Do Earth Tides Change Gravity Down a Borehole?

The Challenge

Earth tides are a familiar part of gravity measurements at the surface presenting themselves as oscillations of ~100–200 µGal every 12 and 24 hours, caused by the gravitational pull of the Moon and Sun. But what if your gravimeter is deployed kilometres down a borehole?

Does the surrounding rock shield the signal? Does the amplitude diminish with depth? And what about ocean loading? Does it vanish below ground? These are important questions for anyone planning long-term monitoring underground.

Earth Tides in Context

• Cause: Gravitational attraction of the Moon and Sun deforms the Earth’s crust by tens of centimetres.

• Effect on gravity: At the surface, the main lunar component is about 110 µGal (rigid Earth). Accounting for Earth’s elastic response (via Love numbers), the amplitude is closer to 128 µGal.

• Predictability: Tides follow precise astronomical cycles (semi-diurnal and diurnal), and models can compute their contribution to within a few µGal.

Do Tides Diminish Underground?

The tidal potential scales with distance from Earth’s centre.

At depth z: Δg(z)/Δg(0) ≈ 1 – z/a

where a ≈ 6371 km (Earth’s radius).

• At 2 km depth, the amplitude reduces by just 0.03% — around 0.04 µGal on a 128 µGal tide.

• Even at 10 km depth, the reduction is <0.2 µGal.

• There is no shielding by the overlying rock; the tide is a planetary-scale phenomenon that penetrates through the crust essentially unchanged.

What About Ocean Loading Underground?

Ocean loading is different from body tides. It arises from:

1. The direct gravitational pull of moving ocean water, and

2. The elastic flexing of the crust beneath that load.

• A gravimeter in a borehole still “feels” both the attraction of nearby water and the crustal deflection, as with the body-tide it is almost the same as at the surface.

• Going down a few kilometres does not remove ocean loading. The signal persists through the crust, because both the gravitational attraction and the deformation act on the whole solid Earth.

• The only reduction is the same geometric factor (1 – z/a) that applies to body tides, which is negligible at km-scale depths.

Practical Applications & Best Practice

• Borehole Gravimetry: SMGs downhole MEMS gravimeters will record the same tidal signatures as at the surface.

• Drift Correction: In long-term monitoring, the tidal cycle can be used to track instrument performance at depth.

• Treat underground tide signals as if you were at the surface - no special correction is needed beyond standard tidal and loading models.

• Use standard tidal prediction tools with optional ocean-loading corrections for coastal or offshore sites.

Conclusion

Earth tides remain robust and essentially unaltered underground. At 2 km depth, the signal is reduced by just 0.04 µGal, far below the noise floor of any gravimeter. Ocean loading, also, does not vanish with depth: it must still be considered near coasts and offshore. So, for most exploration or monitoring applications, you can be confident that the pull of the Moon and Sun follows you down the borehole!