Gravity: It’s heady stuff!

TAG Oil’s Gravity Survey of the Puka permit has been completed.  The purpose of this testing is to measure small variations in the earth’s gravitational field, as little as 0.01%, which the TAG team uses as a tool to interpret the subsurface geology.

Here’s how the gravity survey procedure works:

  • The base plate is pressed into the ground
  • Gravity Meter is placed on the base plate and levelled using the adjustable feet.
  • Once levelled the user lines up a light beam within the cross hairs of the lens.
  • This is repeated four 4 times at each station for accuracy.
  • Time, Location and Elevations are recorded.
  • Field terrain estimations are taken.

Before the data is useful we must take into account the factors that affect earth’s gravitational field.

  • Calculations must be made to correct for: Instrument Drift, Latitude, and Elevation
  • Instrument Drift refers to a gradual change in meter readings over time. This is due to imperfect elasticity of the springs as well as tidal and temperature variations.
  • Drift is corrected for by taking repeat readings at a station several times a day. It’s assumed to be linear and therefore can be graphed simply.
  • Solving for Y results in a drift correction at each station based on the time of the readings.
  • Absolute gravity values for each station can then be calculated by applying the drift curve as well and the known base absolute gravity value to our meter reading.

In Pukatea’s case, we are looking for anomalies in rock units that differ in density from the surroundings. This will help us define an edge of Pukatea’s intended target reservoir and basement rocks.  

The TAG team took 171 survey points, which covered 47.25 km of the permit, focusing on the southern line nearest to the Pukatea Prospect. With this newly gathered data we can now combine it with the gravity data from GNS in order to produce a gravity model over the permit. It may not sound all that exciting, but it’s this kind of careful foundation laying that’s music to our ears.