Prior to leaving Adelaide, one of the participants there mentioned to us that on our flight to Brisbane, we would fly over the Murray River. In the course, we present a case history, Bookpurnong, which is aimed at understanding and managing salinization of the Murray River (we will discuss this case history in more detail later on in the blog). So we asked the flight attendants when we should expect to see the Murray — 15 minutes after take-off, the pilot came over the intercom and announced “For those passengers interested (all 2 of us!!), we are coming up to the Murray River,” and Doug and I were able to get a couple photos!
Brisbane had the largest number of participants for the Australian leg of the DISC and among the most engaged groups we have connected with. Many were from the Brisbane area, but there were also several who made the trip from Canberra to attend. There was a mix of backgrounds, some work daily with electromagnetics, and others came with backgrounds in seismic or geology and an interest in using EM. Several attendees were graduate students or professors at the University of Queensland and a number were with Geoscience Australia, the Geologic survey of Queensland or CSIRO. There was also a variety of industry-oriented backgrounds, mining was the largest, and there was a good showing from groundwater and environmental applications (we will discuss these further in the DISC Lab section). In a lot of ways, this was exactly the type of audience we are hoping to gather at each location: diverse in backgrounds and engaged in the content!
Magnetotellurics (MT) was a topic of wide interest — seeing beneath Australia’s conductive regolith is a challenge for EM and MT essentially provides a giant transmitter and low frequencies that enable signal to penetrate deeper into the earth.
Of the case histories, the Lalor case history had a particular connection with some of the participants who work with GAP, the company that developed and deploys the HeliSAM system used in that study.
From the participants
“Incredibly enthusiastic and great detailed presentations. Thoroughly enjoyed it! Looking forward to playing with the apps” — Dan Eremenco, Geophysicist
“Good overall coverage. Nice to see good graphs and pictures. Great capability of explaining the difficult things in simple terms!”
“Fantastic. Best course ever! Highly appreciated”
“Favourite aspect is how up-to-date and modern the course feels. The Apps make me want to go home and experiment.” — Nick Josephs, Geophysicist
“A good overview of EM methods from first principles. I really liked Doug’s practical explanation without heavy use of equations. Looking forward to discovering the open source modelling codes.”
“Good refresher of key concepts, well presented and explained. Grounded sources was very interesting; it is not widely publicized by contractors. Good to see the tools students are using now compared to ~15 years ago. I will start to use these. I would like to see the mineral exploration industry embrace the open source approach to solving geophysical problems.” — James Alderman, Geophysicist
The course material from Brisbane are available at: http://disc2017.geosci.xyz/brisbane
We started the day with two additional presentations from Doug Oldenburg discussing: (1) EM coupling issues and how it relates to IP, which is particularly of interest when conducting IP surveys and contending with the conductive regolith over Australia, and (2) the inversion of airborne geophysical data over TKC, which is an example where a rock model is created from geophysical data.
Following those presentations, we had a good showing of participants willing to present what they are currently working on! Presentations included a range of topics from instrumentation, groundwater applications, magnetotellurics, mineral exploration and agricultural applications. Below we share a few highlights from each.
Chris Parker (slides), with GAP Geophysics, started us off. He presented a case study showing electromagnetic (EM) and magnetometric resistivity (MMR) data collected over two sites in Western Australia. A time-domain grounded source is used and total field magnetic data are collected over the survey area. Data are sampled during both the on and off times, so both MMR and EM datasets are obtained. Currently, they are inverted independently, but there is interest in jointly inverting the MMR and EM data — a research opportunity!
David Allan (slides), with Groundwater Imaging, presented the AgTEM system he developed (MacGeyvered!). It is a towed transient EM system. At the back, there is a large loop, which is the transmitter. The receiving loop is positioned in the centre, as is a bucking coil to cancel the primary field at the receiver. The arms which hold out the transmitter loop are flexible so that if it hits a tree, it will bend and spring back. In order to be portable, it needs to pack up to be compact; David has included a number of images in his slides. In his presentation, he also showed data that have been collected with the AgTEM for groundwater applications. He spoke to some of the challenges that geoscientists need to be aware of when using EM for groundwater. As for future work, he is interested in conducting 3D surveys, where stationary receivers are laid out over the region of interest and the transmitter towed through the survey area.
