SOES6002: Modelling in Environmental and Earth System Science

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

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

4. Recap from yesterday

5. Boundary conditions • Apart from the general form of the governing equations, we haven’t gone deeply into the mathematics • But two useful boundary conditions are useful: • Eparallel is continuous • Jnormal is continuous

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

7. Poynting Vectors

8. Sensitivity • Poynting vectors show local direction of transport of energy • Another way of investigating this is to look at sensitivity • 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?

9. Sensitivity pattern

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

11. 4 models • 50 ohm-m half space • 200 ohm-m half space • 1 km thick layer, 50 ohm-m overlying 200 ohm-m half space • 1 km thick layer, 200 ohm-m overlying 50 ohm-m half space

12. 1 layer and 2 layer models

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

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

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

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

17. The skin depth

18. Skin depth (m) in various materials

19. 3 models at 8 Hz

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

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

22. 2 models, 2 frequencies

23. Thin layer model • 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 • It does have an evident effect on the data, but the effect depends on frequency

24. Effect of shallow water

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