Building Geological Cross-Sections from Borehole Data: A Step-by-Step Workflow

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Building Geological Cross-Sections from Borehole Data: A Step-by-Step Workflow

Geological cross-section being created from borehole data showing correlated soil and rock layers between multiple boreholes.

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Introduction

Geological cross-sections are one of the most important tools used in geology, geotechnical engineering, hydrogeology, and environmental investigations. These diagrams provide a visual representation of subsurface geology by showing how soil and rock layers extend beneath the ground surface.

Because most of the subsurface cannot be observed directly, engineers and geologists rely on borehole drilling to gather information about underground conditions. Each borehole provides a vertical record of geological materials encountered during drilling.

However, interpreting subsurface geology requires more than simply reviewing individual borehole logs. Geological layers must be correlated between boreholes in order to understand how they extend across a site.

Geological cross-sections transform discrete borehole data into continuous interpretations of subsurface stratigraphy. These diagrams help engineers understand soil conditions, identify geological hazards, and design safe infrastructure.

Although modern geological software can generate cross-sections automatically, it is important to understand the underlying workflow involved in constructing them. Cross-section interpretation still requires geological reasoning and careful data preparation.

This article explains the step-by-step process used to build geological cross-sections from borehole data, including data preparation, section design, lithological correlation, and visualization techniques.

Step 1: Collect Borehole Data

The first step in creating a geological cross-section is collecting borehole data from a site investigation.

Boreholes are typically drilled using geotechnical drilling rigs that penetrate soil and rock layers beneath the ground surface. As drilling progresses, geologists or geotechnical engineers record observations about the materials encountered.

Typical borehole logs include:

  • lithology descriptions
  • depth intervals
  • soil classifications
  • groundwater observations
  • sampling information

Each borehole log represents a vertical profile of subsurface conditions at a specific location.

Accurate borehole logging is essential because cross-section interpretation depends entirely on the reliability of the borehole data.

Step 2: Organize Borehole Data in a Database

Before constructing a cross-section, borehole data must be organized into a structured format.

Modern geological workflows typically use borehole databases that store information in tables.

A typical borehole database contains two main components.

Borehole Collar Data

This table stores general information about each borehole, including:

  • borehole ID
  • coordinates (easting and northing)
  • ground surface elevation
  • drilling date

These values define the spatial location of each borehole.

Lithology Interval Data

This table describes the geological materials encountered in the borehole.

Typical fields include:

  • borehole ID
  • from depth
  • to depth
  • lithology description

This structured data allows geological software to interpret borehole logs and generate cross-sections.

Proper database preparation is essential for reliable cross-section modeling.

Step 3: Select a Section Line

A geological cross-section represents a vertical slice through the Earth along a specific line.

This line is called the section line.

The section line determines which boreholes will appear in the cross-section and how they will be positioned relative to each other.

When selecting a section line, geologists often consider:

  • project layout
  • geological features
  • infrastructure alignment

For example, in a highway project, cross-sections may be drawn along the proposed road alignment.

Multiple section lines may be used across a site to analyze subsurface conditions in different directions.

Step 4: Project Boreholes onto the Section Line

Once the section line has been defined, boreholes must be projected onto that line.

This process determines the position of each borehole within the cross-section.

Boreholes located directly along the section line appear in their true position.

Boreholes located slightly off the line may be projected onto the section if they fall within a specified distance from the section line.

Projecting boreholes allows geologists to include nearby boreholes that provide useful information for interpretation.

However, excessive projection distances should be avoided because they may distort geological interpretation.

Step 5: Plot Borehole Logs

The next step is plotting the borehole logs within the cross-section.

Each borehole is drawn vertically with depth or elevation values along the vertical axis.

Lithology intervals are then plotted according to their depth ranges.

For example, a borehole may display layers such as:

  • clay from 0–3 meters
  • sand from 3–7 meters
  • gravel from 7–12 meters

Plotting these intervals creates a visual representation of each borehole log.

At this stage, the cross-section displays individual boreholes but does not yet show continuous geological layers between them.

