Looking inside the planet Capturing the underground with geophysical measurements
Crude oil and natural gas rarely reach the earth’s surface by themselves. Most reservoirs of these natural resources are hidden at depths of between 500 and 5,000 meters. Hence, to find them, one has to be able to look inside the planet – a challenge that can only be mastered with the help of modern technology. Geologists use findings from geophysics, the science that deals with the composition of the earth and the processes in the earth’s interior, for their own purposes.
One possibility of capturing the inside of the earth’s interior is offered by the earth’s magnetic field. The rock strata underground vary in their magnetic properties, and thus they also influence the strength of the magnetic field at the earth’s surface greatly. The field of geomagnetics is based on this finding. With this method, the fluctuations are measured with the help of so-called “magnetometers” and thus give us information on the characteristics of the ground.
The magnetic field around a reservoir, for example, is weaker: the sedimentary rock containing oil and gas is less magnetic than rock of volcanic origin. With the geophysical measurements method, airplanes search for such fluctuations in the earth’s magnetic field. They fly over the areas in question pulling a magnetometer behind them on a cable. Then maps are made from the data collected. They indicate the ground’s composition as well as which characteristics and rock types it has.
Gravity makes exploration easier Mass matters
The earth’s gravity can also tell us something about how the ground under our feet looks like. Like the earth’s magnetic field, the level of gravity is not the same everywhere. The reason is the density of the rock layers: if the density of one type of rock is very different to that of the surrounding rock, the gravity at the surface is also weaker or stronger. The technique of gravimetric analysis uses this information.
For example, gas and oil reservoirs are often found under salt layers. Since salt has a lower density than many types of rock, a lower level of gravity can be an indication of salt reservoirs deeper down – and thus point geologists towards the resources they’re looking for. These minuscule deviations are measured with a gravimeter. These instruments are extremely sensitive scales carried on board planes that measures changes in gravity over a large area.
A combination for success Gravimetry and magnetics complete seismic
In recent decades the methods for measuring the earth’s magnetic field and gravity have become increasingly advanced. Since it is now possible to measure them from an airplane, this enables very large areas in which hydrocarbons are believed to exist to be studied and analyzed. This gives the Wintershall experts important information about the subsurface and enables them to generate geological maps in which the various types of rock or the thickness of the rock strata, for example, are indicated.
This provides such valuable information particularly when the results are combined with the data from other methods – such as, for example, seismic data. The results of the gravimetric analysis and the magnetic field measurement can be as helpful for producing seismic models as for interpreting the data acquired by the seismic surveys. Furthermore, in places where reflection seismology is only possible on a limited basis – for example at reservoirs under salt layers – both these methods are vital.