# Rock Physics Avo

> |

- **Type:** Skill
- **Install:** `agentstack add skill-steadfastasart-geoscience-skills-rock-physics-avo`
- **Verified:** Yes — security-reviewed for prompt injection and unsafe behavior
- **Seller:** [SteadfastAsArt](https://agentstack.voostack.com/s/steadfastasart)
- **Installs:** 0
- **Category:** [Agent Skills](https://agentstack.voostack.com/c/agent-skills)
- **Latest version:** 0.1.0
- **License:** MIT
- **Upstream author:** [SteadfastAsArt](https://github.com/SteadfastAsArt)
- **Source:** https://github.com/SteadfastAsArt/geoscience-skills/tree/main/workflows/rock-physics-avo

## Install

```sh
agentstack add skill-steadfastasart-geoscience-skills-rock-physics-avo
```

Requires the [AgentStack CLI](https://agentstack.voostack.com/docs/cli). Works with Claude Code, Cursor, and any MCP-compatible agent.

## About

# Rock Physics & AVO Workflow

End-to-end pipeline for rock physics analysis and AVO feasibility studies,
from well log preparation through elastic property calculation, Gassmann
fluid substitution, AVO modelling, and synthetic seismogram generation.

## Skill Chain

```text
lasio / welly          bruges                    segyio / obspy
[Well Log Prep]     --> [Rock Physics]         --> [Synthetics / Tie]
  |                      |                          |
  Load LAS/DLIS          Elastic moduli             Wavelet extraction
  QC & despike           Gassmann fluid sub         Reflectivity series
  Resample curves        AVO intercept/gradient     Convolve synthetic
  Extract Vp, Vs, rho    Backus averaging           Well-seismic tie
```

## Decision Points

| Task | Library | When to Use |
|------|---------|-------------|
| Load well logs | lasio / dlisio | Always the first step |
| Curve QC and management | welly | Multi-curve processing, despiking |
| Elastic moduli, AVO equations | bruges | Core rock physics calculations |
| Gassmann fluid substitution | bruges | Predict fluid replacement effects |
| Wavelet extraction from seismic | segyio + bruges | When tying to seismic |
| Synthetic seismogram | bruges | Generate reflectivity and convolve |
| Dispersion curves | disba | Surface wave rock physics |

## Step-by-Step Orchestration

### Stage 1: Well Log Preparation (lasio + welly)

```python
import lasio
import numpy as np
from welly import Well

# Load well with sonic, density, and shear sonic
las = lasio.read('well.las')
df = las.df().dropna()

# Extract elastic logs
depth = df.index.values
vp = 1e6 / df['DT'].values       # P-wave velocity (m/s) from sonic (us/ft)
vs = 1e6 / df['DTS'].values      # S-wave velocity (m/s) from shear sonic
rho = df['RHOB'].values * 1000   # Density (kg/m3) from g/cc

# QC: check ranges
assert np.all(vp > 1500) and np.all(vp  500) and np.all(vs  1500) and np.all(rho  0, G  0  (very soft, dim with offset)

# Zoeppritz exact for full offset range
rc_exact = bruges.reflection.zoeppritz(
    vp1=vp[i], vs1=vs[i], rho1=rho[i],
    vp2=vp[i+1], vs2=vs[i+1], rho2=rho[i+1],
    theta=np.arange(0, 50, 1)
)
```

### Stage 5: Synthetic Seismogram (bruges + segyio)

```python
# Create reflectivity series
rc_series = bruges.reflection.reflectivity(vp, rho)

# Create wavelet
duration = 0.128  # seconds
dt = 0.002        # sample rate (2ms)
wavelet = bruges.filters.ricker(duration=duration, dt=dt, f=25)

# Convolve to create synthetic
synthetic = np.convolve(rc_series, wavelet, mode='same')

# If tying to seismic, extract wavelet from seismic trace
import segyio
with segyio.open('seismic.sgy') as f:
    near_trace = f.trace[0]  # Nearest trace to well
    # Extract statistical wavelet from trace
    # Or use bruges.filters for analytic wavelets
```

