uu.seUppsala University Publications
Change search
ReferencesLink to record
Permanent link

Direct link
Joint inversions of three types of electromagnetic data explicitly constrained by seismic observations: results from the central Okavango Delta, Botswana
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
Show others and affiliations
2015 (English)In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 202, no 3, 1429-1452 p.Article in journal (Refereed) Published
Abstract [en]

The Okavango Delta of northern Botswana is one of the world’s largest inland deltas or megafans. To obtain information on the character of sediments and basement depths, audio-magnetotelluric (AMT), controlled-source audiomagnetotelluric (CSAMT) and central-loop transient electromagnetic (TEM) data were collected on the largest island within the delta. The data were inverted individually and jointly for 1-D models of electric resistivity. Distortion effects in the AMT and CSAMT data were accounted for by including galvanic distortion tensors as free parameters in the inversions. By employing Marquardt–Levenberg inversion, we found that a 3-layer model comprising a resistive layer overlying sequentially a conductive layer and a deeper resistive layer was sufficient to explain all of the electromagnetic data. However, the top of the basal resistive layer from electromagnetic-only inversions was much shallower than the well-determined basement depth observed in high-quality seismic reflection images and seismic refraction velocity tomograms. To resolve this discrepancy, we jointly inverted the electromagnetic data for 4-layer models by including seismic depths to an interface between sedimentary units and to basement as explicit a priori constraints. We have also estimated the interconnected porosities, clay contents and pore-fluid resistivities of the sedimentary units from their electrical resistivities and seismic P-wave velocities using appropriate petrophysical models. In the interpretation of our preferred model, a shallow ∼40 m thick freshwater sandy aquifer with 85–100 Ohmm resistivity, 10–32 per cent interconnected porosity and <13 per cent clay content overlies a 105–115 m thick conductive sequence of clay and intercalated salt-water-saturated sands with 15–20 Ohmm total resistivity, 1−27 per cent interconnected porosity and 15–60 per cent clay content. A third ∼60 m thick sandy layer with 40–50 Ohmm resistivity, 10–33 per cent interconnected porosity and <15 per cent clay content is underlain by the basement with 3200–4000 Ohmm total resistivity. According to an interpretation of helicopter TEM data that cover the entire Okavango Delta and borehole logs, the second and third layers may represent lacustrine sediments from Paleo Lake Makgadikgadi and a moderately resistive freshwater aquifer comprising sediments of the recently proposed Paleo Okavango Megafan, respectively.

Place, publisher, year, edition, pages
2015. Vol. 202, no 3, 1429-1452 p.
Keyword [vi]
Inverse theory; Electrical properties; Magnetotellurics; Hydrogeophysics; Acoustic properties; Africa.
National Category
Research subject
Geophysics with specialization in Solid Earth Physics
URN: urn:nbn:se:uu:diva-257713DOI: 10.1093/gji/ggv184ISI: 000360625500001OAI: oai:DiVA.org:uu-257713DiVA: diva2:840372
Available from: 2015-07-08 Created: 2015-07-08 Last updated: 2016-01-12Bibliographically approved

Open Access in DiVA

No full text

Other links

Publisher's full text

Search in DiVA

By author/editor
Kalscheuer, ThomasWagner, Frederic
By organisation
In the same journal
Geophysical Journal International

Search outside of DiVA

GoogleGoogle Scholar
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

Altmetric score

Total: 271 hits
ReferencesLink to record
Permanent link

Direct link