Ever experienced to stumble and tumble?
The reason is Gravity (and maybe clumsiness in the first place)!
Gravity is one of the four fundamental interactions of nature. Objects with mass are attracted to each other and the amount of attraction is proportional to their masses and the inverse square of their distance.
Considering the size of earth and all its mass there is very strong gravitation or a gravitational field, being the reason that we are attracted to earth. (There are other attractions like love who don’t belong here…) Now Gravimetry is the field of Geophysics that measures the gravitational field. You might say ‘I heard that earths gravitational acceleration is g=9.81 m/s2!’ But fact is that depending on mass variations in the subsurface, the latitude and the tides this value varies slightly. With Gravimetry we are able to measure these small variations which can be smaller as 10^-8 m/s² or 1 µGal (Often the Unit Gal is used with the definition 1 Gal=1 cm/s²). A couple of corrections are made to the measured values since one wants to have a field that is constant in time and referenced to a constant field. Usually the reference ellipsoid of the WGS-1984 coordinate system is used. The corrections are in this order:
Drift correction for the Gravimeter which tries to capture drifts induced to the device by temperature and material changes. However most devices already reduce this to a minimum by an added cooling unit. The occurring drift is can be derived by measuring at a fixed base point at the begin and at the end of your survey.
The tidal correction i.e. due to the movement of the moon around the earth; While the gravitational field of the moon affects and interacts with the gravitational field of earth and sun it moves especially the water masses. These effects have to be removed since they are time dependent and distort the actual information of the local field, that we want to have. Luckily these changes can be measured/simulated and therefore removed. Fun fact: There are points on the earth where the change due to tides is zero over time.
Due to the fact that the earth is no perfect sphere (it is flattened with approximate equatorial radius of 6,378.1km and polar radius of 6,356.8km) the gravitation increases with increasing latitude since on gets closer to the earths gravitational center (Gravity is proportional to the inverse distance squared). Usually the effect is a change of 0.8mGal/km along the latitude. Since we want to have a map of anomalies independent of this effect a latitude correction is done.
Similar height dependent issues occur locally if strong topography is involved. Because all values should be related to the same horizontal plane a free air correction is done, removing a gradient of 0.3086 mGal/m * height from each measurement to get at sea level. For this correction the additional measurements of heights at each point are usually done with a good GPS system (i.e. differential) to get as accurate as one cm.
The (last) step is the Bouguer correction which refers to the correction of mass induced changes due to height differences. A topographic density model is needed for this correction. But usually a density of 2.67 g/cm³ which is the density of granite are used.
Having all these corrections done, one is able to map anomalies independent of time effects and referenced to one horizontal plane (There are other corrections like the Eötvös Correction i.e. which are only important in special cases).
Now back to the start, having the anomalies plotted we are having an idea of density inhomogeneity in the subsurface. Different sizes in area and amplitude of anomalies give us information about the depth and the material that causes anomalies. Like smaller anomalies which could be possible resource deposits or cavities or large anomalies which are a sign for deeper crustal anomalies.
In the second part I will go more into detail about the device of the Gravimeter itself and other special corrections.
Have fun experiencing gravity!