Integrated interpretation of helicopter and ground-based geophysical data recorded within the Okavango Delta, Botswana

Joel E. Podgorski, Alan G. Green, Thomas Kalscheuer, Wolfgang K.H. Kinzelbach, Heinrich Horstmeyer, Hanruedi Maurer, Lasse Rabenstein, Joseph Doetsch, Esben Auken, Tiyapo Ngwisanyi, Gomotsang Tshoso, Bashali Charles Jaba, Onkgopotse Ntibinyane, Kebabonye Laletsang

Research output: Contribution to journalArticleResearchpeer-review

17 Citations (Scopus)

Abstract

Integration of information from the following sources has been used to produce a much better constrained and more complete four-unit geological/hydrological model of the Okavango Delta than previously available: (i) a 3D resistivity model determined from helicopter time-domain electromagnetic (HTEM) data recorded across most of the delta, (ii) 2D models and images derived from ground-based electrical resistance tomographic, transient electromagnetic, and high resolution seismic reflection/refraction tomographic data acquired at four selected sites in western and north-central regions of the delta, and (iii) geological details extracted from boreholes in northeastern and southeastern parts of the delta. The upper heterogeneous unit is the modern delta, which comprises extensive dry and freshwater-saturated sand and lesser amounts of clay and salt. It is characterized by moderate to high electrical resistivities and very low to low P-wave velocities. Except for images of several buried abandoned river channels, it is non-reflective. The laterally extensive underlying unit of low resistivities, low P-wave velocity, and subhorizontal reflectors very likely contains saline-water-saturated sands and clays deposited in the huge Paleo Lake Makgadikgadi (PLM), which once covered a 90,000km2 area that encompassed the delta, Lake Ngami, the Mababe Depression, and the Makgadikgadi Basin. Examples of PLM sediments are intersected in many boreholes. Low permeability clay within the PLM unit seems to be a barrier to the downward flow of the saline water. Below the PLM unit, freshwater-saturated sand of the Paleo Okavango Megafan (POM) unit is distinguished by moderate to high resistivities, low P-wave velocity, and numerous subhorizontal reflectors. The POM unit is interpreted to be the remnants of a megafan based on the arcuate nature of its front and the semi-conical shape of its upper surface in the HTEM resistivity model. Moderate to high resistivity subhorizontal layers are consistent with this interpretation. The deepest unit is the basement with very high resistivity, high P-wave velocity, and low or complex reflectivity. The interface between the POM unit and basement is a prominent seismic reflector.

Original languageEnglish
Pages (from-to)52-67
Number of pages16
JournalJournal of Applied Geophysics
Volume114
DOIs
Publication statusPublished - 1 Mar 2015
Externally publishedYes

Keywords

  • ERT
  • Hydrogeophysics
  • Okavango Delta
  • Seismic reflection
  • Seismic refraction
  • TEM

Programme Area

  • Programme Area 2: Water Resources

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