The Elastic properties wizard calculates elastic and acoustic properties of the rock from the sonic and density curves of a well. Every method reads the same three inputs — compressional slowness, shear slowness and bulk density — and each run produces one output curve for the one method that is selected.

It runs at the selected well or, with Batch calculate, at a group of wells, and it shares the well selection, depth range, unit handling and run reporting common to the calculators of the Petrophysics bar.

Launching the wizard

Open the Petrophysics ribbon bar and click Elastic properties in the GeoMech section; on the button the caption is shortened to Elastic. Select the well, limit the interval on the MD range, TVD range, TVDSS range, Markers range or Zones range tab, pick a method, check the input curves and their units, and then Run.

The dialog carries two dividers: one between the depth-range tabs and the Method group, and one between the left pane and the quality-control panel on the right. Drag either of them to give more room to the part that is being worked on.

Methods

The Method list is grouped by property family, and the group headers are labels that cannot be selected. Choosing a method rebuilds the input rows, the equation shown under them and the Output curve name and unit list, while the curves and units already assigned are kept — so one input assignment can be run through one method after another without being set up again.

Velocities

• P-wave velocity (Vp) — the compressional slowness converted to velocity. Output curve Elastic - Vp, in m/s or ft/s.
• S-wave velocity (Vs) — the shear slowness converted to velocity. Output curve Elastic - Vs, in m/s or ft/s.
• Vp/Vs ratio — the ratio of the two velocities, taken directly from the slownesses, so it needs no density curve. Output curve Elastic - Vp over Vs, without a unit.
• Predicted shear slowness — writes the shear slowness predicted from the compressional log as an ordinary curve, so it can be inspected, plotted and reused. Output curve Elastic - DTS predicted, in µs/ft or µs/m.

Impedances

• Acoustic impedance (P) — density multiplied by the compressional velocity. Output curve Elastic - Acoustic impedance, in (g/cc)*(m/s) or (kg/m3)*(m/s).
• Shear impedance (S) — density multiplied by the shear velocity. Output curve Elastic - Shear impedance, in the same units.

Elastic moduli (dynamic)

• Poisson's ratio — ν = (Vp² − 2·Vs²) / (2 · (Vp² − Vs²)). It is a velocity ratio, so it needs no density curve. Output curve Elastic - Poisson's ratio, without a unit.
• Young's modulus — E = 2 · μ · (1 + ν), with μ = ρ · Vs². Output curve Elastic - Young's modulus, in GPa, MPa or Mpsi.
• Shear modulus (μ) — μ = ρ · Vs². Output curve Elastic - Shear modulus, in GPa, MPa or Mpsi.
• Bulk modulus (K) — K = ρ · (Vp² − 4/3 · Vs²). Output curve Elastic - Bulk modulus, in GPa, MPa or Mpsi.
• Lamé's first parameter (λ) — λ = ρ · (Vp² − 2·Vs²) = K − 2/3 · μ. Output curve Elastic - Lambda, in GPa, MPa or Mpsi.
• P-wave modulus (M) — M = ρ · Vp² = K + 4/3 · μ. Output curve Elastic - P-wave modulus, in GPa, MPa or Mpsi.
• Bulk compressibility — Cb = 1 / K. Output curve Elastic - Bulk compressibility, in 1/GPa or 1/MPa.

AVO attributes

• Lambda-Rho (λρ) — λρ = AI² − 2 · SI². Output curve Elastic - Lambda-Rho, in GPa*g/cc.
• Mu-Rho (μρ) — μρ = SI². Output curve Elastic - Mu-Rho, in GPa*g/cc.

Both AVO attributes are built from the impedances, so they agree with the impedance curves that the same input assignment produces.

Input curves

Only the rows that the selected method uses are shown. Each curve list contains all curves of the well and carries its own unit list; on first opening, a row is filled from the well's standard mnemonics and the unit follows the one recorded on the chosen curve.

