E. Other Southern Studies

Saskatchewan Geological Survey 191

Summary of Investigations 1993 192

Computer-generated Regional Geological Maps of Southern Saskatchewan: Preliminary Report

L.K. Kreis

Kreis, L.K. (1993): Computer-generated regional geological maps of southern Saskatchewan: Preliminary report; in Summary of Investigations 1993, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 93-4.

In the spring of 1993, the Petroleum Geology Branch in­itiated a project aimed at producing computer-gener­ated structure and isopach maps at 1: 1 000 000 scale of every major subsurface stratigraphic unit in southern Saskatchewan from the Precambrian surface to the Up­per Cretaceous Belly River Formation. This paper sum­marizes the methodology adopted.

1. Previous Work The first published set of computer-plotted isopach and structure maps by Saskatchewan Energy and Mines was undertaken by Paterson (1973) for the major De­vonian formations in the province. This was followed by a similar set of maps for the major lower Paleozoic for­mations (Paterson, 1975). These maps will be super­seded by maps produced from this project.

2. Overview of Methodology Four common methods used for manual contouring are: 1) mechanical contouring, 2) parallel contouring, 3) equal-spaced contouring, and 4) interpretive contouring. Interpretive contouring is the only me1hod that permits the geologist to impart geological bias to the map. Simi­larly, it is the goal of the computer mapper to employ computer algorithms and mapping procedures, which re­sult in the best fit to the geologic interpretation that the geologist has envisioned for the data. The production of a computer-generated geological map can be divided into three major steps: 1) geological consideration, 2) data procurement and processing, and 3) execution of the mapping program to best honor the geological inter­pretation (Jones and Hamilton, 1992).

a) Geological Consideration

The goal of the computer mapper is to produce the most realistic interpretation of the subsurface. The first step is to consider, if possible, the geological nature of the surface or body being mapped. This may dictate how the data is to be processed and which algorithm should be used. Knowledge of both local and regional geology will be considered in the construction of com­puter-generated maps for the province. This will include geological aspects such as depositional environments, erosion, salt-solution effects, and faulting.

b) Data Procurement and Processing

Observed measurements are the second requirement. Three pieces of information, required for each data

Saskatchewan Geological Survey

point, are commonly expressed in terms of X Y, and Z They represent the geographic location (X }? of a data point and it's Z value which in a structure map is a depth value relative to a datum such as sea-level, and in an isopach map a thickness value of a stratigraphic unit. Geographic locations can be expressed as latitude/ lon­gitude or as northing/ easting UTM co-ordinates. The X Y, and Zdata are entered into a spreadsheet (Lotus 123 is used in this project).

Geological tops of the subsurface stratigraphic units are derived almost exclusively from petroleum, natural gas, and potash wells. The data for these wells are stored in the Well Information System (WIS) under the manage­ment of the Sedimentary Geodata Division. Exceptions to this would be data from publications and from wells which were not drilled under the Petroleum and Natural Gas Regulations and are not included in WIS. The geo­logical tops in WIS have, over the years, been picked by different geologists and changes in interpretation over that time have resulted in many of the earlier picks be­ing of questionable value. Therefore, all data are first checked for accuracy and consistency.

Various data processing techniques can be employed in computer mapping of geological data to make a com­puter-generated map which honors geological insights. For example, mapping complex geological surfaces such as reefs, which have two distinct features, reef crests and inter-reef areas, requires separation of the geologic data into two distinct populations. Each popula­tion would then be mapped independently (using an ap­propriate algorithm) and later combined to produce a more realistic representation of the reef distribution. An­other technique used in computer mapping is substitut­ing zero values with negative values or no data (2NON). This technique is used to ensure a correct zero-line pro­jection of a unit. These are but two data processing tech­niques that enable the computer-generated map to re­flect geological knowledge.

c) Execution of Mapping Program

In order to draw a contoured map, most computer pro­grams must first convert scattered data into an ordered fashion commonly referred to as a grid. Therefore, the third step involves selecting an algorithm that is most consistent with the geologic interpretation and data char­acteristics. The present project uses a software program by Zycor Inc. called Personal Computer Mapping Sys­tem (PCMS). PCMS supports nine different gridding methods which have been designed for irregularly spaced data. The program allows substantial refinement

193

of the initial grid so that the chosen grid gives the best possible representation of the data.

The structure and isopach maps of the Middle Cam­brian Earlie Formation are examples of this mapping project (Figures 1 and 2). Some minor modifications have been made using Autocad to enhance the geologi­cal accuracy of the map. The physical limitations of a page-size map do not allow the township/ range grid to be shown, therefore only a latitude and longitude grid is used. Published maps will be larger and include both grids.

194

3. References Jones, TA and Hamilton, O.E. (1992): A philosophy of con­

tour mapping with the computer; in Hamilton, O.E. and Jones, TA (eds.), Computer modelling of geologic sur­faces and volumes, AAPG Computer Applications in Geol­ogy, No. 1, 296p.

Paterson, D.F. (1973): Computer plotted isopach and struc­ture maps of the Devonian formations in Saskatchewan; Sask. Oep. Miner. Resour., Rep. 164, 15p.

_ _ __ (1975): Computer plotted isopach and structure maps of the Lower Paleozoic formations in Saskatche­wan; Sask. Dep. Miner. Resour., Rep. 165, 15p.

Summary of Investigations 1993

,. . ., ...... ----+-

,, ...

_ ,. ...

; ·----

I ('51lV \

Figw, 1 • Struc""" map of"'• top of"'• Farlie Fo,ms,On (Cambrian) ;n Saska~hewan (oonto<K infe""1 ~ 10 m)

195

Saskatchewan Geological Survey

~·-·...----1-

~-,·oo·

Hgu,o 2 - 1,opach mBP o/ /he B,rl/e '°"""'on (Cambaan) In ""''°'~hBWon (oontoudnl8<V"i ;, 10 m).

Summary of Investigations 1993

196