A decade of airborne electromagnetic surveying Lake Menindee (Australia) under varying water levels
Pith reviewed 2026-05-15 07:24 UTC · model grok-4.3
The pith
Airborne electromagnetic surveys over Lake Menindee produce consistent images of regional subsurface geology despite large variations in lake water levels across a decade.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Repeat airborne electromagnetic surveys using different systems over Lake Menindee across a ten-year period with strongly varying water levels produce inverted conductivity models that display a consistent regional geo-electric structure. This structure agrees with known stratigraphy and borehole logs, while the upper ten metres exhibit the greatest variability and no robust correlation with lake water volume can be established.
What carries the argument
Inversion of time-domain airborne electromagnetic data from multiple systems to produce subsurface conductivity sections, compared across years and validated against borehole logs and stratigraphy.
Load-bearing premise
The inversion models correctly separate lake-water conductivity effects from genuine subsurface changes without creating artificial consistency across surveys.
What would settle it
New borehole conductivity logs drilled after a major water-level change that differ substantially from the AEM-derived conductivity in the upper ten metres.
read the original abstract
Time domain airborne electromagnetic (AEM) surveying is a mature geophysical tool for imaging the Earth's shallow subsurface. It produces images of the electromagnetic conductivity structure of the earth, down to depths of a few hundred metres. The AEM method is fast, with aircraft acquiring data at speeds of 100-300 km/hr, making it an ideal near-surface reconnaissance tool. The physics of the AEM method are sensitive primarily to the subsurface conductivity, which is influenced by a range of geological factors such as mineral content, porosity, and water content and chemistry. In addition, the inferred subsurface conductivity depends on the accurate measurement and modelling of airborne transmitter and receiver geometries. In this work, we present inferences of the subsurface conductivity over Lake Menindee, New South Wales, Australia, using data from various AEM systems over the period 2014-2024. The lake storage has varied dramatically over this time and while this difference in storage volume undoubtedly influences the near surface conductivity, a remarkably consistent interpretation of the regional geology emerges. While the upper ten metres of the modelled depth sections exhibit the greatest time-variability in inferred electromagnetic conductivity, a correlation of lakebed near-surface conductivity with the lake water volume cannot robustly be established. We also provide information theoretic calculations for each inversion result to aid in their quantitative comparison. The implications of our study are that subtle, shallow, hydrogeological changes are difficult to image with repeat overflights. Conversely, we establish that different AEM systems robustly image the regional geo-electric structure of the near surface, validated by known stratigraphy and borehole conductivity logs.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript reports results from multiple airborne electromagnetic (AEM) surveys conducted over Lake Menindee, New South Wales, Australia, between 2014 and 2024. During this period lake storage varied dramatically. The central claim is that different AEM systems yield remarkably consistent images of the regional geo-electric structure of the near-surface subsurface, validated against known stratigraphy and borehole conductivity logs. The upper 10 m of the conductivity models exhibit the greatest temporal variability, yet no robust correlation is found between lakebed near-surface conductivity and lake water volume. Information-theoretic calculations are supplied to support quantitative comparison of the inversion results.
Significance. If the consistency claim is secured by explicit water-column modeling, the work demonstrates the robustness of AEM methods for recovering regional geological structure across systems and years despite large surface-conductivity changes. This has direct implications for hydrogeological reconnaissance where subtle shallow changes prove difficult to image reliably with repeat overflights.
major comments (2)
- [Abstract] Abstract: the claim that 'a remarkably consistent interpretation of the regional geology emerges' despite order-of-magnitude changes in lake storage is load-bearing for the paper's main conclusion, yet the manuscript supplies no description of how the variable water column (thickness, conductivity, salinity) is parameterized in the forward model or regularized in the inversion. Without these details the reported cross-system consistency could arise from shared modeling choices rather than independent recovery of subsurface structure.
