SOES6002: Modelling in Environmental and Earth System Science CSEM Lecture 3 Martin Sinha School of Ocean & Earth Science University of Southampton

Recap and plan: l Yesterday: basic principles of CSEM sounding. Modelling for uniform seafloor resistivity l Today: sensitivity patterns, boundary conditions vertical variations in resistivity, CSEM sounding

Recap from yesterday

Boundary conditions l Apart from the general form of the governing equations, we haven’t gone deeply into the mathematics l But two useful boundary conditions are useful: l E parallel is continuous l J normal is continuous

Transport of energy l As we see, different resistivities for a uniform seafloor lead to different patterns of amplitude vs range l What happens if seafloor resistivity varies? l First step – what is the path taken by the flow of energy?

Poynting Vectors

Sensitivity l Poynting vectors show local direction of transport of energy l Another way of investigating this is to look at sensitivity l For a given transmitter position and receiver position, if we make a small change to the resistivity of a small element of the sea floor, how much does this affect the measured amplitude?

Sensitivity pattern

l Sensitivity follows a broadly U- shaped region between source and receiver l At longer source receiver offsets, sensitivity extends deeper beneath the seafloor – so ‘averages’ over a greater depth range l Hence we can perform a ‘sounding’ study by increasing the offset

4 models l 50 ohm-m half space l 200 ohm-m half space l 1 km thick layer, 50 ohm-m overlying 200 ohm-m half space l 1 km thick layer, 200 ohm-m overlying 50 ohm-m half space

1 layer and 2 layer models

Behaviour: l At long offsets, the slope of the curve corresponds to the effect of the deeper layer l Amplitudes are shifted up and down by the effect of the shallower layer l At short offsets, would see only the shallow layer effect

SOES6002: Modelling in Environmental and Earth System Science CSEM Lecture 4 Martin Sinha School of Ocean & Earth Science University of Southampton

Lecture 4 l The importance of frequency l Can we detect isolated, thin, conductive layers? l The air wave problem

What about frequency? l Our choice of frequency depends on skin depth l We need to choose f so that skin depth is comparable to our scale of investigation l But higher f means shorter skin depths, so high frequencies intrinsically see less deep than low frequencies

The skin depth

Skin depth (m) in various materials

3 models at 8 Hz

Frequency issues l Higher frequencies have better resolution l But they also have poorer penetration depth l So we always face a trade-off between these two l In real surveys, it’s often useful to collect data at multiple frequencies

Thin layers l Can we detect, e.g., the presence of a thin conductive layer (for example a melt lens) within the sea bed? l There’s clearly going to be a resolution problem – diffusive signal propagation is not necessarily a good way of finding thin layers

2 models, 2 frequencies

Thin layer model l Model consists of a 50 ohm m half- space, with a 100 m thick conductive layer (2 ohm-m) embedded in it at a depth of 1 km l It does have an evident effect on the data, but the effect depends on frequency

Effect of shallow water

The ‘Air wave’ interaction l In deep water, very little of the signal reaches the sea surface – so the surface has little effect on signal propagation l In shallow water, the surface does have an effect l The ‘Air Wave’ – propagation up, along and down again – can be a problem