Automatic Geological Structure Reconstruction

INTRODUCTION

In this article, a new geological structure reconstruction method is described, based on information regarding the occurrence of geological horizons obtained by exploration.

A number of terms used in the description of the technique need to be determined. Under the wells in this note seem geotechnical boreholes that determine the physical and mechanical properties of soils. A borehole provides information on the vertical distribution of layers through the soil depth. The layers of materials around the drilling wells are shown by the segments in Figure 1.

Fig. 1: Layers of materials revealed by boreholes

When creating the geological models in specialized software packages, some additional steps are required in most cases. For example, when reconstructing geological layers using borehole data, the grouping of layer segments is performed manually. This can be a very complicated process for sites with a large number of boreholes and layers.

In this newly developed solution, the assignment of segments to corresponding geological layers is performed automatically. Here, the user can refine the results by specifying additional input parameters regarding geological information above and beyond the borehole data.

INPUT DATA

Borehole sets

For correct geological structure analysis, boreholes should be located along the borders of the modeling area, as well as at the points of sharp capacity differences in geological layers (e.g., ice layers). Borehole data:

1) Borehole coordinates
2) Wellhead mark
3) List of materials in geological layers
4) List of layers capacity

Between two boreholes, it is possible to create cross sections describing the capacity of the layers. Sections cannot have overlapping capacities (they cannot cross each other).

Fig. 2: Cross section

Describing a cross section:
1) A pair of boreholes
2) List of non-intersecting lines connecting joining points of layers.

OUTPUT DATA

Geological structure:
1) The set of geometries in the form of closed triangulated surfaces that represents geological layers, without crossing or voids between.

Fig. 3: Geological structure of a simulation area

2) Cross sections between the two boreholes.

THE CONSTRUCTION METHOD
There are four main steps in automatic geological structure reconstruction.

Step 1 – construction of idle cross sections:
The Delaunay triangulation is used based on boreholes (vertices) and user cross sections (structural ribs). The resulting sections are reconstructed by the edges of the generated triangulation. Among the reconstructed sections, an "empty" one will appear (see. Fig.4). “Idle cross sections” are defined as those that were not initially set by the user. The layer distribution in such sections is unknown.

Fig. 4: Generation of borehole triangulation

Step 2 – filling the idle cross sections:
Cutpoints of segments are connected in order to form the correct distribution of materials in idle cross sections between boreholes.

Multi-criteria optimization with linear scalarization is used in the described method. Two criteria are optimized:
1) The number of layers, filling the idle cross sections.
2) Total layer inclination.

It can be solved with sequence alignment with a recurrent formula:

Where:
d_ij– subject value for i and j borehole segments.
m_i, m_j– segment layers i and j in boreholes 1 and 2 respectively.
S_ij – layer inclination i and j.
g,h– criterion weight.

Criterion weighting can be specified by user.

This method allows splitting the borehole segments into several parts and can improve the quality of the simulation through an increase in the method operation time.

Step 3 – construction of the coarse layers
After filling in all the sections in the overall structure of the cuts, a rough reconstruction of the geological ground structure is performed on the basis of the approximate geometries (hereinafter - the “coarse” layers), describing the layers of materials.

Fig. 5: Coarse layer creation

Coarse layers are the valid geometries, composed of lines that constitute cross sections. Upper and lower bounds of coarse layers are smoothed after interpolation.

Step 4 – interpolation of coarse layers:
Various interpolation schemas were used: kriging, inverse distance weighted interpolation (Shepard's method), radial-basis function interpolation.
The user can customize these as required, selecting the most suitable one for the model reconstruction. Interpolation is carried out for the points that make up the upper and lower bounds of coarse layers.
Intermediate points are uniformly distributed in the areas of coarse layers, according to user defined distribution densities. The data points obtained thus form the basis of the upper and lower triangulated surfaces generated. These surfaces are connected to the side faces and form a geological layer.

Conclusions
This technique represents a unique solution for geological structure reconstruction.
Its distinguishing feature is that it allows the automatic construction of geological horizon 3D models involving a large number of boreholes and a large variety of soils, thus minimizing the amount of routine work for the user. The user can also create cross sections manually.
This technique is popular in creating models for the geological structure of the soil on long runs, as it significantly reduces user time for the reconstruction.