Introduction To March Focus: Back to Basics

I was walking down the street one day, thinking about how I would explain to someone how geophysicists use gravity to explore the subsurface. We measure slight changes in gravity along the surface of the Earth. Of course, like seismic data, we have to process the raw data to remove the effect that topography (like hills and valleys) has on the gravity measurements. Elevation and latitude also affect gravity, since gravity varies with distance from the center of the earth and the Earth is shaped more like a clementine (a slightly squashed ball; the actual shape is called an oblate spheroid) as opposed to an orange. We’re primarily interested in the ore body, fracture zone, or channel system that we want to learn about – whatever geology might be changing the density of the subsurface volume.

“It’s a force field”, I mutter to myself. “Hmm… wait. Gravity isn’t a force. It’s an acceleration. We know that from the math, specifically Newton’s Law of Gravitation.”

My muttering is getting louder as I recall details from university, and an episode from The Big Bang Theory television show. I’m starting to get looks from other people on the sidewalk.

Even though I’m slightly embarrassed by my audible muttering, I’m also getting excited. As I recall my education and field experience, I am also recalling the excitement of learning about earth and physics. Fun, fun, fun.

Like any knowledge or skill, if you don’t use it, you lose it. And, I’ve forgotten quite a bit. However, each time I review geophysical concepts, I learn those concepts a little better than I did last time. I gain knowledge as I add new experience to memorized theory.

The idea for this FOCUS came from a member who replied to one of the CSEG’s member surveys concerning the RECORDER. So, thanks to that member who let us know, and who gave us the great title “Back to Basics.” Many other members also asked for technical content that was a little more basic than usual, and for articles from new authors. So, thanks to everyone. I don’t know if we’ve been successful at meeting your requests, but hopefully, we’re on the right path.

As I searched for submissions, I asked each potential author if there was some knowledge that they 1) wished to review for themselves by writing about it, 2) felt that fellow colleagues would benefit from learning, or 3) thought new members to the geophysics community should know as they enter industry. These questions leave a lot of wiggle room as to the technical level of the articles, and the available topics.

Adam Thomas, in his article, “Datums, Projections and Coordinate Systems”, explains how the surface of the Earth is calculated, and locations on its surface determined. The NAD27 and NAD83 datums are described, and the differences between them illustrated using colour maps. Datums are just one component used to determine well locations, exclusion areas, or survey line locations on the Earth. Projections and coordinate systems are also necessary.

Carl Reine has expanded his GeoConvention extended abstract, “A Rock-physics Tutorial”, (2015). He describes a quantitative interpretation process to predict rock and fluid properties of the subsurface, and explains the additional information output from the various modeling steps. He connects the rock properties to the physical properties sampled by seismic and well data.