## Common Pipelines

### AVO Feasibility Study
```
- [ ] Load well logs (Vp, Vs, Rho, porosity) with lasio
- [ ] QC logs: check ranges, despike, fill gaps
- [ ] If no Vs log: estimate from Castagna or Greenberg-Castagna
- [ ] Calculate elastic moduli and impedances with bruges
- [ ] Run Gassmann fluid substitution (brine to gas/oil)
- [ ] Compare Vp, Vs, density, impedance before/after fluid sub
- [ ] Compute AVO response at target interface (Shuey or Zoeppritz)
- [ ] Classify AVO response (Class I-IV)
- [ ] Generate synthetic seismograms for both fluid scenarios
- [ ] Plot AVO crossplot (intercept vs gradient)
```

### Well-Seismic Tie
```
- [ ] Load well logs and seismic trace at well location
- [ ] Create time-depth relationship from check shots or sonic
- [ ] Convert logs to time domain
- [ ] Extract wavelet from seismic (statistical or deterministic)
- [ ] Generate synthetic seismogram from reflectivity * wavelet
- [ ] Cross-correlate synthetic with seismic trace
- [ ] Adjust stretch/squeeze to optimize tie
- [ ] Report correlation coefficient
```

### Backus Averaging (Upscaling)
```
- [ ] Load thin-bed well logs at fine sampling (0.5 ft)
- [ ] Define averaging window (e.g., quarter wavelength at target frequency)
- [ ] Apply Backus averaging to get effective anisotropic elastic properties
- [ ] Compare fine-scale vs upscaled reflectivity
- [ ] Assess thin-bed tuning effects
```

## When to Use

Use the rock physics & AVO workflow when:

- Performing AVO feasibility studies for exploration prospects
- Running Gassmann fluid substitution to predict fluid effects
- Generating synthetic seismograms for well-seismic ties
- Calculating elastic properties from well logs
- Classifying AVO response at target horizons

Use individual domain skills when:
- Only loading well logs (use `lasio` alone)
- Only computing dispersion curves (use `disba` alone)
- Only creating wavelets or filters (use `bruges` alone)

## Common Issues

| Issue | Solution |
|-------|----------|
| No shear sonic log | Estimate Vs from Castagna mudrock line or Greenberg-Castagna |
| Gassmann gives unrealistic velocities | Check porosity and mineral modulus inputs; phi must be > 0 |
| Negative Poisson's ratio | Usually indicates bad Vs data; QC shear sonic |
| Poor well-seismic tie | Check time-depth relationship; try different wavelets |
| AVO effect too small | May be real; check impedance contrast and Vp/Vs ratio |
| Fluid sub in shales | Gassmann assumes connected pore space; not valid for shales |

## Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

- **Author:** [SteadfastAsArt](https://github.com/SteadfastAsArt)
- **Source:** [SteadfastAsArt/geoscience-skills](https://github.com/SteadfastAsArt/geoscience-skills)
- **License:** MIT

Install and usage instructions live in the source repository linked above.

## Pricing

- **Free** — Free

## Security capabilities

Automated source analysis of v0.1.0 — what this tool can access:

- **Network access:** no
- **Filesystem access:** no
- **Shell / process execution:** no
- **Environment & secrets:** no
- **Dynamic code execution:** no

*"Yes" means the capability is present in the source — more access means more to trust, not that it is unsafe.*


## Versions

- **0.1.0** — security scan: passed — Imported from the upstream source.

## Links

- Listing page: https://agentstack.voostack.com/l/skill-steadfastasart-geoscience-skills-rock-physics-avo
- Seller: https://agentstack.voostack.com/s/steadfastasart
- Browse the marketplace: https://agentstack.voostack.com/browse

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