Eotvos (unit)

The eotvos is a unit of acceleration divided by distance that was used in conjunction with the older centimetre–gram–second system of units (cgs). The eotvos is defined as 10⁻⁹ galileos per centimetre. The symbol of the eotvos unit is E.^[1][2]

In SI units and in CGS units, 1 eotvos = 10⁻⁹ second⁻².^[3]

The gravitational gradient of the Earth, that is, the change in the gravitational acceleration vector from one point on the Earth's surface to another, is customarily measured in units of eotvos.

The eotvos unit is named for the physicist Loránd Eötvös, who made pioneering studies of the gradient of the Earth's gravitational field.^[4]

In free space, the gravitational gradient tensor has trace 0 by Poisson's equation, so the sum of gravity gradient along any 3 perpendicular directions is 0.

Near the surface of earth, the equipotential surface is nearly spherical, following the near-spherical shape of earth. Thus, the gravity gradient tensor has its 3 principal directions: a tensile direction along the local vertical direction, and 2 compressive directions perpendicular to the local vertical direction. The vertical tensile component gradient is ~3,080 E (an elevation increase of 1 m gives a decrease of gravity of about 0.3 mGal). For a perfectly spherical earth, this value is theoretically ${\displaystyle |\partial _{r}g|={\frac {2GM}{R^{3}}}\approx 3080~\mathrm {E} }$. Since the equipotential surface is nearly spherical, the two compressive gradients are roughly 1/2 of that, at ~1,540 E. Gravity gradiometry usually measures the perturbation away from this ideal value.

The effect of Earth's rotation creates an acceleration value of ${\displaystyle \omega ^{2}R\cos \phi }$, where ${\displaystyle \omega }$ is its angular velocity, and ${\displaystyle \phi }$ is the latitude. This perturbs the measured gradient. The maximal perturbation is obtained at the equator, with a value of ${\displaystyle \omega ^{2}\approx 5~\mathrm {E} }$.

Geological formations can modify the gradient. A mountain range, or an underground formation with increased density, is a large amount of mass that increases the tensile component of the gradient, and tilts the direction of tensile component towards the mass. An underground formation of decreased density, such as a salt dome or an oil formation, has the opposite effect.

In general, the gradiometric perturbation of a structure falls off as ${\displaystyle d^{-3}}$, so a structure of characteristic size ${\displaystyle r}$ buried ${\displaystyle d}$ underground produces roughly the same amount of perturbation at the ground surface as a structure of characteristic size ${\displaystyle kr}$ buried ${\displaystyle kd}$, for any ${\displaystyle k>0}$.

The Eötvös torsion balance was used in the exploration for oil and gas reservoirs during 1918–1940 period. It could achieve 1–3 E in accuracy, but requires up to 6 h per station.^[7] Modern (2013) airborne systems can reach 3–6 E in rms accuracy at integration time of ~6 seconds. Such a system achieves higher spatial resolution on slower aircrafts, so the gradiometry maps have higher spatial resolution on the zeppelin and the helicopter than a fixed-wing aircraft. Spatial resolution is also higher for lower-flying aircrafts.^[8]

• Gravimetry

• Gravity gradiometry