Fig 1.
Schematic diagram of a borehole gravity sensing survey.
In typical borehole gravity sensing, the sensor is lowered or raised to measurements points (denoted as 1 and 2) at fixed distances apart and measurements are performed when the sensor is stationary, with the data collected sequentially in time (ti, where i = 1, 2).
Fig 2.
Laser system and science package diagram.
a) Schematic diagram of the laser system and the science package (not to scale). b) Arrangement of the optics used to generate and measure the atom cloud (vacuum walls not shown). c) Photo of the science package. The vacuum system, optics and measurement photodiode are contained within the watertight stainless steel casing shown. The top of the science package is connected to the umbilical, which delivers the light and electrical signals required to operate the science package.
Fig 3.
Borehole where the survey was conducted.
a) The location of the test site and route driven to the test site (shown by the blue dashed line). Map data from © OpenStreetMap under the Open Database Licence [53]. The surface of the borehole used for the deployment campaign, is shown in the inset photo (red border). b) Diagram showing the dimensions and lithology of the borehole used in the deployment campaign. The lithological data was obtained from geophysical logging (natural gamma and electrical resistivity), combined with core examination. c) The winch and tripod system used to lower the science package into the borehole. A wire with markings at 1 m intervals was used to record the depth of the device.
Fig 4.
Variation of the atom number at different depths.
The atom number variation with time, relative to the first measurement in each time period, on the surface and at depths of (25.34 ± 0.02) m and (49.34 ± 0.02) m in the borehole. The inset photo shows a photo of the cloud on the surface with a radius (1/e2) of (0.81 ± 0.08) mm in the x-direction and (1.1 ± 0.1) mm in the y-direction.
Fig 5.
MOT parameters as a function of depth in the borehole.
The atom numbers and loading time constants were determined by fitting equation 2 from reference [55] to loading curves taken at each depth, with the leading source of error attributed to the voltage to atom number conversion. The depths were measured with an error of ±0.02 m.