Janelle Simpson (slides) is with the Geological Survey of Queensland. She is working with MT data over a study area at the southern edge of the Mt. Isa region. It is a geologically complex region that is of interest for potential mines, and the goal is to build up a full 3D geologic model of the project area. There has not been much drilling done in the area, so there are very few constraints on the geology from borehole data; thus, the MT data are important for building up a geologic understanding. Using these data, a depth-to-basement study has been conducted, as have more regional tectonic studies. Currently, Janelle is now working on extracting more from the data through inversion. She has built up a synthetic model based on drill hole data and is exploring inversions of these data. The example she showed prompted discussion on the importance of regularization in an inversion, which is the topic of the MT tutorial! Later in the afternoon, we input the model she is working with into the notebooks developed for the MT tutorial.
Jingming Duan (slides) works with the Resources Division of Geoscience Australia. He presented an overview of Magnetotellurics in Geoscience Australia. There are ~7500 MT stations across Australia. Those data are used to ascertain the depth to basement, which is important for building a geologic understanding of prospective resource plays in minerals or oil and gas. Geoscience Australia have partnered with drilling programs to conduct a study which examines estimates of depth-to-basement from MT data. They first predict the depth from MT data and then assess their estimate and identify challenges based on how well the estimate agrees with the true depth to basement found when the drilling was conducted. He also discussed the AusLamp project (Australian Lithospheric Architecture Magnetotelluric Project), which aims to produce a 3D conductivity model from MT data collected over all of Australia. To date ~26% of the stations have been collected. Working with such large areas and data sets requires efficient 3D inversions; Jingming highlighted the need for improvements in the numerical tools for addressing large-scale problems.
Lachlan Hennessy (slides) is a PhD student at RMIT university with James McNay. He is working on incorporating lightning source information into the processing of MT data. Lightning is one of the sources that provide EM energy for an MT survey. Typically, lightning strikes are treated as a random source (unknown location, unknown current); lightning data, however, provide us with information on both the location and current. From this, we can start to predict some things including the azimuth and arrival time of the EM signal from the lightning (called a spheric) at our MT station (we know that the EM signal from lightning travels at the speed of light). This has important implications in processing of MT data — these spherics have a huge amount of signal, particularly in what is known as the “dead-band” in MT, the band of frequencies between 1.5 and 5 kHz, where signals are typically quite low. Now we can pick out the data that are associated with its arrival and use those in the processing workflow (traditionally, they would be filtered out). Lachlan showed an example where traditional processing actually introduced a bias into the processed data and demonstrated that by including signal from lightning strikes, the quality of the processed data can be significantly improved.
Mark Glover (slides) works with CSIRO Land and Water. He discussed an application of EM on a much smaller scale — looking at the top few centimetres of the earth for agricultural applications. This work is part of an Australian Centre of International Agricultural Research Project aimed at supporting cropping system intensification in the salt-affected coastal zone of Bangladesh and West Bengal, India. In both Bangladesh and West Bengal, the wet season brings about 3000mm rain per year(!) which provides a large freshwater influx to the rivers and soil. When the wet season ends, the region begins to dry out, and as it is within 5m of sea level, sea water begins to intrude, significantly increasing the salinity of the soils. Such swings have a huge impact on crop yield potentials and make it very challenging to interpret results from crop trials, tests comparing different crop rotations which are used to help increase the number of crops that can be grown in a season. They are working on including EM as a tool for understanding the variations in salinity so that its impact on crop yield can be better characterized.
Teagan Blaikie (slides), with CSIRO, presented a case study from the southeastern McArthur Basin. She is working with multiple types of geophysical data types, including gravity, airborne EM, magnetics and radiometrics, to build a 3D geologic model of the Caranbirini area. As each method is sensitive to distinct physical properties, each data set provides distinct information about the geology. Starting from a 3D geologic model interpreted from drilling results, Teagan first examined the model in the context of the geophysical data, assessing where they agreed and where the geophysical data show that the model needs to be updated. Taking a “collaborative inversion approach”, she has inverted each dataset independently, but improved the independent inversions by bringing features such as the location of geologic interfaces, found in the airborne EM inversion as a-priori information in the gravity inversion.
