Wide angle reflection and refraction methods for ground penetrating radar

Angelis, Dimitrios (2023) Wide angle reflection and refraction methods for ground penetrating radar. Doctoral thesis, Northumbria University.

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Abstract

Multi-offset ground-penetrating radar (GPR) data acquisition modes such as the common midpoint (CMP) and wide-angle reflection and refraction (WARR) have many benefits over the traditional single-offset common offset (CO) mode, as they allow for each subsurface point to be sensed by multiple transmitter-receiver pairs, and thus by multiple wavefronts. However, despite their benefits, CMP and WARR data acquisition modes have seen limited adoption in recent decades for several reasons, primarily because they require multiple offsets and they were being performed with a single transmitter-receiver pair, thus making them extremely time consuming and expensive.

Recent advancements in GPR technology have led to the development of new GPR systems with multiple concurrent data acquisition receivers. These newly developed GPRs allow for the fast and dense acquisition of CMP/WARR data with the same speed as the traditional CO mode, and thus offer the potential to provide, rapidly and therefore with low survey cost, all the benefits of the multi-offset modes. However, both the character and the large volume of data generated by these GPR systems, as well as the large number of transducers required for their operation, necessitates the development and automation of new processing methods, workflows, and tools, as well as the investigation of new configurations with reduced transducers.

A novel MATLAB-based software with a user-friendly graphical user interface (GUI) has been developed to enable the visualisation and processing of multi-concurrent receiver GPR data, as well as the testing and evaluation of new processing methods and workflows for such data. The software supports GPR file formats from various manufacturers, but most importantly, it supports the ability to manage both single- and multi-offset GPR data, including simulated data and multi-concurrent receiver GPR data. The software provides a large number of processing methods, both simple and advanced, as well as novel processing methods developed exclusively for multi-concurrent receiver GPR data.

Three specific processing methods have been developed to address challenges associated with the character and the large volume of data generated by multi-concurrent receiver GPR systems. The first is a time-zero alignment method for correcting and managing the time misalignments from multi concurrent receivers; the second is a CMP trace balancing method for compensating for the large amplitude versus offset (AVO) differences in the data; and the third is an automatic velocity spectrum picking method for facilitating velocity analysis of large volumes of data. All three methods have been evaluated and validated using both synthetic and real field data, demonstrating their efficiency and performance.

The aforementioned software and specific processing methods have enabled a robust and comprehensive processing workflow for multi-concurrent receiver GPR data to be created, which is capable of producing detailed velocity models, further enhancing data quality, and thus GPR interpretations. The workflow has been evaluated and validated with both synthetic and real field data acquired by different GPR systems from various environments. It is shown for the first time that seven multi-concurrent receivers are sufficient to provide both manually as well as automatically detailed stacking velocity fields and enhanced zero offset stacked time sections, thus also demonstrating the true potential of multi-concurrent receiver GPR systems.

Finally, the system/survey design of multi-concurrent receiver GPR systems has been explored with the primary objective being of making them more practicable and cost-effective. Utilising both simulated data and real field data it is shown that reducing the number of receivers from a configuration with seven equally spaced receivers to a sparse four-receiver configuration can sustain acceptable velocity resolution in velocity spectra panels. It is also demonstrated for the first time that not only seven but also four receivers can provide detailed stacking velocity fields and enhanced zero-offset stacked time sections.

With a robust and comprehensive data processing workflow for multi-concurrent receiver GPR data now established, there will be opportunities to further automate velocity analysis and add processing steps such as depth imaging and migration. Furthermore, apart from moveout-based velocity spectra analysis, other advanced velocity analysis methods such as prestack depth migration velocity analysis or reflection tomography can be explored.

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