Gravimetry, magnetics and electromagnetics

DEA makes use of various geophysical measuring methods that enable us to x-ray the upper layers of the Earth. All these methods have one thing in common: they react to variations in the composition of rock layers. We use a combination of several methods rather than limiting ourselves to a single method. In this way we can eliminate ambiguities and improve our subsurface model. The results of our measurements form the basis for successfully exploring for oil and gas.

Measuring the earth's gravity with a gravimeter

Gravimetry: When gravity becomes a tool

Gravimetry was one of the first geophysical methods to be used in exploring for oil and gas. As the name suggest, this method uses accurate measurements of the Earth's gravitational field to locate horizontal and vertical variations in the density of subsurface rocks. The precondition is that the geological structures have to be distinguishable through clear differences in density. This is the case, for example, in underground locations where there are salt domes, e.g. in the Norwegian Barents Sea. Salt usually has a much lower density than the surrounding rocks and thus generates negative gravity anomalies. Salt dome rims and overhangs are great traps for oil and gas.



Gravimeters measure the density of the rock underground from the surface

The devices used to take such measurements are known as gravimeters. They have become significantly more powerful in recent decades and today's portable devices can come up with a reading within minutes. Gravimeters, which basically work like a pair of ultra-sensitive scales, are employed on the surface and from ships and aircraft. If measurements are taken on the surface, the speed with which an area can be surveyed depends on the terrain. Every measuring point has to be precisely positioned in terms of its exact geographical location and altitude, and each time the instrument has to be carefully calibrated.

Gravity map detail

Increased level of difficulty: Measurement at sea and from the air

Gravimetric measurements at sea require a gyro-stabilised platform that compensates for the ship's movements in the swell. For several years now, satellites have also been used to measure the Earth's gravitational field. Aerial and satellite-based methods have made gravimetric measurements an interesting option for sensitive transition zones in shallow water and remote regions that are difficult to access.

Magnetics: In search of weak magnetic fields

Magnetics and gravimetry are both passive geophysical methods, i.e. they only measure what actually exists, i.e. the Earth's gravitational field, and do not themselves emit any signals, as is the case with seismics.

If we discover variations in the Earth's gravitational field that are not due to external factors, conclusions can be drawn about the existence of magnetic subsurface rocks. Volcanic deposits, for example, generate strong positive variations. And if variations of sufficient magnitude are discovered, they may point to the possible presence of oil and gas deposits. The magnetic field around an oil or gas reservoir is weaker because the sedimentary rocks in which oil and gas are deposited are less magnetic than other rocks.

Investigating magnetic fields from the air

Many years ago, terrestrial measurements were replaced by aeromagnetic surveys where specially equipped planes or helicopters measure the magnetic field in a specified area – with no impact on the environment. Aeromagnetics makes continuous recording of the magnetic field possible – irrespective of any surface factors. Desert, jungle, water – whatever the environment, it can be easily and quickly surveyed.

The measuring and recording device is mounted in the tail of the plane or helicopter, or hung from the aircraft on a cable while the pilot flies over the area to be surveyed. After the results have been evaluated, it is possible to draw up maps that point to specific geological structures and types of rocks. Aeromagnetics is also the preferred method if large areas have to be surveyed in a relatively short time for preliminary exploration purposes.

Electromagnetics: Characterising sediments by means of conductivity

This method is used to investigate how electrical conductivity is distributed under the surface of the Earth. Electrical conductivity, or its reciprocal value electrical resistance, is an important parameter for characterising sediments. Natural hydrocarbons, such as oil and gas, and gas hydrants are of low conductivity. Such accumulations in the pores of rocks increase the overall resistance of the rock formation.

Sensor for measuring electromagnetic fields

Changes in electrical resistance indicate different rock strata

Variations in electrical resistance thus allow our specialists to determine the presence of underground stratifications and various rock parameters as well as to investigate water-bearing strata and pore structures. In this way, it is possible to determine the existence of oil and gas deposits with a relatively high degree of accuracy. For such measurements DEA also makes use of the active controlled source electromagnetic method (CSEM) involving an electromagnetic transmitter and the respective sensors. In CSEM electromagnetic signals are transmitted by specially designed equipment, just like seismics. Seismic and electromagnetic methods penetrate the subsurface to similar depths and are increasingly being used in tandem. Whilst seismic methods record the structural boundaries between different strata, electromagnetic methods supply information about the volumetric characteristics.