Understanding Time-Depth Conversion

Tips for success and suggested workflows

The Basics of Time-Depth Conversion

In seismic surveys, sound waves are sent into the earth, and their travel times are recorded as they reflect off various subsurface layers. These travel times help create seismic “time sections,” showing layers and potential reservoirs in the subsurface. However, because these time sections are in seconds rather than meters or feet, geophysicists convert this data from time to depth for accurate drilling and development.

Key Factors in Time-Depth Conversion

The conversion process relies heavily on knowledge of seismic wave velocities in subsurface rocks. The depth of a feature depends on both travel time and the speed at which waves travel through different layers. Variations in lithology, porosity, and fluid content can impact these velocities. By creating a velocity model, geophysicists refine time-depth conversions, adjusting for these variables.

Methods for Time-Depth Conversion

There are several approaches to time-depth conversion, including:

1. Interval Velocity Models: This method involves calculating the velocity of seismic waves through each layer and using these velocities to transform time measurements into depths.
2. Layer-Stripping Techniques: By adjusting for each layer’s velocity successively from the surface down, this method accounts for velocity variations in each geologic layer.
3. Vertical Seismic Profiles (VSP): Directly obtained from boreholes, VSP data provides accurate depth and velocity measurements that greatly improve time-depth models.

Tips for Successful Time-Depth Conversion

SeisWare’s approach to time-depth velocity modeling is done by creating interval velocity model layers. The most common input options are time horizons, and formation tops, in depth.

To ensure a smooth and accurate conversion, users should ensure that they are using a consistent datum and that they have enough data coverage for an accurately calibrated model. Failure to do this step might result in inaccurate depths, which can raise drilling risk.

It is also recommended that SeisWare software users QC results. SeisWare’s Geoscience Exploration solution provides several avenues to do this:

• Use the crossplot to search for and correct any anomalous points
• Use velocity, time, and depth histograms to examine the distribution of data any data that may be out of range and in need of correction
• QC well points through the cross-section viewer and automatically apply changes to your time-depth model to avoid any mismatches.

Suggested Workflows

• Output velocity and mistie values as tops for each well and display these as postings or ribbon values on the basemap
• Stretch/squeeze velocity curves so that they tie to horizons, if appropriate. This is an automated way to bulk generate velocity curves for your wells that are not tied to your seismic data.

Applications for Exploration and Development

Accurate time-depth conversion helps geophysicists build more precise structural maps and define potential reservoirs with greater accuracy. This process also mitigates drilling risks, ensures that wells are drilled to the correct target depths and avoids costly deviations.

In summary, time-depth conversion is a vital step in transforming seismic reflections from abstract time data into actionable geological insights. By bridging the gap between time and depth, this process enables more effective exploration and resource management.