• Compressional slowness — the sonic log, in µs/m or µs/ft, preselected from DT, DTCO, DTC or DTP. It is shown for every method except Shear impedance (S) and Mu-Rho (μρ), which read the shear velocity only — and it appears there as well whenever the shear velocity is derived from the compressional log.
• Shear source — where the shear velocity comes from: DTS curve adds a Shear slowness row with its own unit list, preselected from DTS, DTSM or DTSH; Predicted from Vp derives it from the compressional log; and Vp/Vs constant applies a single typed Vp/Vs ratio, 1.90 by default. The row opens on DTS curve when the well carries a recognised shear log and on Predicted from Vp when it does not, so the wizard opens in a state that produces a result. It is hidden for the methods that do not use the shear velocity — P-wave velocity (Vp), Acoustic impedance (P) and P-wave modulus (M) — and for Predicted shear slowness, which always predicts.
• Bulk density — the density log, in g/cc, kg/m3 or lb/ft³, preselected from RHOB, RHOZ or DEN. It is shown only for the methods that read density: both impedances, all moduli except Poisson's ratio, and both AVO attributes.

Supply each curve in the unit it was logged in and let the wizard convert it; the unit lists follow the project measurement system where the curve itself carries no unit.

Predicting the shear velocity

With Predicted from Vp the shear velocity is calculated from the compressional velocity through Vs = c · Vp² + a · Vp + b, with both velocities in km/s. The Lithology list fills the Coefficients from a published relation, and they stay editable afterwards, so a locally derived relation can be entered instead:

• Mudrock line (Castagna) — a = 0.8621, b = −1.1724, c = 0
• Sandstone — a = 0.80416, b = −0.85588, c = 0
• Shale — a = 0.76969, b = −0.86735, c = 0
• Dolomite — a = 0.58321, b = −0.07775, c = 0
• Limestone — a = 1.01677, b = −1.03049, c = −0.05508

A predicted shear velocity is always marked: the line Shear velocity is predicted, not measured. appears under the coefficients, and the preset and coefficients used are stored with the output curve. Choosing the Predicted shear slowness method instead writes the prediction out as a curve of its own.

Output curve

The Output curve field carries the default name of the selected method, listed above, and can be edited before the run. The unit list beside it offers the units that suit the property. The calculation is evaluated in SI — velocities in m/s, density in kg/m³, moduli in Pa — and converted once into the selected output unit, which the note under the equation repeats; the same note states that these are dynamic properties, obtained from sonic-frequency measurements and not from laboratory loading.

The method, the input curves with their units, the shear source — including the preset and coefficients where the shear velocity was predicted — and the calculated interval are stored together with the curve. The Show elastic properties settings command on that curve in the Data manager reopens the wizard with those values.

Running and saving

The wizard has the standard row of buttons along the bottom of the dialog:

• Save to workflow — adds the wizard with all of its current settings to the project Workflow as a task that can be re-run later.
• Run — calculates the curve with the current settings and shows it in the preview panel.
• Save — stores the calculated curve in the well. A curve that already carries the output name is replaced after a Confirm prompt; if nothing has been calculated yet, the wizard reports Nothing to save.
• Close — closes the wizard.

Changing the method, a curve, a unit, a coefficient or the zone selection discards the previous result, so the preview never shows a curve that no longer matches the settings.

Results

After Run the wizard reports for how many wells the calculation completed and lists them, then the number of samples that produced no value, broken down by cause:

• a required input that is null or not positive;
• Vp ≤ Vs, where the shear arrival outruns the compressional one, which points at bad data or a mispicked curve;
• Vp ≤ √2 · Vs, the boundary below which Poisson's ratio is no longer positive, so every method that reads the shear velocity nulls the sample;
• a bulk modulus of zero, for Bulk compressibility;
• a result that is not a finite number.

A large count is usually the sign of a mispicked curve or a wrong unit assignment rather than of the rock, which is why the causes are counted separately. Wells that were skipped or only partly calculated are reported after the run summary.

Quality control

The panel on the right of the dialog carries two tabs:

• Input/Output — carries the name of the selected well in its title and plots the input curves that the current method uses next to the calculated Output track.
• Crossplot — plots one of the pairs offered by the Pair list: ν vs Vp/Vs, AI vs Vp/Vs or λρ vs μρ. This is where a wrong unit assignment becomes visible: curves that look plausible on their own land in an impossible place on the plot. The caption states how many points are plotted and from which curves, and, where the shear velocity was not logged, whether it came from the Vp/Vs constant or from the prediction, with the preset and coefficients used.

See Also

• Petrophysics
• Base properties
• Pore pressure
• Workflows