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Modelling the Earth through Inversion

Interview with Geosoft Chief Technologist Ian MacLeod on the benefits of 3D inversion modelling and the company’s plans to introduce voxel inversion modelling technology as an online service in 2011.

November 5, 2010

How would you describe the principle of 3D inversion as it applies to geophysical exploration?

When we conduct geophysical surveys we seek information about the underlying and hidden geology that gives rise to the values we observe on the Earth’s surface. For example, the observed gravity field varies as a function of the underlying rock density, and a magnetic survey varies as a function of rock susceptibility. But the 3D relationship is very complex. What we see at the surface depends on the depth and shape of various features, and there are many distributions of density or susceptibility or other rock properties that can explain what we observe.

The process of 3D inversion seeks to produce the most likely distribution of physical rock parameters that explain what we have observed. As the process is non-unique, the ability of the inversion process to produce reasonable results, combined with reasonable constraints, and the experience of the interpreter is important.

How much new information about the Earth’s geology can be gleaned using inversion?

Perhaps the most important advance is our ability to visualize and describe the Earth as it is – a 3D volume of complex geology. The process of 3D inversion describes the earth as a collection of small rectangular cells (typically 5 to 50 metres in size), each with a different physical parameter (density, susceptibility, magnetic vector, etc.). We call this a voxel model of the earth. This is becoming a common model that can be used in a variety of visualization systems and combined with other geological information and interpretive processes. The ability to share these types of 3D geophysical models with other exploration disciplines is key to modern exploration success.

Geophysical interpretation has always been about understanding the subsurface geology. This is not new. What makes Geosoft’s approach different?

Historically, the skills and visualization abilities of the interpreter together with manageable 2-D and simpler 3D tools have been used to target features of interest and help resolve geologic uncertainties. However, the available tools have been limited or very difficult to use. While 3D inversion using voxel earth models has been around for well over 10 years, it has only been accessible to highly-trained specialists with powerful computer technology.

At Geosoft, we’ve focused on making complex techniques simpler and more accessible to all geoscientists. Our latest developments in inversion modelling combine a well-integrated workflow and simplified interpreter interface with network technology that harnesses internet-based servers to perform computer-intensive inversions. The workflow will allow interpreters to efficiently get information into the inversion process, apply constraints and evaluate results. They will be able to easily export and share results with team members. The delivery of inversion capabilities as an online service will give interpreters access to the most advanced 3D inversion modelling capabilities available today, without consuming desktop processing power.

What types of constraints enhance the effectiveness of geophysical inversion modelling?

While the earth can be infinitely complex, when we conduct exploration we generally have pretty good models of what we are looking for and the environments in which we are working. For example, in petroleum exploration we seek petroleum traps in sedimentary geology, and in hard-rock mineral exploration we understand the basic geologic plays – things such as strike and dip of rocks, and the style of complexity for the target deposits, and so on. We also have other information to help guide us – seismic interpretations in petroleum, or surface mapping, depth of overburden and possibly drilling information in mineral exploration plays. All this information allows us to build constraints that help improve the accuracy of 3D inversions.

However, the workflow to define and introduce constraints continues to be a challenge. With Geosoft voxel inversion technologies, we have taken a big step in making this as simple as possible, and in on-going development over the coming months and years we will continue to improve in this area.

How would you describe inversion’s cost and time benefits?

Understanding the time and cost benefit depends on the perspective you’re taking, whether petroleum exploration or mineral exploration.

Petroleum exploration today is really about deep drilling in frontier areas of the world. In these environments the costs of drilling exploration wells can be extremely expensive – 10’s to 100’s of millions of dollars. With such a significant investment at stake, explorers need to apply every possible piece of information to give exploration wells the best chance of success. Of course, 3D seismic surveys continue to be the primary source of information, but increasingly gravity and magnetic interpretations are being used to resolve doubts or verify seismic interpretations. In the end, the best interpretation of the earth explains everything, including the gravity and magnetic field. In petroleum exploration, 3D modelling and inversion around a voxel model of the earth is particularly powerful as this is the way petroleum explorers have become accustomed to thinking about the subsurface. Removing the barriers to integrating gravity and magnetic inversion with seismic interpretation environments is adding to the over-all confidence of exploration decisions, and hence the cost/benefit is well justified.

In mineral exploration, particularly greenfield exploration, one of the important drivers of success is the ability to make good decisions, test the potential of a property through drilling, and move on to the next property. Success over time (among other things) is proportional to the number of targets tested. In many cases, these time limitations have prevented the use of complex 3D inversion interpretations, because producing inversions currently takes a lot of time, and there are a limited number of experts available to do them. As a result 3D inversion has been rarely used. We believe that making 3D inversion fast and accessible to more explorers will change the cost/benefit scenario significantly, and this will lead to more exploration success through targeting decisions that benefit from an improved three dimensional understanding of the subsurface.

What specialized skills and knowledge are required to use the inversion modelling technique?

While our goal is to make access to the technology as simple as possible, geophysical inversion still requires both practical experience and a good understanding of both geology and physics. Having removed the accessibility barriers we will be supporting the education of interpreters through good help systems, on-line training and the development of community knowledge that can be shared among interpreters. Here again is where we see now-common internet technology and collaboration helping a great deal. We want to make it easy and natural for professional explorers to collaborate and share knowledge, skills and information.

How can inversion be incorporated into the early stages of planning an exploration program? How can it be applied to programs already under way?

Once an exploration team has a good idea of the type of target they seek, what we call “forward modelling” of the target is an important step to determine the value of conducting various geophysical surveys. While this is not inversion, this is a capability that comes from the types of systems we are building. An exploration team can create a model of the 3D earth, that they expect to find, and from that predict what a survey would measure. This gives the team the confidence that a particular method is appropriate for what they seek.

As for exploration programs already under way, geophysical surveys – particularly magnetic and gravity surveys - are today already standard methods that provide both direct targeting information and are useful as an aid to general geologic interpretation.  Because this data already exists it is often useful to use 3D inversion of select parts of existing prospects to add to the existing knowledge.  So we have the data – what is it telling us in 3D?