Determining sensor orientation

A Matlab extension for Scream! lets you easily determine the exact orientation of a sensor relative to a surface reference sensor, which can be installed on an accurately-surveyed N/S line.

The extension uses the Blacknest correlation method to determine relative orientation. This method was developed at AWE Blacknest in collaboration with Guralp Systems.

Signals received by the N/S component of the reference sensor are correlated with those received at the N/S and E/W components of the sensor being studied, after different amounts of mathematical rotation. The highest correlation will occur when the N/S component of the reference sensor matches the rotated N/S component of the borehole sensor.

Once you know the deviation of the borehole components from the compass points, you can instruct the digitizer to rotate the signals algorithmically.

Installing the Scream! extension

The Relative Orientation extension is supplied in the standard Windows distribution of Scream! 4.2 and later.

The extension uses Matlab libraries, which are currently only available for Windows. However, you do not need the full Matlab package to use the extension. The Matlab runtime libraries are also included in the Scream! distribution.

Installing the reference instrument

To measure the orientation of a sensor, you will need a second instrument which is known to point precisely North. It should be located on a solid surface as close to the other instrument as possible. Most boreholes are constructed with a concrete base around the top of the borehole. If this is present, we recommend installing the reference sensor there.

Ideally, the two sensors will be directly connected to the same 6-channel digitizer. If you are using separate digitizers, you will need to ensure they are exactly synchronized. This can be done by connecting GPS receivers to both digitizers and waiting for the control system of each one to settle. This process takes at least 12 hours.

Measuring the orientation

1. Run Scream!. Open the File – Setup window, and select the Display tab.
   
2. Under Stream Buffering, increase the buffer size to an amount which will hold all your experimental data. Click OK.
3. Drag the data files you have recorded into Scream!. A Replay Control window will open.
4. Click the Increase Speed icon until the legend (128x in the picture above) reads Max.
5. Click the Pause icon to begin replaying.Scream! will be able to replay data faster if you are not currently displaying it. When Scream! has finished, the Replay Control window will disappear.
6. Hold down CTRL and select the N, E, and X streams from the digitizer at the correct sample rate.The N and E streams are the “North/South” and “East/West” components of the downhole instrument. The X stream is the North/South component of the reference instrument.If you are using a separate digitizer, the reference instrument will appear on the North/South component of the other digitizer, instead of the X stream. To select this at the same time as the other streams, make sure Network is selected in the left-hand pane of Scream!’s main window, to display all the streams from your seismic network.
7. Double-click on one of the selected streams, or press ENTER. A WaveView window will open.
8. Drag the streams across the window so that the reference stream is at the top, the N stream in the middle, and the E stream at the bottom.
9. Click the Pause icon to stop the traces moving, and zoom in and out until you can see a suitable data range. You should use a period of at least an hour, and preferably longer.
   
10. Hold down the SHIFT key and drag across the WaveView window with the left button until all three streams are selected for the whole range.Make sure there are no gaps in the data you select. In Scream! 4.3 and later, the selection will be shown with hatched lines if there is a gap:
    
    When you are happy with the selection, release the mouse button, but keep SHIFT held down.
11. When the menu appears, release the SHIFT key. Select Relative Orientation from the menu that appears.Two small windows will appear: a small progress window, and a warning with a legend like

    Assuming DEMOX3 is reference N/S

    Scream! produces this warning because the reference sensor is not using a standard N/S channel, but the auxiliary (X) channel.

    If you are using a separate digitizer, the warning will not appear.

    If you get an error, make sure the streams are in the right order in the WaveView window. If you still have problems, you may have selected too few data points for it to be confident about the orientation; you should try again with a larger selection, or when more data is available.

12. After a few seconds, the calculation should finish and two windows will appear. (One may obscure the other.)The top window is a graph of Coherence vs Angle:
    
    The two-stage algorithm rotates the N/S and E/W components of the sensor being tested in small steps.It measures first the amplitude similarity, and then the coherence between this new N/S component and the reference N/S component, for a number of rotation angles.The error in the final calculation is around 2.5 °.

    The peak of the coherence curve (upper graph) therefore corresponds to the angle of rotation which best matched the reference component. This angle is shown in the title bar, together with an estimated error.

    You should see a coherence curve which is smooth and symmetrical. If the curve is distorted, either the surface data is too noisy or the data selection is too short.

    The lower graph shows the overall amplitude similarity of the rotated signal. This provides an idea of the sign of the coherence (since signals in perfect antiphase have a high coherence as well as those in phase). If there are two peaks in the coherence graph, the correct one is where the amplitude similarity is most positive.

    The sample plots show that the borehole instrument is installed with its N/S axis at a bearing of –90 ° from true North.

13. The second window shows the result of applying the rotation to the signal, i.e. the time series that a sensor in perfect N/S orientation would have produced:
    
14. You can perform more accurate calculations by narrowing the search range. This is done in the two entry boxes on the Coherence vs Angle window: the first denotes the centre of the new search, and the second its range.The program suggests suitable values for you, so in most cases you can just click Calculate to perform another iteration.
15. A new graph will be displayed showing the results.
    
    Our sample instrument is thus aligned at –90.35 ± 0.07 °.
16. The error given is only a rough estimate.You should repeat the orientation experiment several times using different data sets.

The true error in the computed orientation can be determined by observing the spread of the results.

The Blacknest orientation method generally provides a reliable indication of the sensor’s orientation. In most cases, the greatest source of error is in the installation of the reference sensor.

Applying automatic rotation

You can configure a DM24 mk3 digitizer to apply an automatic rotation to the digitized data and output streams representing ground motion on true North/South and East/West axes.

This is done within the DSP to minimize the reduction in data quality.

To set up the rotation:

1. Open a terminal session with the digitizer. You can do this with a program such as minicom (for Linux) or hypertrm (for Microsoft Windows). Alternatively, you can access the digitizer’s console through Scream! by right-clicking on its icon and selecting Terminal….
   You should see an ok prompt, indicating that the digitizer is ready to receive commands.
2. Type

   0 rotation AZIMUTH

   where rotation is the angle of deviation from true North that you measured earlier, as a whole number of tenths of a degree. This is the same angle (with the same sign) as that given by the orientation program.

   The 0 tells the digitizer to apply the rotation to instrument number 0 (the first, or only instrument.)

   Thus in the example above, you would type 0 -903 AZIMUTH to make the digitizer rotate signals by –90.3 degrees.

3. Reboot the digitizer with the command re-boot.
4. Collect some more data with the transformation active, and carry out another orientation calculation. The data from the downhole instrument should now have a maximum coherence with the reference sensor at 0 °. Check in particular that the sign of the rotation you have applied is correct.