A new approach to integrate passive seismic HVSR depth models in magnetotelluric (MT) 1D inversion to characterize the cover-basement interface.

Suriyaarachchi, Nuwan1,2 , Giraud, Jeremie1,2, Seille, Hoel3 , Jessell, Mark1,2, Lindsay, Mark1,2, Hennessy, Lachlan4, Ogarko, Vitaliy5

 1Centre of Exploration Targeting, School of Earth Sciences, University of Western Australia, 35 Stirling Highway, Perth  6009 WA, Australia; 2Mineral Exploration Cooperative Research Centre (MinEx CRC), School of Earth Sciences, University of Western Australia, 35 Stirling Highway, Perth 6009 WA, Australia; 3CSIRO Deep Earth Imaging FSP, Australian Resources Research Centre, 26 Dick Perry Avenue, Kensington 6151 WA, Australia; 4Anglo American, Group Discovery and Geosciences, 201 Charlotte Street, Brisbane 4000 QLD, Australia; 5International Centre for Radio Astronomy Research, University of Western Australia, 35 Stirling Highway, Perth 6009 WA, Australia

Electromagnetic methods are useful tools to understand cover-basement interface in many aspects. They could provide valuable information to exploration targeting, mineral prospectivity mapping and in some cases. They can also provide volume constraints for groundwater and energy resource estimates with a fraction of the cost of drilling. In this study, we use passive seismic Horizontal to Vertical Spectral Ratios (HSVR) to reduce the uncertainties of detecting cover-basement contact in Magnetotelluric (MT) depth-resistivity models. 

We invert MT apparent resistivity and phase using Occam’s inversion algorithm to obtain resistivity-depth model. Generally, Occam’s inversion produces the smoothest model that fits the observations (data) with a certain target misfit. But the smoothest model may not fully describe a realistic resistivity structure, such as thin resistivity layers and sharp resistivity contrasts (eg. impermeable basement), which is critical to identify the cover-basement transition and thus interface. For this study, we expect to control the MT depth-resistivity model smoothness (or roughness) without producing unrealistic models. For that, we brought forward the importance of detection of cover-basement interface prior to 1D MT inversion. Our approach is to use a prior interface-depth model to support interface prediction by adjusting the model roughness values in Occam’s 1D inversion. This prior interface-depth model is generated from co-located or reasonably placed passive seismic-HVSR models, which give robust results to detect possible interfaces up to 1500m depths.

To test our approach, we created a synthetic homogenous two-layer cover-basement case with 20 Ω m cover resistivity, 1000 Ω m basement resistivity and a cover-basement interface at 1000m depth. The 1D forward response (from 104 to 10-3 Hz) was calculated and 5% random noise was added to the synthetic data. Firstly, a cover-basement interface depth interval is estimated using synthetic HVSR model data. Then we performed 1D MT inversions. The inversions are regularized using a series of roughness penalty values ranging from 0 (no penalty) to 1.00 (maximum penalty) within this hypothetical cover-basement transition depth interval.  Thinner MT depth mesh was used at the HVSR derived cover-basement interface region to identify accurate resistivity variations.

Preliminary results on the synthetic test show that the depth roughness penalty values between 0.05-0.25 reveal sharp resistivity contrasts consistent with the true depth to cover basement interface. We tested a spectrum of cover-basement depths and cover basement resistivity scenarios. We expect to test the procedure further for multi-layer synthetic cases and analysed the results statistically to validate our approach. Additional priori information, such as borehole data and stratigraphy data will be used to improve the HVSR-based constrain for the 1D MT (real data) inversion.

We acknowledge the support of the MinEx CRC and the Loop: Enabling Stochastic 3D Geological Modelling (LP170100985) consortia. The work has been supported by the Mineral Exploration Cooperative Research Centre whose activities are funded by the Australian Government’s Cooperative Research Centre Programme. This is MinEx CRC Document 2020/xxx.


Biography

Nuwan is a PhD candidate working with Mark Jessell, Jeremie Giraud, Mark Lindsay, Hoel Seille, Lachlan Hennessy and Vitaliy Ogarko. He is working to Integrate magnetotelluric data and Passive seismic data to characterize the cover-basement interface. This project is a collaboration between MInexCRC and Loop consortium

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