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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. 10.7.2 Continuous mode Attitude data derived using continuous gyro survey systems have a tendency to drift exponentially with time. In many gyro systems, it is common practice to compensate for this effect by forming estimates of the drift at regular intervals during the survey. This is achieved by holding the tool stationary for short periods of time, and subtracting the s m d compon n s of E h’s so wh ch h ool s subj c d h cu n loc on. h d f s m s generated by this process are the accumulated effect of all physical errors at the given interval, and are used to implement a real-time re-calibration of the tool. The quality and effectiveness of this re-calibration are dependent on many factors that are difficult to predict, including the change in tool-face. It is almost impossible to keep track of what happens to the different physical sources of error when drift compensation is applied. Studies of field data, where comparisons of in-run and out-run surveys have been made, indicated that accumulated azimuth error in most continuous surveys can be estimated using four simple empirical parameters. The four empirical parameters are: • the error of the initial reference • a term proportional to measured time (gyro drift) • a term proportional to the square root of measured time (random walk) • a random error which is irrelevant for position error calculations. Whilst the use of empirical error sources is a departure from the usual form of error model linked to physical uncertainties, it does allow realistic uncertainty estimates to be produced. Gyrodata has developed a new method for final calculation of continuous surveys called Continuous Drift Correction (CDC). It is logically equivalent to averaging in-run and out-run surveys, but adopts a more complex approach which facilitates the estimation of error model terms, including the linear drift and random walk components. CDC also checks for tool misalignments provided that the in-run and out-run high-side tool-faces are not the same throughout the survey. The misalignment correction compensates for systematic misalignment for the whole survey and provides additional quality control to the data. The use of CDC provides several benefits compared to a simple in- run/out-run comparison. It corrects for linear drift and systematic misalignment and allows QC checks to be implemented by providing estimates of residual errors that can be checked against the tool error model. 10.7.3 Summary Stationary gyro-compassing and continuous survey methods have been outlined along with procedures developed by Gyrodata for quality control of the resulting surveys. The powerful MSC and CDC methods provide comprehensive quality control, for stationary and continuous surveys respectively, and tool performance characteristics linked directly to the respective tool error models.
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