Seismic Exploration: Data Acquisition to Interpretation

Posted on Aug 27, 2024 in Geology

A) Data Acquisition Phase

B) Data Processing Phase

Once the data acquired are sent to the processing center through a specialized program for analysis, this involves a sequential series of mathematical manipulations applied to the seismic data to obtain complete 2D or 3D seismic sections, depending on the acquisition technology.

Stages of Processing

1. Basic Stage

a) Preliminary Step

  • Review of supporting material
  • Format ID

b) Demultiplexing and Editing

  • An analysis process demuxes certain records, using cross-correlation testing, to study the profit curve.
  • Characteristics of the demultiplexing process include: all information demultiplexed, edited, added, defining the profit curve, and cross-correlation.

c) Generate Geometry

To effectively implement dynamic correlations, it’s necessary to define the geometry of the line for the following parameters:

  • Offset Distance between stakes set
  • Distance between shots
  • Coverage
  • Map Reference of Cota
  • Speed Stakes weathered layer or depth of the seabed
  • Average depth of marine cable-laying
  • Possible changes in the direction of the line
2. Analysis of Implementation Parameters
  • Deconvolution: To undo the effect of another filter, which eliminates reverberation of the seismic signal and improves the wave spectrum, potentially balancing the amplitude of the seismic trace.
  • Filter: Takes into account the filters used in the field or range of the sweep.

Bandpass Filter: Eliminates noises outside the desired seismic signal frequency range.

Stacking: Allows significant suppression of coherent noises in the data, applied after geometric corrections and mute (with mute, noisy peaks with large amplitudes are eliminated).

The static corrections have two purposes:

  1. Refer the recording time to an imaginary plane of constant height, called the datum.
  2. Subtract the time delay caused by the seismic signal traveling through the low-velocity layer (Love waves).
3. Evaluation of the Stacked Section
  1. Check for problems with static corrections.
  2. Assess if the speed function was optimal (quality of consistency of events).
  3. Determine if the points along the line where speeds were chosen were sufficient for a good stack of information.

The Stacking

The process that allows diffractions to collapse and moves dipping events to their true subsurface position.

Types of Migration

  • Diffraction: Considers parameters of speed and ΔT
  • Wave Equation: Considers speed and time. ΔT is a multiple of the sampling rate (average speed).
  • Frequency Domain: Based on the wave equation (deterministic methods).

Time to Depth Conversion

To perform this process, knowledge of the velocity distribution in the study area is required, including:

  • Integrated sonic log
  • Speed Stacking
  • Seismic Speed outline
  • Vertical seismic profiles

C) Phase Interpretation

An analysis of the recorded field data, resulting in a seismic image or 3D cube. This phase traditionally involves mapping geological structures, calibrated with well data. It requires broad geological knowledge to understand basin sedimentation and structural evolution. The interpreter must be familiar with geology and seismology concepts and the latest algorithms and software for efficient interpretation.

Types of Interpretation

  • Structural Interpretation (Reservoir Limits, compartmentalization)
  • Stratigraphic Interpretation (depositional environments)
  • Reservoir Characterization (lithologies, fluids, porosity, fracturing, etc.)

Objectives of Seismic Interpretation

a) Exploration: Mapping potential prospects, fluid contacts, and estimating potential hydrocarbon volumes. Locating exploratory wells.

b) Delineation: Reassessing hydrocarbon volumes. Locating delineation wells.

c) Production: Monitoring (4D seismic) fluids and/or pore pressure, concluding on reservoir connectivity and undrained hydrocarbon potential. Locating production and injector wells.

Conventional Example of a Flowchart in Seismic Interpretation

  1. Data collection
  2. Definition of project cargo data
  3. Data condition (scale, areas of interest, etc.) and quality control
  4. Calibration of wells to the seismic data
  5. Seismic Interpretation: Horizons, faults, and calculation of surface and volume attributes
  6. Construction of the velocity model range (or average velocity maps)
  7. Time-depth conversion. Estimation of uncertainty
  8. Refining the interpretation and/or velocity model
  9. Mapping (contouring and gridding)
  10. Volume calculation
  11. Geological Summary (Sedimentation in the basin, structural evolution, etc.)

Seismic Wells

Traditionally, seismic data acquisition involved using energy sources and detectors on the surface. However, using seismic techniques in the well provides more and better information about the seismic area. These techniques include the synthetic seismogram and Vertical Seismic Profile (VSP). These methods share seismic data through electrodes placed in the borehole, close to lithological changes, complementing surface seismic information. The data provides information about fundamental seismic wave propagation properties and contributes to understanding reflection and transmission processes within the Earth. This helps improve the structural, stratigraphic, and lithological interpretation of seismic records.

The Synthetic Seismograms

Reconstruction of a seismic trace along the well’s direction using density and sound recordings.

Helps with:

  • Identifying multiple events
  • Continuous and accurate time-depth conversion
  • Analyzing real seismic amplitude
  • Lithological correlation of the surface seismic with well logs
  • High-resolution seismic

Elements constituting the synthetic seismogram:

  • The velocity curve (m/s) obtained from the sonic log.
  • The solid density curve (g/cm3) obtained directly from the density log.
  • Seismic verification shooting to measure seismic pulse travel times from the surface to various depths, generally spaced at 250m or at major lithological changes.

VSP Applications are categorized into two types:

  1. Supporting and complementing surface seismic data.
  2. Applications during well drilling.

Types of Seismic Sources

Two main types of sources are used in well seismic exploration:

  • Air Cannon: A chamber, usually 200cc, filled with air by a compressor at 2000psi. Once ready, a valve releases the high-pressure air, causing expansion in the external liquid medium and generating a wave that travels through the subsurface.
  • Vibrator: A mechanical wave generator mounted on a truck, attached to the Earth’s surface by a steel plate. It generates and transmits a long traveling wave whose frequency gradually increases over time for up to 14 seconds.

Seismic Receivers

The tool receiving the seismic signal in the well must have specific characteristics for optimal performance, including:

  • Small diameter
  • Short length and light weight
  • Pressure resistance
  • Rounded terminations at the ends for well connection

Geometry of Seismic Surveys

In conventional or surface seismic exploration, both sources and receivers are placed on the Earth’s surface. In well seismic, the source is on the surface, but the receivers are placed in the well.

Surface seismic provides considerable lateral subsurface information for several miles but loses vertical resolution at depth.

Well seismic provides high-resolution vertical penetration information, lithological information to total depth and below, but with limited lateral information.

Both surveys complement each other, providing complete vertical and horizontal subsurface information.

Vertical Seismic Profile (VSP)

The main objective is to obtain a high-quality seismic section with good resolution and penetration at the well location.