10.7.1 Gyro-compassing mode

Gyro biases, which have an unpredictable behaviour, are measured and corrected for directly at each gyro-compassing station though a process of indexing the gyro. This involves mounting the gyro on a rotatable platform and driving it between two positions that are 1800 apart. Measurements of turn rate are taken when the gyro is stationary at each index position. Whilst the turn rate to which the gyro is subjected is reversed between the two index positions, any bias which is present in the measurements remains fixed. Hence, an estimate of the measurement bias can be obtained by summing the two measurements and dividing the result by two. Any residual bias which remains can still be significant and must therefore be estimated in the field.

Gyro mass unbalance is stable when the gyro is at rest. However, as discussed earlier, it may change significantly if the gyro is exposed to a mechanical impact, as can occur during transportation or surface handing. The average mass unbalance for the entire survey should therefore me estimated and corrected in the field.

Accelerometer calibrations are usually very stable, but they can change over time or as a result of temperature exposure. It is therefore important that the performance of the accelerometer pack is always verified for every recorded measurement and for the survey as a whole.

Gyrodata has developed a method for field calibration while surveying known as Multi-Station Correction (MSC). It is impossible to determine accurately all of the calibration terms in the field, the goal of MSC is therefore to correct those terms that are more likely to change, namely gyro fixed biases and mass unbalance, whist at the same time minimising the effect of residual errors in other terms. In addition, a MSC accelerometer test has also been included, to check the accelerometer measurements throughout the survey; only applicable for survey tools containing three accelerometers. MSC is a very powerful tool that updates the calibration values of residual biases and direct mass unbalance for the gyro and checks the performance of the accelerometer package. Additionally, since MSC is based on a least-squares adjustment technique, the standard deviations of the x and y gyro biases and mass unbalance are generated. This information is checked against the tolerance defined by the gyro error model and forms an essential part of the quality control (QC) procedure that is implemented each time a survey tool is run.