Posted: November 28, 2010
An important part of natural gas development is seismic surveying. Seismic surveys provide the data that geologists use to model the Earth’s subsurface conditions as deep as two miles or greater. In Pennsylvania, geologists are using these surveys to characterize or “map” the Marcellus and other shale targets.
Seismic testing does not specifically tell geologists whether or not there is gas present but does help identify the better areas for potential well sites. The data also provide a picture of what to expect when companies decide to drill such as whether there are faults or other geohazards present, so these can be avoided during drilling. It also tells them the angle they need to be drilling the horizontal lateral for maximum effectiveness during the process.
There are two kinds of seismic surveys—2-D and 3-D. Two-dimensional seismic surveys use variable signal frequencies to “shake” the ground. Vibroseis or “thumper” vehicles send seismic pulses into the ground that are reflected off the various geologic layers and features. 2-D surveying is mainly done along public roadways with permits from PennDOT or the appropriate township. The trucks run in a series of 3 or sometimes four with the related ground crews for cabling and traffic control. Vibrations are not normally detected 30 feet away from the truck when it is working.
3-D surveys typically employ charges set off in shallow borings to produce seismic reflections. Collected by an array of geophones, these reflections are modeled to form an image of the subsurface. There is very little surface disruption, and companies are willing to work with landowners. The explosive charges used for 3-D are generally small (approx. 2 lbs. of dynamite) and placed 20 feet in the ground. ATVs are often used in setting up for 3-D surveying, and in remote areas, helicopters may be used to transport the cables and equipment needed.
Companies doing seismic normally are under contract to one or more energy companies to conduct the study. It is very expensive and is a multi-million dollar effort for a most 3-D shoots. This is not a random activity. Seismic companies sometimes will do the studies independent of an energy company with the hope of selling the very valuable information on the open market to one or more companies. Energy companies will sometimes pay for seismic, then decide against an area, and later sell the seismic information to another energy company with a different point of view on the potential geology. Because the information has cost to derive and value to sell, it is not given freely to landowners even if their land was included. It would be very difficult to understand except to a trained geologist. It is not placed in the public domain due to the companies having paid for the information.
So why all the seismic activity in PA? Energy companies with large Marcellus acreage are looking at the best places to exploit the shale and retrieve the natural gas at the lowest cost to produce. They are looking for shale thickness, geologic faulting, and confining layers of other types of rock as well. And they are also looking for other shale targets. Much has been spoken about the Marcellus because that is what is driving the economics and what will get the infrastructure built in many parts of PA. But there are other shales and the Trenton Black River formation in areas. Although they might not have been commercially viable in the past, they could become so with new pipelines, compressor stations, and water facilities built nearby based on Marcellus economics. The confining layers are important for drillers to know because that is what contains the frac energy during that part of the process. Faulting is generally seen as a negative because it could allow the frac energy to be lost without breaking the target shale rock. Seismic can also potentially discover PA geology to allow deep well injection of the waste water and several companies are intensely working on this issue, some are even now leasing acreage they feel is promising.
Do seismic companies need landowner permission to conduct seismic testing? Companies leasing the gas rights may have the right to do the seismic testing as part of a signed lease agreement. It is advisable to consult an attorney to learn what is and what is not covered in a lease. If it is not covered, there are pros and cons for allowing the testing. Leasing rates are in the $5/acre range in PA. An attorney that has been working on much of this advises the use of addendums to protect your unique interests and also suggests a limit on the time to conduct the study vs. open ended agreements. Many existing leases already allow seismic testing. If you don’t allow seismic, it creates a hole in the map. Companies don’t like holes and won’t likely be back to fill them at a later date due to the cost. If the hole is small enough it doesn’t really matter because the map will still give an overall picture with enough data for a company to make a decision on where or where not to drill and where to place the well pad. If you do allow the seismic, your parcel will have data on it for future consideration. If your goal is a gas well, you probably want to be in the seismic shoot. But what if they do the seismic and they find geologic conditions, i.e. major faulting, which is problematic for drilling? They will likely not be interested in continuing work in your area and move on to other more promising areas. The value of your lease could diminish greatly or drop to zero but this is more likely an issue for larger parcels vs. smaller ones. If they do the seismic, especially 3-D, and come back to you for leasing, it is likely you are in an area with a higher probability of commercial success. Your opportunity to negotiate better lease terms and value could potentially rise. Always remember that although millions of acres have been leased, not all will be drilled on. Seismic will determine largely in the end, where much of the drilling activity will occur.
Written by Tom Murphy, Penn State Marcellus Center for Outreach and Research