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10.4 How to Measure Azimuth
So how does the ability to measure rates of rotation help us to determine borehole azimuth? For ease of explanation and understanding of the methods used in a gyro survey system, consider first the simple case in which a dual-axis gyro is mounted with its spin axis vertical so that its input axes measure the horizontal compon n of E h’s s d p c d n F gu 53. Calculate the gyro measurement about the axes - x-axis; G x = Ω H sin α y-axis; G y = Ω H cos α . It can be seen that the ratio of the x-axis measurement to the y-axis measurement defines the tangent of the direction ( α ) in which the y axis points with respect to the true north. In most practical constructions of rate gyro survey tools, sensors are attached rigidly to the tool with the result that the input axis of the rate gyro takes measurements in the plane perpendicular to the tool axis; not in the local horizontal plane. Therefore, direct measurements of the horizontal components of E h’s only g n d wh n h ool s vertical. In general, the output from the gyro measures compon n s of E h’s wh ch nclud bo h vertical and horizontal components. In order to define the azimuth direction of a bore-hole in which the tool is located, it is necessary to have knowledge of the direction in which the gyro input axes are pointing with respect to the horizontal E h’s c o . h s m y b sp c f d n terms of the inclination of the well and the orientation of the gyro input axes with respect to the high-side of the borehole. This information can be determined using accelerometers installed in the tool with their input axes aligned parallel to the axes of the gyro as shown schematically in figure 54.
Figure 53: Measuring azimuth.
Figure 54: Instrument configuration with accelerometers.
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