Tying the well: proving the wiggle is the rock
Learning objectives
- Build a synthetic seismogram from velocity and density logs
- Apply a bulk shift and read the correlation between synthetic and seismic
- Explain what makes a correlation peak sharp or broad, and why it matters
- Say why a well tie must precede horizon naming rather than follow it
You have been calling something "top reservoir" for four sections. Nobody has demonstrated it.
The well knows where the reservoir is, in metres. The seismic knows where its reflectors are, in milliseconds. Those are different axes measuring different quantities, and the only bridge between them is a synthetic seismogram: take the velocity and density logs, multiply them to get acoustic impedance, difference that to get reflectivity, convolve with the wavelet, and compare the result against the trace recorded at the well.
If they match, the name on the log and the wiggle on the section are the same rock. If they do not, everything downstream is a guess with a confident label on it.
Read the correlation curve, not the wiggles
The panel on the right is correlation against bulk shift, and it is more informative than the traces beside it. On this survey it is a sharp spike: 0.809 at the tie, 0.016 two samples away, 0.157 a full wavelet period away.
That sharpness is a property of the reflectivity, not of the quality of the work. This volume carries about twenty-four interfaces over its recorded length, so the reflectivity is dense and the correlation function is nearly a spike. A thick, clean sand sequence gives you the opposite: few strong reflectors, a broad correlation function, and side lobes almost as tall as the true peak.
That second case is where cycle skips come from. A tie one wavelet period out still correlates well, still looks convincing, and puts every horizon you subsequently pick one loop wrong, which at this depth is around thirty metres. The defence is not a better eye. It is looking at the shape of the correlation function and asking whether the peak you have chosen is really the only candidate.
Why the tie comes before the picking, not after
It is tempting to pick first and tie later, because picking is satisfying and tying is fiddly. The trouble is that a horizon picked on the wrong loop is internally consistent: it tracks beautifully, closes its loops, and grids into a perfectly plausible map of the wrong surface. Nothing downstream will catch it. The tie is the only place in the whole workflow where the seismic is compared against something that was measured rather than interpreted.