Analysis of induced electromagnetic signals from elementary geometries in the subsurface

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Analysis of induced electromagnetic signals from
elementary geometries in the subsurface
Applied Geophysics Potential Field Methods, May
12th 2005

• Alejandra Reynaldos, Christina Schell,
• Corinne Bachmann, Laura Alisic Tonie Davidsen

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(No Transcript)
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Introduction

• Necessary to recognize shapes in EM signals to
  delineate geometries in subsurface
• No standard shapes due to different methods and
  layouts
• Case study of some signals / geometries in FD and
  TD

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Sphere in Frequency Domain

• HLEM, frequency domain. T R L
• 5 setups with sphere at increasing depth (L/8,
  L/4, L/2, L, 2L)
• Perfectly symmetric with shoulders on 3 (4) of
  the responses
• Below L you can not see the sphere
• Difficult to determine the exact shape

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Sphere in Frequency Domain
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FDEM System over Dipping Sheets

• Dip-Angle fixed-transmitter system over dipping
  sheets
• Upper picture dipping sheet, lower vertical sheet
  with varying depth
• Real Crossover over top of sheet, false crossover
  from bottom of sheet or resistive structures
• Variance with depth perfectly asymmetric

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FDEM System over Dipping Sheets
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Two Semi-Infinite Vertical Sheets

• Multiple conductors
• Frequency domain by means of VLF
• 3 configurations of spacing
• Curves show wiggles around midpoint
• caused by mutual coupling influence
• false crossover
• Influence disappears with distance

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Two Semi-Infinite Vertical Sheets
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Time Domain EM

• Main difference with FDEM two measure-ments
  available, early and late stage
• Early stage profile is only dependent on the size
  and shape of the conductor
• Late stage profile can be used for estimating the
  conductivity
• Some examples of vertical component signal for a
  number of simple shapes

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TDEM profiles (normalized)
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Discussion Conclusions

• Different methods and layouts cause different
  responses for same geometry
• A signal can be explained with different
  geometries
• Thorough study of EM results is necessary to come
  to the right conclusions in a research