Reprocessing of high‐resolution seismic data for imaging of shallow groundwater resources in glacial deposits, SE Sweden

Reprocessing of high-resolution seismic reflection data over groundwater-bearingglacial deposits near Heby, southeastern Sweden, improved the images of near-surface structure at this site. Post-stack time migration and pre-stack depth migrationwere tested and compared to determine the improvements on imaging an undulatingbedrock surface. The pre-stack depth migration image displays better continuity ofthe dipping structures within the glacial sediments and provides a more detailed to-pography of the bedrock reflector. First-arrival picks were used to define an initialmodel for input into tomographic inversion. The tomography result then formed thebasis for building the migration velocity model. The final pre-stack depth migrationimage shows a strong reflection at around 35 m elevation (about 9 m below thesurface) that can be correlated to a thin (0.2 m) hard silt layer. The upper 20 m ofoverburden is interpreted to consist of clay, and the seismic images show weakersub-horizontal reflections within this unit, except for the strong silt reflection, con-sistent with our modelling results. Below 20 m, sand/gravel sediments are presentand overlay the bedrock. Forward modelling based on the pre-stack depth migrationimage and subsequent processing shows that pre-stack depth migration provides ahigher resolution image compared with post-stack time migration. Our study showsthat pre-stack depth migration is preferable to post-stack time migration even for theshallow near-surface seismic data acquired at Heby, and that integrating tomography,migration, modelling and geological information provides a better understanding ofthe structure which these groundwater resources are contained in

High Resolution Seismic Reflection PP and PS Imaging of the Bedrock Surface below Glacial Deposits in Marsta, Sweden

Multi-component high resolution seismic reflection data were acquired at the UppsalaUniversity field test site in Marsta, Sweden, in March 2019 with the aim of obtainingan improved understanding of the subsurface structure in the area. An advantage of thesite is that a number of boreholes have been drilled there for both hydrogeological andgeophysical purposes, allowing surface geophysics to be compared with downholeinformation. The presence of a low velocity layer above the water table generatessignificant processing challenges due to trapped waves. A further complication with thedata is that the upper part of this layer was frozen at the time of the survey, resulting inthe sediments just below the surface having a significantly higher velocity than thosebelow. By analyzing and processing both the vertical and radial component data it waspossible to build a velocity model that is consistent with the observed data. A strong PSconverted reflection allows the bedrock to be imaged on the radial component stacks tomuch higher resolution than on the vertical component stacks. Both commonconversion point binning and pre-stack depth migration were used to process the radialcomponent data. We confirmed that our processing strategy was effective with thegeneration of synthetic data were processed in a similar manner. The PS images indicatea step in the bedrock of about 2m, depth increasing from 15m to 17m, close to one ofthe boreholes. This step is not observed on the PP stacks, due to their lower resolution

Subsurface seismic imaging with a hammer drilling source at an exploration drilling test center in Örebro, Sweden

Seismic imaging while drilling (SWD) technology offers possibilities of imaging ahead of the drill-bit, which could be useful for determining when to go from hammer drilling to core drilling. Also, seismic images of the surrounding rock can improve geological models which could be then used to guide drilling programs. An SWD field test was carried out in August 2020 at an exploration drilling test site in Örebro, Sweden, with the aim to determine if the signals from hammer drilling can be used for seismic imaging around the drill-bit in a hard-rock environment where the strong drill-rig noise interference is one of the main challenges. The test site had previously been investigated with various geophysical methods, geological mapping and diamond core drilling, and it therefore represented an ideal location to perform this feasibility study. After data pre-processing and cross-correlation with the trace from the geophone closest to the rig, the shot-gathers were vertically stacked over the length of a drill pipe to achieve further signal improvement. A comparison with the active seismic data shows reasonable agreement, in spite of the fact that the noise level is significant even after careful processing. However, the lack of clear reflections in the active seismic data, indicating no detectable changes in the bedrock lithology in the near surface, hinders the full assessment of the seismic signal generated with hammer drilling at this site

Reprocessing of high‐resolution seismic data for imaging of shallow groundwater resources in glacial deposits, SE Sweden

