Sunday 22 November 2009

Interview with Dr. Randy Marrett

If you regularly read this blog (I guess that is something boring, because I don't regularly upgrade it...), you have noticed that I have changed, once again, the layout of the page. Well, this time is quite a definitive change, I like it now, finally. It is clear and neat. Please, allow me a few days to make all work properly!.

I have thought that it would be interesting to put in the right side bar some videos from time to time related with structural geology. I will start this series of videos with an interview with Dr. Randy Merrett, associate professor in the Department of Geological Sciences at the University of Texas.

As the introduction of the video states:

Dr. Marrett's research concentrates on deformation processes in the upper continental crust where folds, faults, and opening-mode fractures are the most important products. This work is applicable to a wide range of human concerns such as natural hazards (e.g., earthquakes, volcanoes) and the flow of fluids through rock (e.g., hydrocarbons, water, contaminants). Some common themes that relate his disparate interests are quantitative field observations and analysis, especially using techniques that address spatial and size scaling of structures. Dr. Marrett currently has on-going projects that address active faulting and geyser eruption patterns in the Central Andes of Argentina and Chile, detachment folding and curved orogenic belt development in the Sierra Madre Oriental of Mexico, and opening-mode fracturing in numerous areas.

Quite basic, but always useful for forwarding it to people who want to know more about what we do.

Enjoy it!

Saturday 21 November 2009

Geo-Logic Systems, bought by Halliburton

Halliburton has recently acquired Geo-Logic Systems, the producers of Lithotect, the balancing and restoring software. It has been announced in several E∓P websites, and also in the corporative websites of both companies:

On Tuesday, October 27, Halliburton announced its acquisition of Geo-Logic Systems, LLC. “The industry faces increasingly difficult exploration and drilling scenarios,” said Paul Koeller,Hal liburton’s vice president of Software and Asset Solutions. “The integration of Geo-LogicSystems offers ge oscientists advanced modeling solutions to address the technical challenges of exploring in complex petroleum regimes such as overthrust belts and the pre-, syn- (including sub-salt) and post-ri ft portions of extensional basins. In these complicated, poorly imaged plays risk reduction is of paramount importance.”

That is probably a big step for Lithotect, a great piece of software. Now, being part of such a huge company like Halliburton, surely the rivarly with MVE's Move (formerly know by its different components: 2DMove and 3DMove) will tight up, and this will be clearly in benefit of the customers.

Halliburton owns already the giant of seismic software, Landmark, and this acquisition points out to the direction that E&P is taking in the last years: Structurally complex scenarios, fold and thrust belts, deep water belts, sub-salt environments, etc, which need serious expertise on structural geology and risk reduction. Well, let's revisit our notes and books because there is a lot of hard work waiting for us...

Monday 2 November 2009

Recording planes with strike or dip? The problem of the right-hand rules

When we want to record the orientation in space of a geological surface (e.g. a stratigraphic body), we have two different ways of doing it:
a) Dip direction and dip angle: Dip direction is just the direction toward which the plane is inclined. Dip angle is the angle of inclination of the plane. In the example included in this article, we would write it down as 090/45; the plane is inclined 45º towards the east (090º). Easy.
b) Strike and dip angle: Strike is the direction of the imaginary line which would represent the intersection between the plane and a horizontal surface. But here it comes the problem: the strike, as we know, can be represented by two different conjugated angles, and we need to follow some kind of convention; typically, the strike is expressed as the acute angle between the intersection line and the north direction. In our example, it would be 0ºN. But how which convention we take for indicating towards which direction is dipping the bed? For that we use the right hand rule.

There are two different right hand rules... Did you know that? Oh, that is not very convenient...
  • American right hand rule: Looking to the strike direction, the bed dips to the right. In our example, that would fit with 000/45. So, looking to the north, the bed dips to the right, to 090 (east).
  • British right hand rule: The thumb of the right hand indicates the dip direction, and the heal points to the strike direction. So in our example, we would record the orientation as 180/45. That is: Strike towards 180 (perfect south bearing), and dip of 45º towards 090.
Usually, if the person who took the measurements knew that other people MAY HAVE different criteria expressing the same results, the data can be recorded as


Which would mean, no doubt, a strike of 180 and a dip direction towards the east, with a magnitude (dip angle) of 45. If you use strike, please remember to write always as a companion of the dip angle the quadrant where the dip direction would be. For example, 045/56SE. In this case we are talking of 135/56 (dip direction/dip angle). 045/56NW; now we would be talking of 315/56 (dip direction/dip angle).

But what happens with measures like:


What does that mean? I have seen it. Does it mean 45º from north towards west, so 315? or does it mean strike of 045 and an a dip angle of 34 towards W? Is that? Uh, I don't think so.

The truth is that, besides right-hand rules, many geologists use other conventions, sometimes personal ones, and maybe in a wrong way. Perhaps using only NW and NE quadrants for the strike. Perhaps not.

It is clear that using strike values when representing the orientation of a bed we are introducing a risk factor if we don't state clearly which right hand rule we are using. Of course, for some people in some situation the usage of strike/dip angle may be very useful:
  • When we speak of general trends in a big area (e.g. an orogen, in order to show that a trend is parallel or not to the structure).
  • In a mine, in order to visualise quickly if the strike direction is the same as certain mineralisation or lineaments.
  • In engineering geology, for the same reason, in order to know if the bedding/joints have the same orientation than a well, a tunnel, etc.
I would suggest another right hand rule... If you use strike for describing the attitude of a plane, your right hand should be cut!.
But perhaps that is very extreme... so far.