by MICHAEL MACKENZIE, GSNI
Antrim’s Gravitational Pull
GEMINI’s Gravity Survey Underway
17 July 2025
It’s been a busy summer of fieldwork for
the GEMINI teams in County Antrim with three separate geophysics surveys
currently on the go in the area; in addition to the two surveys led by DIAS,
magnetotellurics and passive seismic, GSNI geologists have been on the ground
undertaking a gravity survey, rounding out the fieldwork scheduled for this
summer.
Images - Top: Alex from GSNI recording gravimeter readings during the GEMINI gravity survey in County Antrim; Bottom: Local wildlife has been taking a keen interest in the gravity survey, but we’ve had to politely decline when they try to hitch a ride with us.
What does a Gravity Survey measure? While unnoticeable to us in our day to day
lives, Earth’s gravitational field isn’t uniform: it changes at different
locations across the planet’s surface. These variations are tiny and can only be
registered on highly sensitive specialist geophysical equipment.
These variations can be caused by differences
in the subsurface, such as densities of different rock types buried underground,
with denser rocks (e.g. basalt) exerting a stronger gravitational pull than
less dense rocks (e.g. limestone). By conducting a gravity survey, we can
measure these variations and make inferences about the different rock types and
structures beneath our feet, and how they could be used for geothermal energy.
The actual measurements are taken by a
device called a gravimeter - there are a few different types of
gravimeters, but we are using a spring-based model which contains a mass
suspended by a spring. Gravitational force pulls the mass down and stretches
the spring, which is then measured to determine the gravitational acceleration
at that location.
What does the GEMINI Gravity Survey
involve?
The survey will collect measurements at 606
sites across an area of about 393 km2, extending from south of
Antrim to north of Ballymena, plus numerous repeated measurements to ensure
quality control of the data we collect. Before we commenced collecting site
measurements, we established two base stations within the survey area.
Measured several times throughout the day, these base stations act as a control
that help minimise the effects of external factors (see below) on our collected
data.
On each day of the survey, the team acquires
data in the following pattern:
measurement at a base station
before starting a new loop of measurements,
measurements of planned sites
in the survey area,
repetition of a site
measurement from the previous loop (for quality control),
return to local base station to
take a measurement to close that loop and begin the next one.
In addition to the measurements taken using
the gravimeter, we also use a highly accurate GPS to record the coordinates
where the gravity measurement was taken, and a thermometer to record the
temperature at the time of measurement. These help with quality control and
data processing.
We typically take measurements along flat
roadside verges, trying to take measurements away from features like
underground services and during quiet times with less traffic, as these can
affect the accuracy of the gravimeter’s readings.
We take multiple readings at each site
location, which takes about 10 minutes, to ensure that readings are consistent
and therefore accurate. Once this is complete, we pack up our kit and move to
the next site, leaving no trace on the local environment.
The GSNI team has been surveying every
other week since mid-May and is due to complete data acquisition by early
September 2025.
did you know?
The
gal is a unit of acceleration used in gravimetry. It is 1 second per
centimetre squared (1 cm/s2); a milligal (mGal) is one-thousandth of a gal.
