Controlled Source Electromagnetic (CSEM) Survey for Seabed Mineral

Exploration GUANREN WANGMSci Geophysics

With special thanks to Professor Tim Minshull

SOES6030 ADVANCED INDEPENDENT RESEARCH PROJECT

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Blue Mining Project: break through solutions for sustainable deep-sea mining

Objective: Using 1D & 2D synthetic modelling to determine the optimal acquisition parameters for a deep-towed CSEM survey to detect eSMS deposits.

DASI

Image (above right & left) from Eric Attias’ (University of Southampton) project poster.

Image (above middle) after Rona (2008) and the http://www.bluemining.eu/ website.

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Target Area: Trans-Atlantic Geotraverse (TAG) Hydrothermal Field 26°N, 45°W

It is estimated that TAG hosts over 1x106 tonnes of SMS deposit, comparable to quantities found in volcanogenic massive sulphide sites on land (Humphries et al, 1995).

23/05/2016 AAPGPlot from topographic codes provided by Dr.Romina Gehrmann.

Method: 1D forward modelling

4-layer SMS model (Sa) 3-layer background model (Ra)

The aim of 1D modelling is to determine which tow-height, offset distance and frequency range combine to give the strongest electric field amplitude for different thicknesses of the sulphide and sediment layer.

𝒅𝒊𝒇𝒇 %=𝑺𝒂−𝑹𝒂

𝑺𝒂×𝟏𝟎𝟎%

eSMS/

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Method: 2D forward modelling

In-line tow configuration.

20 DASI (TX) and Vulcan (RX) positioned on either side of the midpoint of the SMS mound structure.

2D eSMS model 2D background model

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Target mounds 2m gridded bathymetry of the study area showing the location of the active TAG and the target eSMS mounds.

Plot after topographic codes provided by Dr. Romina Gehrmann using plot3 on Matlab. 23/05/2016 AAPG

1D Results: tow-height variation

• The magnitude of the threshold of detectability is assumed to be 10% this means for all frequencies, any offset distance which gives a % difference < -10% should not be used in the CSEM survey.

• The offset distances for the largest % differences increases with tow-height for all frequencies.

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1D results: combining thickness and resistivity variations

• For a thin (1 m) SMS layer, with low resistivity towed 100 m above the seafloor, we cannot distinguish the SMS layer from the basalt background.

• The amplitude contours for the 4 layer and 3 layer model overlap for all frequencies and offset distances.

2D resultsAmplitude decay and phase change at a fixed frequency for 20 receivers positioned across the profile.

1 Hz 2 Hz

5 Hz 10 Hz

Summary

• 5-0.5 Hz are the transmission frequencies that should be used in the CSEM survey. More beneficial for the detection of thicker and deeply buried SMS deposit.

• The largest phase divergence between the two models indicate either the presence of an exceptionally conductive zone or the thickest conductive layer within the mound.

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Increases the % difference (amplitude anomaly) for all offset distances and transmission frequencies.

Low DASI-Vulcan tow-heights SMS layer: thicker + more conductive

1D modelling summary

• Higher tow heights increase the minimum offset distance that can detect the SMS deposit.

2D modelling summary

Future work

• % diff plots for the 2D SMS % background model.

• 1D & 2D inversions to test the reliability of 1D & 2D forward modelling, i.e. whether forward modelling results can output the forward modelling parameters through inversion.

• Proceed to 3D forward modelling for SMS models of increased complexity.

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Thank-you & I welcome your questions

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1D results: sediment thickness Changing the more resistive sediment layer has minimal effect on the output result, especially for low frequencies.

Thin sediment coverage

References citedKey. K., 2009, ‘1D inversion of multicomponent, multifreqeuncy marine CSEM data: Methodology and synthetic studies for resolving thin resistive layers’, Geophysics, 74(2), p. F9-F20, doi: 10.1190/1.3058434

Rona. P. A., 2008, ‘The changing vision of marine minerals’, Ore Geology Review, 33, p. 618-666

Website used: http://www.bluemining.eu/

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Appendix : Skin depth • Inductive techniques (CSEM) rely on both geometries and frequencies

to determine depth. Inductive, plane wave EM fields will attenuate as they propagate away from the source in a uniform conductive medium.

• Where

• Signal amplitude decrease exponentially as the attenuation behaviour for amplitude decay is defined by, known as the skin depth. The skin depth is the distance at which the field strength is reduced by a factor of 1/e or a shift of 1 radian length of the signal.