. and Lal, T., 1972b, Calculation of the vertical gradient of the gravity field using the Fourier transform, Geophysical Prospecting, 20, 448-458.
Ansari, A., H. and Alamdar, K., 2010, 3-D depth and susceptibility estimation of magnetic anomalies using Local Wavenumber (LW) method, Iranian Journal of Science & Technology, Transaction B: Engineering, 34(B5), 567-575.
Bulent, O., Ertan, P. and Serafeddin, C., 2010, Interpretation of magnetic anomaly in the south of lake Sapanca using an Enhanced Local Wave number method, Journal of Engineering Science and Design, 1(2), 87-90.
Golub, G., 1965, Numerical methods for solving linear least squares  problems, Numerische Mathematik, 7, 206-216.
Keating, P. and Pilkington, M., 2004, Euler deconvolution of the analytic signal and its application to magnetic interpretation, Geophysical Prospecting, 52, 165-182.
Ku, C. C. and Sharp, J. A., 1983, Werner method for automated magnetic interpretation and its refinement using Marquardt inverse modeling, Geophysics, 48, 754-774.
Ma, G. Q., 2013, Improved local wavenumber methods in the interpretation of potential field data, Pure and Applied Geophysics, 170, 633-643.
MA, G. Q., DU, X. J. and LI, L. L., 2012, Interpretation of potential field tensor data using the tensor local wavenumber method and comparison with the conventional local wavenumber method, Chinese Journal of Geophysics, 55(4), 380-393.
Murthy, K. S. R. and Mishra, D. C., 1980, Fourier transform of the general expression for the magnetic anomaly due to a long horizontal cylinder, Geophysics, 45, 1091-1093.
Nabighian, M. N., 1972, The analytic signal of two-dimensional magnetic bodies with polygonal cross-section: its properties and use for automated anomaly interpretation, Geophysics, 37, 507-517.
Naudy, H., 1971, Automatic determination of depth on aeromagnetic profile, Geophysics, 36, 717-722.
Peters, L. J., 1949, The direct approach to magnetic interpretation and its practical application, Geophysics, 14, 290-320.
Reford, M. S., 1964, Magnetic anomalies over thin sheets, Geophysics, 29, 532-536.
Ridsdill-Smith, T. A. and Dentith, M. C., 1999, The wavelet transform in aeromagnetic processing, Geophysics, 64, 1003-1013.
Salem, A., Ravat, D., Smith, S. and Ushijima, K., 2005, Interpretation of magnetic data using an enhanced local wave number (ELW) method, Geophysics, 70, L7-L12.
Smith, R. S., Thurston, J. B., Dai, T. F. and Macleod, I. N., 1998, iSPI the improved source parameter imaging method, Geophysical Prospecting, 46, 141-151.
Thompson, D. T., 1982, EULDPH—a new technique for making computer assisted depth estimates from magnetic data, Geophysics, 47, 31-37.
Thurston, J. B. and Smith, R. S., 1997, Automatic conversion of magnetic data to depth, dip, and susceptibility contrast using the SPI method, Geophysics, 62, 807-813.
Thurston, J. B., Smith, R. S. and Guillon, J. C., 2002, A multimodel method for depth estimation from magnetic data, Geophysics, 67, 555-561.