Analysis procedure and equipment for deep geoelectrical soundings in noisy areas

Alfano, L.; Carrara, E.; Pascale, G. De; Rapolla, A.; Roberti, N.

doi:10.1016/0375-6505(82)90033-5

Cited by 8 publications

(3 citation statements)

References 2 publications

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“…Inset: geological sketch map of the western Bohemia-Vogtland area and the Cheb Basin near the German-Czech border in Central Europe, modified from Flechsig et al (2008), Dahm et al (2013), and Bussert et al (2017). mented by Alfano (1974) and Alfano et al (1982). Because of the logistical effort of large-scale ERT, just a few experiments with exploration lines up to approximately 20 km are documented.…”

Section: Introductionmentioning

confidence: 99%

Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an International Continental Drilling Program (ICDP) monitoring site to image fluid-related structures

Nickschick¹,

Flechsig²,

Mrlina³

et al. 2019

Solid Earth

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Abstract. The Cheb Basin, a region of ongoing swarm earthquake activity in the western Czech Republic, is characterized by intense carbon dioxide degassing along two known fault zones – the N–S-striking Počatky–Plesná fault zone (PPZ) and the NW–SE-striking Mariánské Lázně fault zone (MLF). The fluid pathways for the ascending CO2 of mantle origin are one of the subjects of the International Continental Scientific Drilling Program (ICDP) project “Drilling the Eger Rift” in which several geophysical surveys are currently being carried out in this area to image the topmost hundreds of meters to assess the structural situation, as existing boreholes are not sufficiently deep to characterize it. As electrical resistivity is a sensitive parameter to the presence of conductive rock fractions as liquid fluids, clay minerals, and also metallic components, a large-scale dipole–dipole experiment using a special type of electric resistivity tomography (ERT) was carried out in June 2017 in order to image fluid-relevant structures. We used permanently placed data loggers for voltage measurements in conjunction with moving high-power current sources to generate sufficiently strong signals that could be detected all along the 6.5 km long profile with 100 and 150 m dipole spacings. After extensive processing of time series for voltage and current using a selective stacking approach, the pseudo-section is inverted, which results in a resistivity model that allows for reliable interpretations depths of up than 1000 m. The subsurface resistivity image reveals the deposition and transition of the overlying Neogene Vildštejn and Cypris formations, but it also shows a very conductive basement of phyllites and granites that can be attributed to high salinity or rock alteration by these fluids in the tectonically stressed basement. Distinct, narrow pathways for CO2 ascent are not observed with this kind of setup, which hints at wide degassing structures over several kilometers within the crust instead. We also observed gravity and GPS data along this profile in order to constrain ERT results. A gravity anomaly of ca. −9 mGal marks the deepest part of the Cheb Basin where the ERT profile indicates a large accumulation of conductive rocks, indicating a very deep weathering or alteration of the phyllitic basement due to the ascent of magmatic fluids such as CO2. We propose a conceptual model in which certain lithologic layers act as caps for the ascending fluids based on stratigraphic records and our results from this experiment, providing a basis for future drillings in the area aimed at studying and monitoring fluids.

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Section: Introductionmentioning

confidence: 99%

Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an International Continental Drilling Program (ICDP) monitoring site to image fluid-related structures

Nickschick¹,

Flechsig²,

Mrlina³

et al. 2019

Solid Earth

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“…First theoretical considerations and practical tests for deep electrical sounding with dipole-dipole arrays are documented by Alfano (1974); Alfano et al (1982). Because of the logistical effort of large-scale ERT, just a few experiments with exploration 10 lines up to 20km are documented.…”

mentioning

confidence: 99%

Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an ICDP monitoring drill site to image fluid-related structures

