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Induced polarization of volcanic rocks. 8. The case of intrusive igneous rocks

Geophysical Journal International - Fri, 03/21/2025 - 00:00

SummaryIn the previous papers of this series, we have developed an in-depth analysis of the low-frequency complex conductivity response of volcanic (extrusive) rocks. We showed that the alteration of these rocks plays a key-role in determining their induced polarization properties, especially regarding the formation of smectite in response to the thermo-activated alteration of the volcanic glasses. We also considered the effects associated with the presence of magnetite and pyrite. In the present paper, we look at the induced polarization properties of igneous rocks like granites and granitoids. Usually, the alteration path of these rocks leads to the formation of kaolinite, a clay mineral with a much lower Cation Exchange Capacity (CEC) than smectite. Thirty-three core samples from 3 sites in France are saturated with NaCl solutions at 3 salinities (pore water conductivity of 0.1, 1.0, and 10 S m−1, 25°C) and their complex conductivity spectra are measured in the frequency range 0.01 Hz-45 kHz. As observed for volcanic rocks, the surface conductivity, normalized chargeability, and quadrature conductivity depend strongly on the CEC of the rock, which is independently measured with the cobalt-hexamine method. The (intrinsic) formation factor follows an Archie's type relationship with the connected porosity with a porosity (cementation) exponent of m = 1.70 ± 0.02, much smaller than for volcanic extrusive rocks. Like for volcanic rocks, a dynamic Stern layer model can be used to illustrate the behavior associated with the clay-minerals (mostly kaolinite). A field investigation is conducted in the Vosges (France) using a deep time-domain induced polarization survey reaching at a depth of investigation ∼400 m. We show how the electrical conductivity and the normalized chargeability can be used to image the water content and CEC of the granitic substratum. The conductivity of granite is found to be dominated by surface conductivity rather than by bulk conductivity and therefore Archie's law cannot be used as a conductivity equation to interpret field data as commonly done in ElectroMagnetic (EM) surveys.

On the variability of sources in ambient seismic noise source inversion

Geophysical Journal International - Fri, 03/21/2025 - 00:00

SummaryWe present a method for ambient noise cross-correlation modelling and source inversion, which accounts for spatio-spectral variability in noise source distributions. It is based on numerical wavefield simulations in 2-D acoustic media. The source power spectral density is parameterized by a sum of a small number of spatial source distributions, each with a corresponding frequency spectrum held fixed during the inversion. Algorithmically, this is an extension of our previous work which assumed spatially homogeneous source spectra. In this paper, we use it to study the impact of incorrectly estimating source spectra from observed data. This is done using synthetic tests involving sources with closely spaced frequency spectra. The tests demonstrate that when the spatial variability of sources is either partially or wholly unaccounted for, the recovery of true source locations is compromised.

Not just trees: Most of the carbon sequestered on land is stored in soil and water, study finds

Phys.org: Earth science - Thu, 03/20/2025 - 20:34

Recent studies have shown that carbon stocks in terrestrial ecosystems are increasing, mitigating around 30% of the CO2 emissions linked to human activities.

Scientists in Antarctica: Why they're there and what they've found

Phys.org: Earth science - Thu, 03/20/2025 - 20:20

A media storm blew up in mid-March 2025 when a researcher at South Africa's isolated Sanae IV base in Antarctica accused one of its nine team members of becoming violent.

Hydroacoustic imaging shows human traces on the seabed may be visible for decades

Phys.org: Earth science - Thu, 03/20/2025 - 19:45

Human use of the marine environment of the Baltic Sea is visible on the seabed and can significantly impact the habitat of many marine species. In a new study, researchers from Kiel University have used high-resolution hydroacoustic methods to investigate the seafloor in the southwestern Baltic Sea for human traces.