Question

In regions of lateral conductivity contrasts, the time-independent separation between the subsurface apparent resistivity curves of transverse electric (TE) and transverse magnetic (TM) modes relative to each other arises mainly due to

• A. impedance phase change
• B. amplitude magnification
• C. local distortion of electric field
• D. variation in the frequency of the inducing magnetic field

Hint:

Think about static shift in MT: near-surface charge build-up distorts only the electric field, and this distortion does not change with frequency/period.

Answer 1

The Correct Option is C

The TE and TM apparent resistivity curves are built from two different combinations of the surface electric and magnetic field components, so anything that distorts the electric field alone will move the two curves apart.

In areas with lateral conductivity contrasts, small near-surface inhomogeneities (bodies much smaller than the skin depth of the sounding) become galvanically charged as current flows through them. The charge that piles up on their boundaries creates a secondary, essentially static electric field that adds to the regional field. This charge build-up is a DC (galvanic) effect - it depends only on the geometry and resistivity contrast of the near-surface body, not on how fast the inducing field is oscillating. Because of this, the distortion is present at every period equally, i.e. it is time-independent.

Since apparent resistivity scales as \((E/H)^2\), a constant multiplicative distortion of \(E\) shows up as a constant vertical shift of the apparent resistivity curve on a log-log plot, at all periods - the well-known "static shift". Because TE apparent resistivity uses the electric field parallel to strike and TM apparent resistivity uses the field perpendicular to strike, the same near-surface distortion shifts the two curves by different (but each frequency-independent) amounts, so the curves sit apart from one another by a fixed, time-independent separation.

Impedance phase change, amplitude magnification, and variation of the inducing frequency are all inductive, frequency-dependent phenomena - they would produce effects that change with period, not a fixed offset. The observed time-independent separation is therefore caused by local (galvanic) distortion of the electric field around near-surface conductivity heterogeneities.

Hence the correct answer is \(\boxed{\text{local distortion of electric field}}\), option (C).