Drone-Borne Electromagnetic (DREM) Surveying in The Netherlands
Speaker: Marios Karaoulis, Deltares (the Netherlands)
Summary: In the past decade drones have become available and affordable for civil applications, including monitoring with geophysical sensors. In 2017 and 2019 the feasibility of executing frequency domain electromagnetic (FDEM) method surveys using an off the shelf drone, with a maximum payload capacity of 5 kgs, was investigated at Deltares. This paper reports on the preparatory tests executed to determine the optimal configuration, processing and inversion scheme and on the field validation tests demonstrating the capability of the drone-borne electromagnetic survey.
The integration tests address mounting stability, electromagnetic interference and flight safety issues. The processing scheme tests resulted in how calibration and altitude corrections can be carried out, similar to helicopter-borne electromagnetic (HLEM) surveys.
One demonstration example shows how the system can efficiently map shallow groundwater and surface water salinity in areas where seepage of saline water occurs. The other demonstration example shows how the system can map shallow lithology as well as buried metal cables and pipelines.
The general findings are that the system provides for the flexibility to (automatically) ﬂy a combination of FDEM soundings, proﬁles or grid patterns and its ability and efficiently to ﬂy over inaccessible areas and surface water. Compared to the HLEM surveys, the spatial resolution is much higher which allows for detailed 3D mapping of subsurface and the costs are, certainly for small study areas, relatively low, which also makes monitoring of changes by (automatic) repeated drone-borne electromagnetic (DREM) surveys affordable.
Sparse 3D Reflection Seismic Survey at Ludvika Mines of South-Central Sweden
Speaker: Magdalena Markovic Juhlin, Uppsala University (Sweden)
Summary: Bergslagen mineral district in south-central Sweden is well known for its iron-oxide deposits. Our study area is situated at the historical mining site of Blötberget-Ludvika, which is known for its production of high-quality iron ores. In the past few years, several geophysical and geological studies have been carried out at this site in order to improve the knowledge of the iron-oxide mineralization and its host rock structures. In this study, we present the latest effort of acquiring a sparse 3D reflection seismic survey and a first-hand view of the dataset and potential it has for deep targeting. Starting with carefully selected acquisition parameters and data pre-processing, we obtained high quality images of the mineralization but also subtle features related to the structures. With only conventional processing approaches and for showcasing the potential of the dataset, using two profiles extracted from the 3D survey, we were able to successfully image the mineralization in its known 3D dimensions. This is encouraging and opens up for both research and up-scaled opportunities for similar commodities elsewhere.
Acoustic Impedance Inversion of High Resolution Marine Seismic Data with Deep Neural Network
Speaker: Jean-Remi Dujardin, Norwegian Geotechnical Institute (Norway)
Summary: We investigate a simple Deep Learning Neural Network architecture to invert High Resolution seismic data for reflectivity and acoustic impedance. We generate synthetic reflectivity model and corresponding seismic traces by a simple convolution with a wavelet for training the machine learning network. Synthetic reflectivity models are correctly recovered when the reflectors separation is in the range of the train set. Noise in the data is correctly handled if the network is trained with adding noise on the data. Finally, we investigate the prediction of the acoustic impedance using the Marmousi model.
Efficient State-of-Art HDR 3D GPR Compared to 2D Traditional Utility Investigations
Speakers: Jaana Gustafsson, Guideline Geo (Sweden)
Summary: The subsurface in urban areas is becoming more and more congested with infrastructure, both underground constructions and utilities. With an ever-increasing urbanization, aging subsurface infrastructure and climate change effects, a need has arisen for more efficient and reliable methods for cost effective management of underground assets.
Traditionally, over the past 20 years, investigations of utilities have been carried out with single channel (2D) GPR measurements (alongside other technologies e.g., EM cable and pipe locators), to ascertain both the location and the depth of buried utilities without the need for extensive excavations. These surveys have been collected with a quite coarse sampling spacing and the identification of hyperbolas in the different reflection profiles has served as the marker for the identification of buried utilities in the ground.
When survey time permits, the single channel instruments have previously also been used to gather densely spaced parallel crosses over utilities. The goal has often been to produce more easily interpretable data images in the form of, so called, time or depth slices. The transect spacing of such surveys has often been to 0.5m or more which often still resulted in quite low-resolution 3D images of buried pipes and cables. Even with dense transect spacings, as 0.25 cm, smaller pipes and cables may be hard to identify and interpret correctly. Such surveys are more often time-consuming and often unsuitable for cost-effective, large area, utility detection projects.
For the last 10 years, and as an alternative to single channel GPR instruments, several different 3D array GPR instruments have been available on the market. These have a dense channel spacing and have often been integrated with high accuracy positioning devices to ensure that the data and subsequent interpretations are mapped as accurately as possible. The first commercial array instrument, i.e., the MALÅ Imaging Radar Array (MIRA), had a minimum channel spacing of 8 cm producing exceptionally high-resolution data compared to the often course 3D images produced by single channel systems at the time. The MIRA was based on the traditional repetitive sampling technique.
Today GPR antennas has been refined, and real-time sampling techniques have been introduced. These HDR (High Dynamic Range) antennas enable significantly faster data acquisitions rates, a greater signal-to-noise ratio (resulting in greater depth penetration and higher resolution data) and an unprecedented dynamic range. For a long time, HDR techniques were only available for single channel GPR antennas, but as of now the technique has been implemented in next-generation 3D array systems; MIRA HDR 500. In line with the rapid development of reliable hardware solutions, cutting edge and stream-lined software packages has followed closely.
The purpose of this presentation is to compare traditional single channel (2D) GPR measurement with results from a state-of-the-art real time sampling 3D array GPR system. A short discussion on data collection time and resulting data quality are presented along with other important considerations.
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