Liejun Wang (slides) works with Geoscience Australia. He presented on geomagnetic induction hazards in Australia (echoing a similar effort being conducted by the USGS, as was discussed in Denver). The electrical grid that supports Australia is fully connected and there is a significant pipeline network. During geomagnetic storms, currents can be induced in these networks and surge in current from one of these events can take the networks down. To understand how much EM energy gets loaded onto a grid in a geomagnetic storm, we need to know the conductivity structure of the earth. Using geomagnetic stations across Australia, they are constructing a large scale conductivity model of Australia. Once the conductivity model has been generated, a geomagnetic storm can be simulated and the resultant electric and magnetic fields observed. Once the local EM fields are known, these are passed into engineering workflow.
Xiuyan Ren (slides) is a PhD student at RMIT; she gave a more theoretical talk on numerical modelling for airborne EM. Her aim is to speed up the forward simulation using a “field separation method.” This is a primary-secondary approach which splits the EM fields into a primary part due to a transmitter over a half-space and and a secondary, due to conductivity structures not captured in the primary. The primary is chosen to be simple enough so that it has a semi-analytical solution, and since it does not change during an inversion, the results can be stored. The secondary problem can be solved on a smaller, computationally cheaper mesh.
Malcolm Cattach (slides1, slides2), the founder of the GAP group, gave two presentations. In the first, he presented an overview of work that has been conducted with Steve Billings on explosive ordnance detection. Former military sites in Australia (and around the world) that have been used for practice ranges are littered with unexploded ordnance. For many sites, magnetic surveys have been the method of choice. However, there are several regions of the world (eg. Laos), which have magnetic soils, making it challenging to detect ordnance objects with magnetic methods. As ordnance objects are also electrically conductive, electromagnetics may also be employed. Mal discussed a time domain EM system, the UltraTEM, which is being used by GAP for ordnance detection. Another application it is being used for it locating steel teeth that may fall off of a ground engaging tool (GET) into rock cuttings during mining. If these are not removed from the cuttings prior to going through the crusher, the crusher can be significantly damaged as it tries to break up a steel object. In his second presentation, Mal in the second, he discussed the motivation for the development of the SAM (Sub-Audio Magnetics) system for mineral exploration. The SAM setup uses a large loop on the ground as the source and a total field magnetometer , which may be deployed on the ground or from a helicopter as a receiver. Positioning a transmitter loop on the ground, rather than being towed in an airborne survey, enables it to be much larger and higher currents to be used, thus increasing the moment of the transmitter and enabling more EM energy to be transmitted into the subsurface. As deeper mining targets are sought, drilling becomes much more expensive — this is where geophysics can play an important role!
Following the presentations from participants, we introduced SimPEG, the open source software used to run many of the simulations shown in the presentation and worked through the Magnetotellurics tutorial which we recently published in the Leading Edge. The tutorial walks through the discretization of the 1D MT problem, considerations for forward modelling such as mesh design, and finally discusses the inversion. We spent the majority of our time on the inversion aspect and explored the impact of the choice of regularization parameters and criteria for stopping the inversion on the recovered model. Each of these seemingly small choices can have a significant impact on the inversion result, and in a typical workflow, they are often overlooked and the “defaults” adopted. The tutorial includes several Jupyter Notebooks where you can explore these choices yourself!
A few adventures
Prior to our journey back to Vancouver, we had two days to explore Brisbane! On our first day, we took a ferry along the Brisbane River, past the University of Queensland and off to the Lone Pine Koala Sanctuary. We arrived around 11am and spent the rest of the day taking way too many photos of koalas, kangaroos, emus, cassowaries, kookaburras, birds of prey, wallabies, lorikeets, wombats, and a sleeping Tasmanian devil.
We spent our last day in Australia walking through the Brisbane City Botanic Gardens, kayaking on the Brisbane river, and enjoying the view and a glass of Shiraz from the patio of our airbnb.
Thanks to the ASEG, and in particular Emma Brand and Mark Kneipp for helping organize the DISC in Bribane.