Line Kilometres Of Data Research Articles

Integrating neural networks in least-squares inversion of airborne time-domain electromagnetic data

Airborne time-domain electromagnetic surveys produce extremely large data sets with thousands of line kilometers of data and millions of possible models to explain the data. Inversion of such data sets to obtain the resistivity structures of the subsurface is computationally intensive and involves calculation of a significant number of forward and derivative responses for solving the least-squares inverse problem. The flight altitude of the airborne system needs to be included in the modeling, which adds further complexity. We propose to integrate neural networks in a damped iterative least-squares inversion framework to expedite the inversion process. We train two separate neural networks to predict the forward responses and partial derivatives independently for a broad range of resistivity structures and flight altitudes. Data inversion is not only used for producing the final subsurface models but also used during data processing, or to produce intermediate results during a survey. With these purposes in mind, we provide three inversion schemes with a tunable balance between computational time and modeling accuracy: (1) numerical forward responses used initially in combination with neural network derivatives, and the derivatives switched to a numerical solution in final iterations, (2) numerical forward responses in combination with neural network derivatives used throughout the inversion, and (3) only neural network forward responses and derivatives used in inversion. Experiments on field data find that we improve inversion speed without any loss in modeling accuracy with our first approach, whereas the second scheme gives a significant speedup at the cost of minor and often acceptable deviations in the inversion results from the conventional nonlinear inversion. The last approach is the fastest and captures the overall resistivity structures quite well. Therefore, depending on the modeling accuracy, inversion speedup factors of up to 50 are realized by using the proposed schemes.

Automated Transient Electromagnetic Data Processing for Ground-Based and Airborne Systems by a Deep Learning Expert System

Modern transient electromagnetic (TEM) surveys, either ground-based or airborne, may yield thousands of line kilometers of data. Parts of these data, especially in areas with dense infrastructure, are often disturbed by electromagnetic couplings due to infrastructure, e.g., power cables and fences. In most cases and in particular when working in a hydro-geological context, such coupled data must be culled before inversion. The process of identifying and culling coupled data is a manual task, requiring specialists to examine and process the data in detail. Manual data processing is subjective, difficult to reproduce, and time-consuming. To automate the complex data processing workflows, we propose an expert system based on a deep convolutional auto-encoder to identify couplings in the data. We configure the auto-encoder to learn an encoded representation of TEM data in a latent space. A reconstruction part that decodes the encoded representation is also trained, aiming to reconstruct input data. If the data unaffected by electromagnetic couplings are observed by the auto-encoder, the reconstructed output will have low error to the input. However, when having couplings in the data, the reconstruction error is elevated, indicating a non-geologic anomaly. The size of the anomaly is based on the relative error between the input data and the reconstructed output normalized by the data standard deviation. We show that the proposed approach displays high quality data processing within a fraction of a second for a ground-based and an airborne system, which is either ready for inversion or requires minimal further quality inspection.

Airborne geophysical surveys in central West Greenland and central East Greenland in 1997

In order to stimulate mining exploration activity in Greenland the Government of Greenland decided in 1993 to finance a five-year programme of airborne electromagnetic surveys over selected regions of Greenland, Project AEM Greenland 1994–1998. By the end of 1996 three surveys had been undertaken in various parts of Greenland (Stemp & Thorning 1995a, b; Stemp 1996a, b; Stemp 1997a, b). In 1992 the Danish Government financed a small aeromagnetic survey (Project Aeromag 1992; Thorning 1993). Regional aeromagnetic surveying was taken up again when the governments of Denmark and Greenland jointly financed two aeromagnetic surveys in 1995 and 1996 – the projects Aeromag 1995 and Aeromag 1996 (Thorning & Stemp 1997). To this suite of airborne geophysical surveys of selected regions in Greenland were added two surveys in 1997, both financed by the Government of Greenland. The fourth year of Project AEM Greenland 1994–1998 encompassed a transient electromagnetic (GEOTEM) and magnetic survey over northern Jameson Land, central East Greenland, while Project Aeromag 97 added a regional aeromagnetic survey over Disko Bugt – Nuussuaq, central West Greenland. As in previous years, commercial geophysical contractors carried out the survey operations in Greenland according to an agreement with the Geological Survey of Denmark and Greenland (GEUS) entered into after international tendering following rules of the European Union. GEUS manages the projects and organises the distribution and use of the results. The new maps and digital data from the two 1997 surveys, a total of 85 252 line kilometres of data covering 51 414 km2, were released to the public on 1 March 1998. This note provides some introductory information about the two surveys. Further information can be found in reports by Stemp (1998) and Thorning (in press); both include a number of full-page colour anomaly maps from the survey areas.
 The airborne geophysical programme will continue in 1998, and the areas to be surveyed have already been selected. The final year of Project AEM Greenland 1994–1998 will include combined GEOTEM and magnetic surveys over two regions in North Greenland: Washington Land in western North Greenland, where operations are expected to start in May 1998 operating out of Alert in Canada, and later in the season over J.C. Christensen Land in central North Greenland operating out of Station Nord in eastern North Greenland. Project Aeromag 1998 will continue the regional aeromagnetic survey programme in West Greenland, extending the coverage by including most of the region from 63°45′N to 66°N in southern West Greenland. This project will be based at Nuuk and start in March 1998. The Government of Greenland will finance all surveys in 1998. Figure 1 shows all survey areas for the electromagnetic and magnetic surveys of Project AEM Greenland 1994–1998 and the aeromagnetic survey areas of Project Aeromag 1992, 1995, 1996, 1997 and 1998.