Evaluation of an Old Coal Mine in the area of GeothermieZentrum Bochum by Geophysical Prospection

Table of Contents

1. Introduction

1.1 Motivation

1.2 Research Area

1.2.1 Geology

1.2.2 Former Studies

2. Surveys and Theoretical Background

2.1 Ground Penetrating Radar Survey

2.2 Geoelectric Survey

2.3 Seismic Survey

3. Field measurements

3.1 Measurements at the western side of GZB

3.2 Measurements at the eastern side of GZB

4. Data Processing and Results

4.1 Analyses of radargrams with ReflexW

4.2 Inversion of pseudo-sections with BERT and DC2DInvRes

4.2.1 Geoelectric data analysis of the western side of GZB

4.2.2 Geoelectric data analysis of the eastern side of GZB

4.3 Seismogram processing with FMTOMO

4.3.1 Analysis of the pre-existing seismic data near GZB building

4.3.2 Analysis of the seismic data from the survey at the eastern side of GZB

5. Interpretation

5.1 West of GZB

5.2 East of GZB

6. Summary and Conclusions

Research Objectives and Core Topics

This master thesis aims to evaluate the subsurface stability of an old coal mine ("Markgraf 2") located beneath the GeothermieZentrum Bochum (GZB). By employing non-destructive geophysical methods, the research seeks to develop an efficient, cost-effective, and safe exploration workflow that can identify structural weaknesses, such as loose material or cavities, to prevent ground collapse and subsidence in former mining areas.

• Geophysical investigation of subsurface stability using non-destructive techniques.
• Application and comparison of Ground Penetrating Radar (GPR), Geoelectric Tomography, and Seismic Refraction.
• Data processing and inversion workflows for subsurface modeling in urban, post-mining environments.
• Evaluation of restoration project effectiveness (1980 and 2010) through geophysical data interpretation.
• Development of a precautionary exploration methodology for historical mining sites.

Excerpt from the Thesis

Summary of Chapters

1. Introduction: Presents the motivation behind the study, highlighting the history of mining in the Ruhr-area, the associated risks of subsidence, and describes the research area and geological context of the GZB site.

2. Surveys and Theoretical Background: Outlines the fundamental geophysical principles of GPR, Geoelectric, and Seismic surveys, establishing the theoretical framework for the study's chosen methods.

3. Field measurements: Details the site-specific setups for geophysical measurements conducted at the western and eastern sides of the GZB, including equipment and survey parameters.

4. Data Processing and Results: Describes the processing workflows for radar, geoelectric, and seismic data, including inversion strategies using software like ReflexW, BERT, and FMTOMO.

5. Interpretation: Synthesizes the results of the geophysical surveys to assess the stability and geological features of the subsurface, specifically looking for indicators of old mining activities.

6. Summary and Conclusions: Evaluates the efficacy of the applied geophysical workflow, offering suggestions for future improvements and summarizing the study's findings regarding the safety of the investigated area.

Keywords

Geophysics, Coal Mine, Subsurface Stability, Ground Penetrating Radar, Geoelectric Tomography, Seismic Refraction, Ruhr-area, Restoration, Subsidence, Data Inversion, ReflexW, BERT, FMTOMO, Mining Hazard, Near-surface exploration.

Frequently Asked Questions

What is the primary purpose of this study?

The study evaluates the subsurface stability of an old coal mine ("Markgraf 2") using non-destructive geophysical techniques to identify potential structural weaknesses and prevent subsidence.

Which geophysical methods were employed in this research?

The author utilized Ground Penetrating Radar (GPR), Geoelectric Tomography, and Seismic Refraction to characterize the subsurface properties.

What is the core research question or goal?

The goal is to determine if geophysical measurements provide a reliable, cost-effective, and non-destructive alternative to traditional exploration drilling for evaluating safety in restored post-mining areas.

Which specific areas were investigated?

The study focused on the western and eastern sides of the GeothermieZentrum Bochum (GZB) in Germany, which is situated over historical mining infrastructure.

How were the geophysical data processed?

The author used specialized software packages, including ReflexW for GPR analysis, BERT and DC2DInvRes for geoelectric inversion, and FMTOMO for seismic processing.

What are the key technical challenges mentioned?

Challenges include distinguishing between geological features and artifacts caused by near-surface obstacles (e.g., metallic tubes, gravel), signal attenuation, and the influence of meteorological conditions like rain on resistivity measurements.

How does the author evaluate the effectiveness of the 1980 and 2010 restoration projects?

The author compares current geophysical results with historical reports and borehole data to check for remaining subsurface anomalies, such as loose material or cavities, which indicate that the restoration might not have guaranteed full stability.

What role does the "safety pillar" play in this study?

The safety pillar is a section of the mine left unexcavated to ensure structural integrity; the study investigates whether the integrity of this pillar at the GZB site could be verified using non-destructive methods.