Inside the 3TB
6 Inside the 3TB
6.1 The sensors
The horizontal and vertical sensors are similar in design. The inertial mass in both
cases consists of a transducer coil and a leaf-spring suspended boom which swings
on a frictionless hinge. A triangular spring supports the weight of the mass; in the
vertical sensor this spring is pre-stressed, with a natural period around 0.5 s, whilst
the horizontal sensor has an unstressed flat spring with a natural period around 1 s.
3TB sensors have no spurious resonances below 140 Hz, and weigh around 180 g.
The small boom size and stiff springs allow three independent instruments to be
mounted within the casing, together with all the associated feedback electronics.
The sensor and hole-lock units are stacked one on top of the other with a set of
accurately-machined stacking tubes, which also form part of the sensor's pressure
housing. Fixing holes provided on each end face allow for simple and accurate
assembly. The base of each unit includes a double “O” ring to isolate the sensor from
any pressure variations in the atmosphere.
The 3TB functions by monitoring the position of each mass with a capacitative
position sensor. The three sensors are identical. Signals from the sensors are fed
into an electronic processing unit, which is mounted in a screened compartment
above the mechanical components (see below for details on the feedback circuitry.)
When the instrument is being transported, the masses are locked securely in their
frames so as to relieve strain on the support hinges. This locking is performed by a
small motor-driven clamp in response to a signal from the surface controller unit.
Before using the instrument, the boom of the vertical sensor must be levelled and the
bases of the horizontal sensors tilted, so that the masses are centred in their
equilibrium positions. These adjustments are made by small DC motors controlled
remotely.
See section 4 on page 21, “Installing the 3TB in a borehole” for detailed instructions
on how to set up your 3TB installation.
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Issue G - November 2019
