کاهش به قطب دیفرانسیلی نقشه مغناطیس هوایی ایران

نوع مقاله: مقاله تحقیقی‌ (پژوهشی‌)

نویسندگان

1 دانشیار، دانشگاه تحصیلات تکمیلی علوم پایه زنجان

2 دانشگاه تحصیلات تکمیلی علوم پایه زنجان

چکیده

نقشه بی‌هنجاری مغناطیسی هوایی ایران (صالح، 1387) با روش کاهش به قطب دیفرانسیلی ارکانی‌حامد (1988) به قطب کاهیده شد. در روش کاهش به قطب دیفرانسیلی، بی‌هنجاری‌های مغناطیسی با در نظر گرفتن تغییرات جهت بردار میدان مغناطیسی در منطقه مورد بررسی به قطب کاهش می‌یابند. عملگر کاهش به قطب دیفرانسیلی، بی‌هنجاری‌های مغناطیسی را در همه عرض‌های جغرافیایی روی منابع تولید کننده منتقل می‌کند و در نتیجه باعث سهولت عرضه تفسیر زمین‌شناسی می‌شود. روش کاهش به قطب دیفرانسیلی ابتدا روی مدل‌های آزمایشی در عرض‌های گوناگون جغرافیایی ایران اجرا و پس از کسب نتیجه مطلوب، روی داده‌‌های مغناطیس هوایی ایران اِعمال شد. مقایسه نقشه کاهش به قطب به‌دست آمده از ساختارها و پهنه‌های زمین‌شناسی ایران و همچنین گسل‌ها و کمربندهای آتشفشانی، تطابق زیادی بین مرزهای مغناطیسی و زمین‌شناسی نشان می‌دهد. به‌‌کارگیری این لایه اطلاعاتی جدید امکان تفسیر بهتری از بی‌هنجاری‌‌‌های مغناطیسی گستره ایران را فراهم می‌کند.

کلیدواژه‌ها


عنوان مقاله [English]

Differential reduction to pole of aeromagnetic data of Iran

نویسندگان [English]

  • Vahid Ghobadian
  • Abdolreza Ghods 1
  • Mahnaz Rezaeian 2
  • Vahid Teknik
چکیده [English]

The aeromagnetic data of Iran was surveyed by Aeroservice Company (Houston, Texas) under auspicious of Geological Survey of Iran during 1974-1977. The survey was done with a two-engine airplane using a cesium vapor magnetometer with a sensitivity of 0.02 gama. The data was collected along flight lines with average line spacing of 7.5 km over 62 flight blocks mostly with a constant barometric flight height. A barometric elevation range of 1000-3600 m was used (Yousefi and Feridberg, 1977) which translated to about a 500-1000 m height from the Earth surface.
    Saleh (2006) used the raw data of the 62 blocks and produced an aeromagnetic composite map of Iran. The raw data had already been corrected for daily variations of the geomagnetic field. To produce a composite grid of the aeromagnetic map of Iran from the original rather raw data, Saleh (2006) first implemented the detailed leveling and micro-leveling procedures for each block. The fully leveled blocks were stitched initially to eight geologically coherent larger blocks and finally the larger blocks were stitched. The final 1 km by 1 km grid of the aeromagnetic map of Iran was produced from the combined data set using a bidirectional interpolation scheme.
    Magnetic anomalies in Iran latitudes do not correlate directly with their corresponding causative magnetic bodies because the direction of the geomagnetic field and magnetization are not normal to the Earth surface. The asymmetry between the magnetic anomalies and their causative magnetic bodies increases from north to south of Iran. The deviation could reach to tens of kilometers for aeromagnetic anomalies located in the south of Iran. For geological interpretation purposes, it is very desirable to derive aeromagnetic anomalies that are positioned over their causative magnetic bodies, quite similar to that expected from gravity anomalies, or an induced magnetic body located in the North Pole.  Baranov and Naudy (1964) introduced a procedure called reduction-to-the-pole (the standard RTP method) which converts magnetic anomalies in mid-latitudes to that produced by the magnetic bodies having vertical magnetization, and lying at the north geomagnetic pole.
    The standard RTP is only valid for regions in which the direction of the geomagnetic field is almost constant. Therefore, the standard RTP method is not applicable to produce an RTP map of the aeromagnetic field of Iran. The RTP methods which allow for variations of geomagnetic field are called differential reduction-to-pole methods (DRTP).
    In this study, the revised aeromagnetic map of Iran (Saleh, 2008) was reduced to the pole considering the variations of inclination and declination of the geomagnetic field over Iran. The new aeromagnetic map was produced using the differential reduction to the pole (DRTP) method developed by Arkani-Hamed (1988). The DRTP operator shifts aeromagnetic anomalies in different geographical latitudes to the top of their causative sources, thus facilitating an easier geological interpretation of the magnetic anomalies.  We first applied the DRTP method to the synthetic magnetic anomalies of three identical spheres lying in north, centre and south of Iran assuming induced magnetization for the spheres. We found that the DRTP method did not show any instability; thus it is appropriate to use in Iran. The DRTP aeromagnetic map of Iran showed significantly a better correlation between the magnetic anomalies and the boundaries of the main tectono-stratigraphic units of Iran (e.g. Alborz, Jazmurain Depression), volcanoes (e.g. SabalanMountain) and major active faults (e.g. Tabriz and Doruneh faults).

