Phase Matters
Learning objectives
- See that rotating a wavelet's phase moves the picked event in time
- Explain why a phase error is a systematic depth error, not random noise
- Recognise that a 180 degree rotation flips polarity and causes gross misties
- Understand that phase errors are invisible in the amplitude spectrum
The Phase You Cannot See
The previous section showed that a wavelet is fixed by its amplitude spectrum and its phase. Amplitude spectrum is the part everyone watches. Phase is the part that quietly moves your interpretation, and this section makes that motion visible.
Set a single reflector at a known time and build its response with a wavelet rotated by a phase you choose. An interpreter picks the event at the crest of the loop. At zero phase, a zero-phase wavelet, that crest sits exactly on the reflector. Now rotate.
A Rotation Is a Depth Error
As the phase turns, the crest walks off the reflector. A rotation of degrees shifts the peak by a fraction of the dominant period, so at 90 degrees the pick moves about a quarter period, and every reflector in the section moves by the same amount. That is a bulk time shift, which converts directly to a depth error. It is not noise you can average away; it is a coherent bias. And because a phase rotation leaves the amplitude spectrum untouched, nothing in the frequency content will warn you it happened.
The Polarity Flip
At 180 degrees the loop inverts: the reflector that was a peak is now a trough. A picker following peaks jumps to a neighbouring lobe, and the mistie is no longer a fraction of a period but a whole event. This is why matching phase, usually by tying the synthetic to a well, is one of the first things done before any interpretation is trusted, and it is why the inverse-crime warning about phase was not academic.