Step 6: Correlate Geological Layers

Lithological correlation is the process of connecting similar geological layers between boreholes.

This step requires geological interpretation.

Geologists analyze lithology descriptions, stratigraphic position, and regional geological context to determine which layers represent the same geological unit.

For example, a sand layer appearing at similar depths in multiple boreholes may represent a continuous depositional unit.

Correlation lines are drawn between these layers to create continuous geological horizons.

However, correlation is not always straightforward.

Layers may change thickness, composition, or elevation between boreholes.

Geologists must consider these variations when interpreting the cross-section.

Step 7: Identify Geological Features

While correlating layers, geologists may identify important geological features such as:

Pinch-Outs

A layer may gradually thin until it disappears between boreholes.

Erosional Surfaces

Older layers may have been partially removed by erosion before new sediments were deposited.

Sediment Lenses

Localized deposits of sand or gravel may occur within otherwise uniform sediments.

Structural Features

Faults or folds may offset geological layers.

These features should be incorporated into the cross-section interpretation.

Recognizing geological structures improves the accuracy of subsurface models.

Step 8: Add Ground Surface Profile

A geological cross-section typically includes the ground surface profile.

This profile represents the topography along the section line.

Ground surface elevation is calculated using borehole elevation data and topographic surveys.

Displaying the ground surface helps engineers understand how subsurface geology relates to site topography.

It also allows cross-sections to be used for engineering analysis such as slope stability or excavation planning.

Step 9: Add Groundwater Information

Groundwater observations are often included in geological cross-sections.

Borehole logs may record water levels encountered during drilling or monitoring.

These water levels can be plotted within the cross-section to illustrate the groundwater table.

Groundwater information is important for:

  • foundation design
  • excavation planning
  • groundwater modeling

Including groundwater data improves the usefulness of the cross-section for engineering applications.

Step 10: Review and Refine the Interpretation

Once the cross-section has been constructed, it should be carefully reviewed.

Geologists must ensure that the interpretation is consistent with:

  • borehole data
  • regional geology
  • depositional environments

If inconsistencies appear, the interpretation may need to be revised.

Cross-section interpretation is an iterative process that improves as additional data becomes available.

Using Software to Create Cross-Sections

Modern geological software allows cross-sections to be generated automatically from borehole databases.

These programs can:

  • import borehole data
  • plot borehole logs
  • generate cross-sections
  • interpolate geological layers

However, automated cross-sections should always be reviewed by experienced geologists.

Software algorithms may produce unrealistic correlations if borehole data contains inconsistencies.

Human interpretation remains essential.

Common Mistakes in Cross-Section Construction

Several common mistakes occur when building cross-sections.

Assuming Layer Continuity

Geological layers are not always continuous between boreholes.

Ignoring Pinch-Outs

Layers may thin and disappear rather than continuing across the entire site.

Misinterpreting Lithology

Inconsistent descriptions may cause incorrect correlations.

Over-reliance on Software

Automated interpretations should be verified manually.

Avoiding these mistakes improves cross-section reliability.

Applications of Geological Cross-Sections

Cross-sections are widely used in engineering and environmental investigations.

Geotechnical Engineering

Cross-sections help engineers design foundations and evaluate soil stability.

Hydrogeology

Hydrogeologists use cross-sections to understand aquifer geometry and groundwater flow.

Environmental Studies

Cross-sections help track contaminant migration through soil layers.

Resource Exploration

Mining geologists use cross-sections to visualize ore deposits.

These applications demonstrate the importance of cross-sections in subsurface interpretation.

Conclusion

Building geological cross-sections from borehole data is a fundamental process in geological and geotechnical investigations. By connecting borehole logs across a site, cross-sections transform discrete data points into continuous interpretations of subsurface geology.

The process involves several key steps, including data preparation, section line selection, borehole projection, lithological correlation, and geological interpretation.

Although modern geological software can assist with cross-section construction, accurate interpretation still requires geological expertise.

When constructed carefully, geological cross-sections provide valuable insights into subsurface conditions and support better decision-making in engineering, environmental management, and resource exploration.

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