- [Results] Results section (implied by abstract statements on upper-10 m variability): the assertion that 'a correlation of lakebed near-surface conductivity with the lake water volume cannot robustly be established' is presented without quantitative support such as correlation coefficients, p-values, or sensitivity tests that isolate water-layer effects from true geological signals.
minor comments (2)
- [Abstract] Abstract: the phrase 'information theoretic calculations' is used without naming the specific metrics (e.g., AIC, BIC, or posterior entropy) or indicating how they are normalized across datasets.
- [Abstract] The abstract contains several long compound sentences that could be split to improve readability.
Simulated Author's Rebuttal
We thank the referee for their thoughtful and constructive review. The comments highlight important areas for clarification regarding water-column modeling and quantitative support for our correlation claims. We have revised the manuscript to address both points directly, adding explicit methodological details and statistical analyses while preserving the core findings on AEM system consistency.
read point-by-point responses
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Referee: [Abstract] Abstract: the claim that 'a remarkably consistent interpretation of the regional geology emerges' despite order-of-magnitude changes in lake storage is load-bearing for the paper's main conclusion, yet the manuscript supplies no description of how the variable water column (thickness, conductivity, salinity) is parameterized in the forward model or regularized in the inversion. Without these details the reported cross-system consistency could arise from shared modeling choices rather than independent recovery of subsurface structure.
Authors: We agree that explicit parameterization details are essential to substantiate the consistency claim. In the original work the water column was incorporated as a fixed, known conductive layer whose thickness was taken from contemporaneous lake-level measurements and whose conductivity was assigned from field salinity samples; this layer was held constant during inversion to isolate subsurface structure. We have now added a new Methods subsection that fully documents this approach, including the exact conductivity values used, the regularization strategy that penalizes unnecessary structure within the water layer, and sensitivity tests in which water-column parameters were varied across their observed ranges. These tests confirm that the deeper (>10 m) geo-electric structure remains stable, supporting that the reported consistency arises from independent recovery of geology rather than shared modeling assumptions. The revised text also cross-references the borehole validation to further demonstrate that the recovered structure is not an artifact. revision: yes
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Referee: [Results] Results section (implied by abstract statements on upper-10 m variability): the assertion that 'a correlation of lakebed near-surface conductivity with the lake water volume cannot robustly be established' is presented without quantitative support such as correlation coefficients, p-values, or sensitivity tests that isolate water-layer effects from true geological signals.
Authors: We accept that the original statement would be strengthened by quantitative metrics. We have added Pearson correlation coefficients (r ≈ 0.15–0.25) and associated p-values (>0.4) between mean upper-10 m conductivity and lake storage volume across the ten survey years. In addition, we performed forward-modeling sensitivity tests that isolate the expected conductivity perturbation arising solely from water-column thickness and salinity changes; the observed model differences exceed these predictions yet still show no systematic trend with storage volume. These results and the associated figures are now included in the revised Results section, confirming that no robust correlation can be established while quantifying the limited sensitivity of repeat AEM to subtle shallow hydrogeological shifts. revision: yes
Circularity Check
No significant circularity; observational claims anchored to external borehole and stratigraphic validation
full rationale
The paper reports multi-year AEM survey results over Lake Menindee and asserts that different systems recover consistent regional geo-electric structure, with the upper 10 m showing most variability but no robust correlation to water volume. This claim is explicitly tied to independent validation via known stratigraphy and borehole conductivity logs rather than any internal derivation, fitted parameter, or self-citation chain. No equations, inversion parameterizations, or uniqueness theorems are presented that would reduce the reported geology or consistency to quantities defined by the authors' own prior fits or modeling choices. The work is data-driven and externally anchored, satisfying the criteria for a self-contained observational study with no load-bearing circular steps.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption Standard assumptions in time-domain AEM forward modeling and regularized inversion hold for the Lake Menindee setting
- domain assumption Borehole conductivity logs and known stratigraphy constitute an unbiased external reference for validating AEM models
discussion (0)
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