Near Surface Geophysics - Volume 21, Issue 5, 2023

Typically, transient electromagnetic (TEM) hydrogeological applications focus on spatial characterization. However, there is scope for utilizing time‐lapse TEM to monitor dynamic processes. Changes related to hydrogeological processes are subtle; consequently, measurements must be of exceptional quality, and the presence and influence of noise sources must be well understood. A potentially problematic noise source is radio waves operating in the 3–300 kHz range, as they introduce correlated error into TEM measurements. For instance, it was anticipated that such radio waves could introduce smooth perturbations in the data that could be erroneously interpreted in the TEM inversion results. This work evaluates the presence of correlated noise sources, their temporal dependence and their significance for TEM monitoring. This work combines fully sampled data and normalized covariance matrices of gated data to identify correlated noise sources. Radio signals operating in the low and very low frequency (LF and VLF) bands were found to vary across the investigation period; however, their influence on the TEM inversion results was minimal. Measurement stacking remains an effective way to improve the signal‐to‐noise ratio of TEM data affected by LF and VLF noise sources as they do not introduce bias, for example smooth perturbation, into the TEM data explored in this work.

Flooding, erosion and increases in the water level in Lake Superior have contributed to changes in the stem location and width of the Grand Portage Creek. Those events threaten parts of the Grand Portage National Monument, a historically significant site on the North Shore of Lake Superior, Minnesota. We performed geophysical surveys to investigate these dynamic effects. We studied the near‐surface geological deposits, the mechanisms associated with creek stem dynamics and sediment transport, as well as deposition along the lakeshore in Grand Portage Bay. We deployed ground‐penetrating radar (GPR), sub‐bottom profiler, side scan sonar, Geoprobe coring and Van Veen grab samplers and evaluated time‐lapse satellite images to assess the interaction of the Grand Portage Creek with the Grand Portage Bay. The onshore GPR surveys next to the national monument, the creek and the shoreline showed the presence of a complex deposition with eroded ground surfaces and sediment layers across the creek valley. Results from the offshore geophysical campaigns and the interpretations of satellite images also document a heterogeneous deposition sequence environment with fine‐grained sediment deposits present south and southwest of the creek mouth. In addition, we documented an exposed boulder bed towards the east of the creek mouth that was exposed by the current and wave‐driven erosion process in the Grand Portage Bay. Time‐lapse satellite images and hydraulic current velocity simulations validate these observations and provide insight into how anthropogenic activities and natural events interact and might contribute to the long‐term stability of a site of historical and cultural importance.

The imaging of geological structure in front of a tunnel face is conducive to the safe construction of tunnels. The reverse time migration (RTM) method is popular for its high imaging accuracy. However, most of the current tunnel seismic RTM methods assume complete elasticity, ignoring the nature of wave propagation attenuation, resulting in a poor imaging outcome, especially when tunnelling in the shallow subsurface. Thus, it is necessary to study tunnel RTM suitable for attenuating strata. First, considering the frequency of tunnel seismic forward‐prospecting is usually higher than that of ground surface seismic exploration, we established a corresponding tunnel viscous media model to enable viscoacoustic forward modelling of seismic wavefield. Then, a simultaneous compensation approach for its amplitude and phase velocity is proposed based on the combination of the real and imaginary parts of the complex modulus. After the compensation of seismic wavefields, the tunnel Q‐RTM imaging method in viscoacoustic media is formed, and its effectiveness is verified by numerical examples. Compared with RTM without attenuation compensation, tunnel Q‐RTM effectively enhances the imaging energy and has higher positioning accuracy, improving the imaging effect of complex structures (especially at larger distances from the tunnel face). In the end, a detailed case study is presented to demonstrat the potential of the proposed method for field application.

Glaciers generate seismic waves due to calving and fracturing, meaning that recording and following event classification can be used to monitor glacier dynamics. Our aim with this study is to analyse seismic data acquired at the seabed and on land in front of Nordenskiöldbreen on Svalbard during 8 days in October 2020. The survey included 27 ocean bottom nodes, each equipped with 3 geophones and a hydrophone, and 101 land‐based geophones. The resulting data contain numerous seismic P‐, S‐ and Scholte wave events throughout the study period, as well as non‐seismic gravity waves. The recording quality strongly depends on receiver type and location, especially for the latter wave types. Our results demonstrate that hydrophones at the seabed are advantageous to record gravity waves, and that Scholte waves are only recorded close to the glacier. The Scholte waves are used to estimate the near‐surface S‐wave profile of the seabed sediments, and the gravity wave amplitudes are converted to wave heights at the surface. We further discuss possible source mechanisms for the recorded events and present evidence that waves from earthquakes, calving and brittle fracturing of the glacier and icebergs are all represented in the data. The interpretation is based on frequency content, duration, seismic velocities and onset (emergent/impulsive) and is supported by source localization, which we show is challenging for this dataset. In conclusion, our study demonstrates the potential of using seismic observations for detecting glacier‐related events and provides valuable knowledge about the importance of survey geometry, particularly the advantages of including seabed receivers in the vicinity of the glacier.