SIMPLE ANALYSIS FOR MAGNETOTELLURIC DEPTH RESOLUTION USING EQUIVALENCE CONCEPT IN 1D

• Main Author: Ekaputri - Nim: 12313015 , Disha
• Format: Final Project
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/21805
• Institution: Institut Teknologi Bandung

The magnetotelluric (MT) method is capable of providing an estimate of the earth's resistivity variation up to a relatively great depth depending on the frequency (or period) of the recorded electromagnetic (EM) fields. The MT investigation depth is well-known and expressed by the skin depth formula, although it is only a rough approximation derived for a homogeneous medium. On the other hand, MT resolution in depth is much more complex even for a simple medium, e.g. 1D. It is widely accepted that MT resolution is decreased with depth due to diffusive character of the EM fields. We studied the resolving capability of MT by using a simple synthetic 1D model where a thin conductive layer is embedded in a homogeneous more resistive medium. In such a case, we consider that the second thin layer can be resolved only if it has a conductance (conductivity-thickness product) at least the same as the overburden layer. For an overburden with a higher resistivity, a deeper thin layer can be resolved, which in agreement with the skin depth principle. Within the equivalence concept, a very much thinner layer can produce the same conductance with the similar model response, i.e. it can be considered as a possible model. However, such situation is less likely from the commonly accepted practical point of view. Although we cannot formally formulate the lower thickness threshold, this exercise can exemplify the limitation of MT method in resolving a thin layer at depth. In Indonesia, there are studies using MT with results beyond its real capability, i.e. models with a very thin layer at depth from unconstrained inversion which are considered as real. A deeper comprehension on the fundamental concept of the method and on inversion modeling is needed to avoid such misunderstanding or even "misconduct”.