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Comparative study between filtering and inversion of VLF-EM profile data

دراسة مقارنة بين الترشيح والنمذجه التراجعية لبيانات القطاعات الكهرومغناطيسية ذاتالترددات المنخفضة جدا"

Abstract

This paper discusses the practical use of filtering and inversion in VLF-EM data processing and interpretation. The advantages and disadvantages of both mentioned techniques were outlined to avoid the misleading interpretation of such data in some case studies. Much concern is taken to show the interval distance effect upon the correct depth identification of the anomalous body by either inversion or filtering. The methodology of the study is going through proposing an initial model, generating the synthetic VLF-EM data of the model by means of forward modeling, filtering (Karous–Hjelt filter) and inversion (Inv2DVLF software) of the synthetic data and comparing between the results of both methods and the initial model. The study reached to (1) Karous–Hjelt filter provides misleading depths for his limited depth of resolution and cannot provide estimates of deep targets if the profile is too small, whereas inversion provides exact results, particularly in case of shallow anomalous target; (2) crossover between in-phase and out-of-phase data could resulted from a small shallow conductive target or a large deep one; (3) selection of a reasonable environmental resistivity has an important impact on the inversion process; and (4) the numerical reflection resulted during VLF-EM data inversion could lead to an erroneous interpretation.

Abstract

تناقش هذه الورقة البحثية الاستخدامات العملية للترشيح والنمذجه التراجعية في معالجة وتفسير بيانات القطاعات الكهرومغناطيسية ذات التردد ات المنخفضة جدا". تم تحديد مزايا وعيوب كل من التقنيات المذكورة لتجنب التفسير الخاطئ لمثل هذه البيانات في بعض الحالات الدراسية. ان تأثير المسافة الفاصلة بين محطات القياس على تحديد العمق الصحيح للشذة قد أخذت بأهتمام كبير سواء بواسطة الترشيح أوالنمذجه التراجعية. تمر منهجية الدراسة بأقتراح النموذج الأولي ، وتوليد البيانات الكهرومغناطيسية الاصطناعية فى التردد المنخفض جدا عن طريق النمذجة التقدمية ، والترشيح (Karous - Hjelt) والنمذجه التراجعية (Inv2DVLF برمجيات) للبيانات الاصطناعية ، ومقارنة نتائج كلا الطريقتين مع النموذج الأولي. توصلت الدراسة إلى (1) النمذجه التراجعية أفضل من الترشيح لتحديد العمق (2) الشذات الضحلة الصغيرة والشذات العميقة الكبيرة ينتجان نفس التقاطع فى البيانات الكهرومغناطيسية (3) اختيار قيمة مناسبة لمقاومة الوسط له تأثير كبير فى عملية النمذجه التراجعية (4) الأنعكاس الأصطناعى للبيانات الكهرومغناطيسية الناتج من النمذجه التراجعية يمكن تفسيرة بطريقة خاطئة على انة شذة كهرومغناطيسية.

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Acknowledgement

The main author is indebted to the Fundação para a Ciência e Tecnologia (Portugal) for his support through the post-doctor fellowship (SFRH\BPD\29971/2006). This work was partly developed in the scope of the scientific cooperation agreement between the CGUL and the NRIAG.

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Correspondence to Mohamed Ahmed Khalil.

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Khalil, M.A., Santos, F.M. Comparative study between filtering and inversion of VLF-EM profile data. Arab J Geosci 4, 309–317 (2011). https://doi.org/10.1007/s12517-010-0168-4

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  • DOI: https://doi.org/10.1007/s12517-010-0168-4

Keywords

  • VLF-EM
  • Filtering
  • Inversion
  • Electromagnetic field
  • Electric field component