Gravity is the force of attraction between any two masses; the study of this force is gravimetry; the instruments used to perform this study are gravimeters (or sometimes gravitometers). In the context of the Earth, when we measure the acceleration due to gravity, the Earth is so massive compared to everything else in the picture that there is little room for variance: we must be able to detect small variations on the scale of 1/1000th of a “gal” (the unit of gravitational acceleration named for Galileo, and equaling 1cm/s^2). Variations in density — the amount of mass per unit of volume — in the Earth lead to subtle variations in the gravitational acceleration at different points around the globe (higher density -> more mass -> more gravity and vice versa). These density variations arise from the composition and structure of the Earth, which are of interest to geologists and geophysicists. Therefore, data on the “gravity anomaly” (the difference between the mathematically expected and actually observed gravitational acceleration) at each location we visit is one of the staples of any marine geophysics cruise.
Most gravimeters cannot report the absolute acceleration due to gravity; instead, they display the difference in gravitational acceleration at their location compared to a reference point. This value is calculated by observing the change in the length of a very sensitive, specially prepared spring. Known reference points, where a more bulky and expensive “absolute” gravimeter has been used to take a definitive reading somewhere that the gravity anomaly is likely to remain constant, are known as “gravity ties”. We took a reading with our portable gravimeter at one such tie in Honolulu. This reading, and another taken a few miles away at the pier where the Sikuliaq was fueled, can be used to calibrate the shipboard gravimeter and calculate the absolute gravity anomaly as we travel.
The ship’s gravimeter takes a reading every second. These data are susceptible to many sources of noise, such as the movement of the ship, and other factors which cause the gravimeter to report accelerations including influences that we would prefer to isolate. These corrections can be applied using some simple formulas which yield coefficients that when multiplied by the raw gravity reading, account for these various effects. For example: the Earth is not a perfect sphere, but rather an “oblate spheroid” (almost spherical, but wider around the equator than it is tall along its axis). This means that when travelling north or south, changing latitude, you are drawing slightly closer to or further away from the center of the Earth and consequently changing your distance from its gravitational center. The distance separating two masses affects acceleration due to gravity, and therefore this must be accounted for in the “latitude correction”.
For a ship moving about the globe, the east-west direction also influences the observed gravity. When traveling with the direction of the Earth’s rotation (east), you experience a greater centrifugal acceleration than normal, which if uncorrected would make the gravity reading appear too low (and conversely, going west inflates the observed value for the same reason) . This is called the “Eötvös effect” and can be corrected with a little trigonometry. Yet another adjustment, the “free air correction,” accounts for the additional vertical distance between the gravimeter and the radius used in the latitude correction.
In practice, applying these corrections involves taking the text files dumped by the instrument to the shipside network and running some FORTRAN or MatLab code to produce the fully corrected result. Finally, these processed data can be used to plot the gravity anomaly on maps.