Introducing Gravity Gradiometry TechnologyTue, 01/22/2013 - 12:46
Geological data acquisition is a vital step in determining the presence of subsurface hydrocarbons, and one of the most revolutionary data acquisition technologies is the gravity gradiometry procedure, which Bell Geospace’s founder brought from defense contractor Lockheed Martin. Scott Hammond, the company’s President and CEO, believes that since the company has successfully introduced full tensor gravity (FTG) gradiometry to the industry, operators such as Pemex will be more open to using it as a complementary technology for complex exploration projects, particularly in the Gulf of Mexico.
“The Gulf of Mexico’s seabed is full of salt,” Hammond explains. “While seismic technologies do a very good job of imaging flat salt layers, gravity gradiometrybased technologies are better at dealing with complex salt modeling studies, which include steep dips and the vertical faces of the salt.” Even though this might suggest that gravity gradiometry is a better fit for acquisition projects in the Gulf of Mexico, Hammond hints that the combined use of both technologies produces a more comprehensive study that minimizes the shortcomings of each individual method.
With that in mind, the technology by itself is excellent for modeling complex salt regions. “We all think of gravity as that force that pulls us towards the center of the earth. That is oversimplifying it. What really happens is that we are being pulled by the mass of the objects that are around us, so the force with which it pulls will change according to the mass of the object underneath; a heavier object will pull harder than a lighter one. However, that change in force is minimal,” Hammond clarifies. “Gravity is composed of the X-component, the Y-component, and the Z-component. There is change in each of those axes as an object moves in three dimensions. That adds up to nine components that comprise the tensor field that gradiometry is able to measure. Going through the LaPlace equations, we can prove that only five of those components are independent. Our FTG technology measures those five independent gradient components plus one extra component. This helps to get rid of all of the noise that comes from the surroundings.”
Bell Geospace can acquire its FTG surveys by both air and sea, using the only gravity gradiometer available in the market. “Air surveys are usually more e·cient, since the speed at which the aircraft moves allows us to cover bigger areas in a shorter amount of time. But the amplitude of the signal will drop o due to the lower sampling rate (speed) and as the airplane gets further away from the source,” Hammond elaborates. “In these cases, we sometimes recommend carrying out a marine survey, which has proved to be very successful at modeling targets that are well below 10,000m, even in the Gulf of Mexico.”
Considering the complexity of the Gulf of Mexico’s geology, gravity gradiometry presents itself as a solution to measure its changing patterns. However, that is not all the technique can achieve. Another interesting potential application for the Mexican market consists of surveys in regions where access to the survey area is not easy. “The aerial version of FTG can be used to produce data from areas where you cannot get to the ground,” Hammond explains. “Bell Geospace provides airborne surveys, flying over rough terrain and acquiring the geological data. Once the data has been obtained, Bell Geospace processes it almost in real-time which removes the noise and enhances the separate tensor components. This helps clients to see results almost immediately and expand the survey or concentrate on specific zones within the region being studied.”
Overall, the use of complementary data acquisition technologies provides companies with a better picture of the subsurface geology that they are studying. “Some objects have density contrast, others are magnetic, and some share both characteristics. If several technologies are used to measure these properties within the same area, it will be easier to determine what is out there,” Hammond concludes. “Gravity, magnetic, and electromagnetic signatures will each describe some characteristics of the objects under study but are based on dierent physical characteristics. Therefore, if you combine all those scans, it will be easier to paint a picture of the underground features that are being studied.”