Reprocessing of high-resolution seismic reflection data over groundwater-bearingglacial deposits near Heby, southeastern Sweden, improved the images of near-surface structure at this site. Post-stack time migration and pre-stack depth migrationwere tested and compared to determine the improvements on imaging an undulatingbedrock surface. The pre-stack depth migration image displays better continuity ofthe dipping structures within the glacial sediments and provides a more detailed to-pography of the bedrock reflector. First-arrival picks were used to define an initialmodel for input into tomographic inversion. The tomography result then formed thebasis for building the migration velocity model. The final pre-stack depth migrationimage shows a strong reflection at around 35 m elevation (about 9 m below thesurface) that can be correlated to a thin (0.2 m) hard silt layer. The upper 20 m ofoverburden is interpreted to consist of clay, and the seismic images show weakersub-horizontal reflections within this unit, except for the strong silt reflection, con-sistent with our modelling results. Below 20 m, sand/gravel sediments are presentand overlay the bedrock. Forward modelling based on the pre-stack depth migrationimage and subsequent processing shows that pre-stack depth migration provides ahigher resolution image compared with post-stack time migration. Our study showsthat pre-stack depth migration is preferable to post-stack time migration even for theshallow near-surface seismic data acquired at Heby, and that integrating tomography,migration, modelling and geological information provides a better understanding ofthe structure which these groundwater resources are contained in

Subsurface seismic imaging with a hammer drilling source at an exploration drilling test center in Örebro, Sweden

Seismic imaging while drilling (SWD) technology offers possibilities of imaging ahead of the drill-bit, which could be useful for determining when to go from hammer drilling to core drilling. Also, seismic images of the surrounding rock can improve geological models which could be then used to guide drilling programs. An SWD field test was carried out in August 2020 at an exploration drilling test site in Örebro, Sweden, with the aim to determine if the signals from hammer drilling can be used for seismic imaging around the drill-bit in a hard-rock environment where the strong drill-rig noise interference is one of the main challenges. The test site had previously been investigated with various geophysical methods, geological mapping and diamond core drilling, and it therefore represented an ideal location to perform this feasibility study. After data pre-processing and cross-correlation with the trace from the geophone closest to the rig, the shot-gathers were vertically stacked over the length of a drill pipe to achieve further signal improvement. A comparison with the active seismic data shows reasonable agreement, in spite of the fact that the noise level is significant even after careful processing. However, the lack of clear reflections in the active seismic data, indicating no detectable changes in the bedrock lithology in the near surface, hinders the full assessment of the seismic signal generated with hammer drilling at this site

Drill‐bit position monitoring using seismic‐while‐drilling data; numerical and field examples from Sweden

The undesired drill-bit deviation is a source of drilling risks and requires monitoring. Seismic-while-drilling is one potential method to achieve this and has been tested in a number of previous studies. In August 2020 in Örebro, Sweden, we conducted an experiment to test the feasibility of seismic-while-drilling drill-bit positioning and other applications of the method. We used the hammer drill-bit signal generated while drilling a 200 m deep well in hardrock conditions and implemented vertical stacking of the subsequent impulsive signals from the hammer source, generating enhanced direct arrivals from the drill-bit. Then, we used the relative arrival times to estimate the drill-bit position for selected bit depths, confirming our methodology with two numerical studies. We successfully estimated the position of the drill-bit for the numerical examples and for several of the real data examples, with the accuracy dependent on the receiver array geometry and the quality of the data. We conclude that this drill-bit positioning method shows potential for near-real-time monitoring in drilling operations that could be applicable for both impulsive and noise-retrieved drill-bit signals

Effect of layered geological structures on borehole heat transfer

Borehole heat exchangers, especially deep ones, are usually drilled through different geological layers havingvarying properties. Homogeneous and layered models can be used for borehole performance predictions. Thehomogeneous model considers all layers as a single layer having effective properties while the layered modelconsiders all layers separately and gives better accuracy, although it is more complicated and time consuming tocalculate. In this study, by considering real geological structures, thermal performance predictions of a deepborehole are made using both homogeneous and layered models and the results are compared to examine howpredictions differ from each other depending on the statistical characteristics of geological structures. Ananalytical expression is derived for the relation between statistical characteristics and deviations from the predictionsof the homogeneous model. The magnitudes of deviations are very small and essentially depend on thevariance of the difference for the thermal properties of the layers and a time decaying function. The results helpto understand how horizontally layered geological structures influence borehole performance and when we needa layered model