کلیدواژه‌ها [English]

  • Aeromagentic field of Iran
  • reduction to the pole (RTP)
  • differential-reduction-to-the-pole (DRTP)
  • geomagnetic inclination
  • induced magnetization
  • geological interpretation

صالح، ر.، 1387، بازپردازش نقشه آنومالی‌‌ مغناطیسی هوابرد ایران، رساله کارشناسی ارشد: دانشگاه تحصیلات تکمیلی علوم پایه زنجان، زنجان، ایران.

Arkani-Hamed J., 1988, Differential reduction-to-the-pole of regional magnetic anomalies: Geophysics, 53, 1592–1600.

Arkani-Hamed J.,and W. E. S. Urquhart, 1988, Reduction to the pole of the North American magnetic anomalies: Geophys. J. Int., 55, 211-225.

Arkani-Hamed J., and G. Celetti, 1989, Effects of thermal remanent magnetization of the magnetic anomalies of intrusives: J. Geophysical Res., 94, 7364-7378.

Arkani-Hamed J.,2007, Differential reduction to the pole: Revisited: Geophys. J. Int., 72, L13–L20, 10.

Barnov V., 1957, A new method for interpretation of aeromagnetic maps: Pesudo-gravimetric anomalies: Geophysics, 22, 359-383.

Baranov V., and H. Naudy, 1964, Numerical calculation of the formula of reduction to the magnetic pole: Geophys. J. Int., 29, 67–79.

Blakely, R. J., and A. Cox., 1973, Identification of short polarity events by transforming marine magnetic profiles to the pole: J. Geophys. J. Int., Res., 77, 4339-4349.

Blakely, R. J., 1996, Potential Theory in Gravity and Magnetic Application: Cambridge University Press, 2nd edition, pp 464.

Bott, M. H. P., and A. Ingles, 1972, Matrix method for joint interpretation of two-dimensional gravity and magnetic anomalies with application to the Iceland-Faeroe Ridge: Geophys. J. Roy. Astr. Soc., 30, 55-67.

Cooper, J. R. C., 2005, Differential reduction to the pole of magnetic anomalies using Taylor's series  expansion: Computer and Geosciences, 45, 359-378.

Emami, M. H., Sadeghi, M. M., Omrani, S. J., 1993, Magmatic Map of Iran 1:1,000,000: Geological Survey of Iran, internal report.

Grant, F. S., and G. F. West, 1965, Interpretation Theory in Applied Geophysics: McGraw-Hill Book Co., pp 583.

Sengor, A. M. C., and Natal’in, B. A., 1996. Paleotectonics of Asia: Fragments of a synthesis, in The Tectonic Evolution of Asia, edited by A. Yin and T. M. Harrison, 486–640, Cambridge Univ. Press, New York.

Shahabpour, J., 2009, Tectonic implications of the geochemical data from the Makran igneous rocks in Iran: Island Arc, 19, 676-689.

Silva, J. B. C., 1986, Reduction to the pole as an inverse problem and its application to low-latitude anomalies: Geophys. J. Int., 51, 369-382.

Von Frese. R. R. B., Hmre., W. J., and Braile, L. W., 1981, Spherical earth gravity and magnetic anomaly analysis by equivalent point source  inversion: Earth and Plan. Sci. Lett., 53, 69-83.

Yousefi E., and Friedberg, J. L., 1977, Aeromagnetic map of Iran: Quaderangle NO. F5., Tehran, Geological Survey of Iran.