Agent skill

bruges

Geophysical equations and rock physics calculations for seismic analysis. Use when Claude needs to: (1) Calculate AVO responses (Zoeppritz, Shuey, Aki-Richards), (2) Perform Gassmann fluid substitution, (3) Generate seismic wavelets (Ricker, Ormsby), (4) Compute reflectivity and synthetic seismograms, (5) Calculate elastic moduli from velocities, (6) Apply Gardner/Castagna empirical relations, (7) Model rock physics effects.

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SKILL.md

bruges - Geophysics Equations

Quick Reference

python
import bruges
import numpy as np

# AVO - Zoeppritz reflectivity
from bruges.reflection import zoeppritz
theta = np.arange(0, 45, 1)
Rpp = zoeppritz(vp1, vs1, rho1, vp2, vs2, rho2, theta)

# Wavelet - Ricker
from bruges.filters import ricker
t, w = ricker(duration=0.128, dt=0.001, f=25)

# Fluid substitution
from bruges.rockphysics import gassmann
vp_new, vs_new, rho_new = gassmann(vp, vs, rho, k_min, rho_min,
                                    k_fl1, rho_fl1, k_fl2, rho_fl2, phi)

Module Overview

Module Purpose
bruges.reflection Zoeppritz, Shuey, Aki-Richards AVO equations
bruges.rockphysics Gassmann, Gardner, Castagna, elastic moduli
bruges.filters Ricker, Ormsby wavelets, convolution

AVO Analysis

python
from bruges.reflection import zoeppritz, shuey, akirichards
theta = np.arange(0, 45, 1)  # degrees

Rpp = zoeppritz(vp1, vs1, rho1, vp2, vs2, rho2, theta)  # Exact
Rpp = shuey(vp1, vs1, rho1, vp2, vs2, rho2, theta)      # Fast (~30 deg)
Rpp, G = akirichards(vp1, vs1, rho1, vp2, vs2, rho2, theta, return_gradient=True)

Fluid Substitution

python
from bruges.rockphysics import gassmann

vp_new, vs_new, rho_new = gassmann(
    vp_sat, vs_sat, rho_sat,   # Original saturated rock
    k_min, rho_min,             # Mineral (quartz: 37 GPa, 2.65 g/cc)
    k_fl1, rho_fl1,             # Original fluid (brine: 2.5 GPa, 1.05 g/cc)
    k_fl2, rho_fl2,             # New fluid (oil: 0.9 GPa, 0.8 g/cc)
    phi                         # Porosity
)

Wavelets

python
from bruges.filters import ricker, ormsby

# Ricker (single frequency)
t, wavelet = ricker(duration=0.128, dt=0.001, f=25)

# Ormsby (bandpass: low cut, low pass, high pass, high cut)
t, wavelet = ormsby(duration=0.128, dt=0.001, f=[5, 10, 50, 60])

Synthetic Seismogram

python
from bruges.filters import ricker
from scipy.signal import convolve

rc = np.diff(impedance) / (impedance[:-1] + impedance[1:])  # Reflectivity
_, wavelet = ricker(0.128, 0.001, 30)
synthetic = convolve(rc, wavelet, mode='same')

Elastic Moduli

python
from bruges.rockphysics import moduli

K = moduli.bulk(vp, vs, rho)      # Bulk modulus (GPa)
mu = moduli.shear(vs, rho)        # Shear modulus (GPa)
E = moduli.youngs(vp, vs, rho)    # Young's modulus (GPa)
nu = moduli.poissons(vp, vs)      # Poisson's ratio

Empirical Relations

python
from bruges.rockphysics import gardner, castagna

rho = gardner(vp)    # Density from Vp (g/cc)
vs = castagna(vp)    # Vs from Vp (mudrock line)

AVO Classification

Class Intercept Gradient Description
I + - High impedance sand
II ~0 - Near-zero intercept
IIp ~0 + Phase reversal
III - - Low impedance sand
IV - + Very low impedance

When to Use vs Alternatives

Use Case Tool Why
AVO modelling and reflectivity bruges Complete Zoeppritz/Shuey/Aki-Richards suite
Gassmann fluid substitution bruges Clean API, validated equations
Seismic wavelets bruges Ricker, Ormsby, and more out of the box
Elastic moduli calculations bruges Bulk, shear, Young's, Poisson's from Vp/Vs
Advanced rock physics models rockphypy More models (Hashin-Shtrikman, DEM, etc.)
Simple impedance/reflectivity Custom numpy Fewer dependencies for basic calculations
Full seismic modelling (FD/FE) Devito / SimPEG Wave equation solvers
Well log processing welly / lasio LAS file I/O and log manipulation

Choose bruges when: You need validated geophysical equations for AVO analysis, fluid substitution, or synthetic seismogram generation. It provides clean, tested implementations of standard rock physics equations.

Choose rockphypy when: You need a broader set of rock physics models beyond what bruges offers, such as effective medium theories or contact models.

Choose custom numpy when: You only need a single equation (e.g., acoustic impedance = Vp * rho) and want to avoid adding a dependency.

Common Workflows

AVO Analysis and Synthetic Seismogram Generation

  • Define layer properties (Vp, Vs, density) from well logs or literature
  • Compute acoustic impedance: AI = Vp * rho
  • Compute reflectivity series using zoeppritz() or shuey() for each interface
  • Classify AVO response (Class I-IV) from intercept and gradient
  • Perform Gassmann fluid substitution to model fluid effects on Vp/Vs
  • Recompute reflectivity with substituted properties
  • Generate Ricker or Ormsby wavelet at target frequency
  • Convolve reflectivity with wavelet to create synthetic seismogram
  • Compare synthetic with observed seismic for well tie
  • Compute elastic moduli (K, mu, E, nu) for crossplot analysis
  • Create AVO crossplots (intercept vs gradient) to identify anomalies

Common Issues

Issue Solution
Rpp blows up at high angles Use Zoeppritz (exact) instead of Shuey above 30 degrees
Gassmann gives unrealistic Vp Check mineral modulus and porosity values
Wavelet length too short Increase duration parameter (try 0.128-0.256 s)
Negative shear modulus Verify Vs < Vp and density is reasonable
Units mismatch Ensure consistent units: m/s for velocity, g/cc for density

Tips

  1. Zoeppritz is exact but slower - use Shuey for angles < 30 degrees
  2. Gassmann assumes low frequency and connected pores
  3. Gardner/Castagna are empirical - calibrate to your data
  4. Angles in degrees for reflection functions

References

  • Rock Physics Equations - Velocity-porosity models and moduli
  • Wavelets - Wavelet types, parameters, and selection

Scripts

  • scripts/avo_analysis.py - AVO modeling and classification

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