beautypg.com

5 inside the 3tb, 1 the sensors – Guralp Systems CMG-3TB User Manual

Page 64

background image

CMG-3TB

5 Inside the 3TB

5.1 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. CMG-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

Chapter 3, “Installing the 3TB in a borehole”

for detailed

instructions on how to set up your 3TB installation.

64

Issue C

This manual is related to the following products: