The original vision for this RECORDER edition’s theme was that we would feature a collection of articles that focused on new, leading edge geophysical technologies in applied use. We fell short of that goal, and that is probably because entirely new geophysical technologies are rare, and most of what is new in geophysics is either incremental improvements of existing methods, or mature technologies being used in new applications. Therefore, this issue offers four articles that span the spectrum from an entirely new technology (muon surveys), through innovations in seismic acquisition, new applications of a mature technology (magnetotelluric (MT) surveys), to advancements from seismic inversion shown through a quantitative comparison of various inversion methods.
The first article, Muon Geotomography: A Novel, Field-Proven 3D Density Imaging Technique for Mineral Exploration and Resource Monitoring, by Doug Schouten, gives an overview of what is essentially an entirely new type of survey. Muons are elementary particles similar to electrons, but with greater mass (≈207 times greater). This greater mass means they are not affected by electromagnetic fields as much as electrons and other particles, however their absorption is proportional to the density of the material they pass though. “Secondary” muons are continually generated when cosmic rays hit the Earth’s atmosphere, and these muons shower the Earth at a variety of angles. The muon mean lifetime of 2.2 μs is enough time for these secondary muons to penetrate deep into the Earth. The development of affordable and easily deployable sensors has made commercially viable muon surveys possible. In his paper, Doug describes the methodology for finding anomalous dense regions and discusses possible applications in mineral exploration, oil and gas reservoir development, subsurface monitoring and geotechnical projects.
Seismic acquisition is one of those fields that one feels must be mature, with no room for more innovations or improvements, and then lo and behold, all sorts of new and better ways of doing things emerge! Andrea Crook, in her article Seismic Acquisition Innovations Applied in Canada, covers some of the more interesting advances in onshore seismic acquisition that we have seen in Canada over the last 10 years. As always, the advances are driven by a desire to record more data, more efficiently, and at a lower cost. New geophones and more advanced “mini-vibes” have stretched the usable bandwidth of recorded data, both at the high and low frequency ends of the spectrum. Innovations in surveying technologies and methods have made possible more efficient “stake-less” surveys. Andrea also covers still evolving simultaneous source methods, which deliver larger and more diverse data sets for costs similar to conventional methods, and compressive sensing techniques that facilitate sophisticated reconstruction methods in the processing shop. She concludes by summarizing the challenges and considerations for acquiring large seismic data volumes and explains how these datasets result in significant interpretation and inversion improvements that can lead to more accurate and efficient drilling operations.
The third article features a method that has been around for a long time, but has recently seen an uptick in use, and a broadening of applications. Currents within the Earth known as magnetotellurics (MT) are constantly being generated by solar wind and lightning strikes. These currents generate secondary electromagnetic fields that can be passively recorded, and the data inverted to create resistivity models. Historically, MT surveys were more at the academic end of the spectrum, their deep penetrative abilities exploited to develop models of the Earth’s inner structure. However, advances in recording technologies mean that MT surveys are now routinely used to conduct deeper (i.e. > 500 m deep) mineral exploration, to locate industrial scale geothermal reservoirs at depth (≈1.5 km), and to help image sedimentary basins in situations where it is difficult to obtain good seismic images. In his article Applications of DC Resistivity and Magnetotelluric Methods in Exploration, Rob Hearst provides a high-level understanding of the science underlying the method, covers a few case studies to show how it is currently being applied, and discusses some possible new applications.
The last article shows how predicting reservoir properties from seismic data is a field of active research where advances continue to be made. In their article Quantitative Comparison of Inversion Methods for Estimating Density from Seismic Data, Jinling Zhang and Kyle McMillan compare three inversion methods: 1) PP pre-stack AVO inversion, 2) PP-PS joint pre-stack inversion, and 3) a statistically driven neural network approach, in an attempt to best estimate density in an Athabasca oil sands reservoir. Estimating densities of the various lithofacies in the subsurface has been and continues to be one of the most important goals of seismic inversion. The authors conclude that with this dataset, using the P-S data resulted in better density estimates than from using just the PP data, when looking at both inversion, and neural network integration with well logs. Practical examples like this are invaluable to geophysicists who need to make decisions around how to conduct their interpretations. Articles such as this are a good reminder that many “break through” advances are actually the result of many incremental improvements over a long period of time.