Which Model? An Advisor

You Have Finished

This is the end of the course. You started at a single puzzle, that two rocks with the same porosity can ring at different velocities, and you have built every tool that resolves it: the moduli and how they set velocity, the bounds that discipline every mixture, the fluids and Gassmann, the contact and inclusion frames, calibration to the Ogbon-1 well, pressure and time-lapse, layering and frequency. More than any single model, you have learned the habit under all of them, to ask which model a given rock and a given question actually need, and to name what that model quietly assumes.

The Advisor, and Its Honest Scope

The widget above is a short interview. Tell it three things, your ROCK (clean sand, shaly sand, carbonate, or a laminated stack), what you KNOW (logs with shear, logs without, core and thin sections, or seismic alone), and your GOAL (fluid substitution, porosity, shear prediction, anisotropy for imaging, or 4D feasibility), and it returns three things back: an ordered CHAIN of course tools naming the parts to work through, the ASSUMPTIONS that chain accepts with the section each one comes from, and which of the Part 12.3 Python presets to start from. Ask for a fluid substitution on a clean sand with shear logs and it lays out bounds, then Batzle-Wang fluids, then the Gassmann inverse and re-saturation, and it tells you plainly that you are accepting the seismic-band limit, one connected pore system, and a fluid-invariant shear modulus.

Be clear about what it is. The advisor recommends a STARTING POINT, not an answer. It is rule-based and runs entirely on this page; your choices never leave the browser. It cannot see your rock, so it cannot know whether your aspect ratios are calibrated, whether your carbonate's pores are connected, or whether your layers are truly thin against the wavelength. What it can do is hand you the right chain to begin, with its assumptions named so you know exactly what you will have to check. That is the same discipline the whole course has taught, made into a tool: choose deliberately, and hold the assumptions in view.

The Road On

Carry one idea out of this course above all the others. Velocity is not a property you look up; it is the prediction of a model you must choose, bound, and calibrate. Choose, because the rock and the data decide which frame fits. Bound, because Hashin-Shtrikman says what is possible before any model speaks. Calibrate, because a model is only worth the well it was tuned to. Every part of this course was one of those three verbs, and every honest velocity you will ever quote will carry all three.

From here the road is open and it is short to the first step. Take a rock you understand and run it through a Python preset. Open a real log and place it on the template. Carry your Thomsen parameters into the Synthetic Seismic Modeling course and watch them bend an image. Whatever you reach for, keep asking the question this whole course was built around, before you trust a single number: which model does this rock actually need, and what is it assuming? Answer that well and you are no longer looking velocities up. You are predicting them. Thank you for taking the course.

References

• Mavko, G., Mukerji, T., & Dvorkin, J. (2009). The Rock Physics Handbook (2nd ed.). Cambridge University Press.
• Avseth, P., Mukerji, T., & Mavko, G. (2005). Quantitative Seismic Interpretation. Cambridge University Press.