Nickschick

Flechsig

Mrlina

et al. 2019

Preprint

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Abstract. The Cheb Basin, a region of ongoing swarm earthquake activity in the western Czech Republic, is characterized by intense carbon dioxide degassing along two known fault zones – the N-S-striking Počatky-Plesná fault zone (PPZ) and the NW-SE-striking Mariánské Lázně fault zone (MLF). The fluid pathways for the ascending CO2 of mantle origin are subject of an International Continental Scientific Drilling Program (ICDP) project in which several geophysical surveys are currently carried out to image the near-surface geologic situation, as existing boreholes are not sufficiently deep to characterize the structures. As electrical resistivity is a sensitive parameter to the presence of low-resistivity rock fractions as liquid fluids, clay minerals and also metallic components, a large-scale dipole-dipole experiment using a special type of electric resistivity tomography (ERT) was carried out in June 2017 in order to image fluid-relevant structures. We used static remote-controlled data loggers in conjunction with high-power current sources for generating sufficiently strong signals that could be detected all along the 6.5 km long profile with 100 m and 150 m dipole spacings. Extensive processing of time series and apparent resistivity data lead to a full pseudosection and allowing interpretation depths of more than 1000 m. The subsurface resistivity image reveals the deposition and transition of the overlying Neogene Vildštejn and Cypris formations, but also shows a very conductive basement of phyllites and granites that can be attributed to high salinization or rock alteration by these fluids in the tectonically stressed basement. Distinct, narrow pathways for CO2 ascent are not observed with this kind of setup which hints at wide degassing structures over several kilometers within the crust instead. We also observed gravity/GPS data along this profile in order to constrain ERT results. Gravity clearly shows the deepest part of the Cheb Basin along the profile, its limitation by MLF at NE end, but also shallower basement with an assumed basic intrusion in SW part of profile. We propose a conceptual model in which certain lithological layers act as caps for the ascending fluids, based on stratigraphic records and our results from this experiment, providing a basis for future drills in the area aimed at studying and monitoring fluids.

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“…In Italy during the last fifteen years, much attention has been devoted to this problem occurring in geothermal projects for which dipolar geoelectrical surveys were proposed. Low-power light generators were utilized and techniques for singling out a low-amplitude periodic signal from noise-corrupted voltage time sequences were developed (Loddo and Patella 19771, Patella, Rossi and Tramacere 1979;Alfano et al 1982;Ciminale and Patella 1982;Marchisio and Costantini 1985). A deeper penetration, to depths of the order of a few kilometres, was the result of this innovation.…”

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confidence: 99%

Statistical analysis of non‐stationary voltage recordings in geoelectrical prospecting¹

Lapenna

Macchiato

Patella

et al. 1994

Geophysical Prospecting

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We study the geoelectrical problem of picking out the useful signal from voltage time series, monitored under conditions of a low signal‐to‐noise ratio and non‐stationary noise. Statistical tests performed at different sites show that geoelectrical noise often belongs to the class of non‐stationary phenomena with non‐Gaussian probability distributions. In such cases, the application of conventional methods of geoelectrical useful signal extraction, based on the stationary white‐noise assumption, gives biased estimates. For the on‐line processing of geoelectrical recordings, we recommend the use of the periodogram technique combined with the Kolmogorov–Smirnov test, a suitable algorithm of which is described in detail. The suggested procedure allows data acquisition to stop as soon as the useful signal power is estimated with a relative error smaller than a predetermined value. Finally, we compare the suggested procedure with the autoregressive approach. The previously used and simpler periodogram method, applied to the solution of problems of this kind, appears to give better performances than the autoregressive analysis.

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Analysis procedure and equipment for deep geoelectrical soundings in noisy areas

Cited by 8 publications

References 2 publications

Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an International Continental Drilling Program (ICDP) monitoring site to image fluid-related structures

Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an International Continental Drilling Program (ICDP) monitoring site to image fluid-related structures

Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an ICDP monitoring drill site to image fluid-related structures

Statistical analysis of non‐stationary voltage recordings in geoelectrical prospecting¹

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Analysis procedure and equipment for deep geoelectrical soundings in noisy areas

Cited by 8 publications

References 2 publications

Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an International Continental Drilling Program (ICDP) monitoring site to image fluid-related structures

Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an International Continental Drilling Program (ICDP) monitoring site to image fluid-related structures

Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an ICDP monitoring drill site to image fluid-related structures

Statistical analysis of non‐stationary voltage recordings in geoelectrical prospecting1

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Statistical analysis of non‐stationary voltage recordings in geoelectrical prospecting¹