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How To Draw A Cross Section Geology

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Geologic Cross Section! How to draw or construct a geologic cross-section!

Sometimes strike and dip are reported without the dip direction specified . When strike and dip are reported like this, there is a convention at play called the right-hand rule. Imagine holding your right hand flat on the dipping surface, with your fingers pointing downhill. If you extend your thumb straight out to the side, thats the strike direction. Based on that rule, a bed striking at 045º could only be dipping to the SE, so its unnecessary to add the SE when you record the orientation.

The problem is, another version of the right-hand rule is that you point your thumb down-dip, and your fingers point in the strike direction. The strike and dip for a bed oriented 045º/19º SE would be reported as 225º/19º using that version. If you saw that notation and assumed the first version of the right-hand rule was in use, you would conclude the bed was dipping NW. So, if someone hires you to do geologic mapping and they want you to use the right-hand rule, make sure you find out which one. To avoid confusion, we will not use a right-hand rule for this lab, and will just have you write down the dip direction instead, like in Figure 7.7.

Vertical And Horizontal Scale

To show significant details of stratigraphic variation, it is usually necessary to exaggerate the vertical scale with respect to the horizontal scale on a stratigraphic cross section. It is important to realize the effect that this distortion has on reservoir geometry and angular relationships of geological surfaces. The small angular differences between stratigraphic horizons that account for thickness variations are strongly exaggerated in such a section. The apparent dip of a bed in a vertically exaggerated cross section is related to true dip by the following equation:

where

  • E = apparent dip in exaggerated section
  • = true dip
  • V = vertical exaggeration, or
, the ratio of vertical scale to horizontal scale

As a result of this relationship, low dips are exaggerated and appear larger, whereas higher dips all appear close to vertical. The effect is illustrated in Table 1, where selected values of true and apparent dip are shown for vertical exaggerations of five and ten times. Note that two horizons differing in dip by only 3° appear to differ by 14° and 22°, respectively, for the two values of vertical exaggeration. Any attempt to render structural form on a stratigraphic cross section is schematic but should take into account this effect. It is also important to remember that the image one creates with a stratigraphic cross section is a distortion of reality.

How Do You Draw A Section On A Map

  • How to Draw a Cross-Section of a Map.
  • Take a thin strip of paper and place it along the transect line on the map.
  • On a piece of graph paper, draw the horizontal axis of your graph exactly the same length as the transect A-B.
  • Draw two vertical axes using a suitable scale.
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    Subset Your Outcrop Data

    The relevance of this section depends on the scale of your cross section and the distribution of surface data on your map. In most cases an even spread of surface data is preferable for geological maps, however, in some large mapping areas it is only practical to collect data along traverses, which are often pre-selected for section building anyway. To make a subset of the data I perform a line buffer. This creates a polygon that will contain all the data you wish to subset and use in your cross section, thus excluding any irrelevant data that will prevent accurate section interpretation:

    • Measure an appropriate distance from your line that will allow the buffer to cover enough of your data for making a subset . The size of the buffer will vary greatly depending on the scale of your mapping, complexity of the geology and density of surface measurements, but this part will be up to you and you may need to have a few attempts with different buffers before you get it just right.
    • To buffer: Vector> Geoprocessing Tools> Buffer
    • Fill the dialogue box, being sure to direct QGIS to the correct input vector . If this vector file has more than one shape in it you will need to select the line you wish to buffer prior to accessing the Geoprocessing Tools, then make sure you tick the Use only selected features box.
    • Add in your buffer distance and type the file path to your output and go!

    Figure 3.Figure 4.

    D Geological Model From Cross Sections Preferably Using Open Source Software

    Drawing a Geologic Cross Section

    I work with engineering geology, and my workflow includes making many cross sections of a given site, with data from maps, fieldwork and boreholes.

    What I’m looking for is a way of taking many cross sections and making a 3D model, with open source software, even with lots of drawing work, but preferably with few code or programming knowledge.

    So, when I first noticed this software , that takes many cross sections and gives, as a result, a 3D model, made of solids and not just surfaces, I thought it would be the perfect software. The image below is from the software website.

    Given that I don’t have the resources to buy it, I was thinking if there wouldn’t be a free alternative to this approach.

    So I think the approach would be:

  • Take the cross sections and combine them all in a GIS software, resulting in a fence diagram
  • Interpolate between all of the cross sections, resulting in surfaces, or even better, solids, each one representing a geological layer or lithotype
  • Make new cross sections between the former ones, to refine and adjust the model.
  • So, I think the 3rd step means that there is no need of a complex interpolation algorithm, because the geological complexity would come from the cross sections, that are human made, not from the interpolation, that would be computer made.

    The approach would be a first, simple, computer made interpolation, refined by manual adjustments.

    I’ve read:

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    Orientation And Layout Of A Cross Section

    Linear cross sections are preferably oriented perpendicular to the major structural trends . Bends in the section can be introduced to accommodate variable structural trends or to show different features. In a straight section, much of the data will usually be projected into the plane of section. Accomplishing this projection requires detailed knowledge of the strike direction. If the structural trend is variable so that the cross section is not everywhere perpendicular to strike, data should be projected along strike onto the section. To fully represent the structure, several transverse sections may be linked by a longitudinal or strike section running parallel to the strike. Strike sections may also be important in showing the plunge of a structure, culminations in a fold, or the importance of secondary structures .

    Strike And Dip: Describing The Orientation Of Rock Layers

    Strike and dip are two parameters used to measure the orientation of rock layers. To get an intuitive sense of what they mean, have a look at the roof of the structure in Figure 7.4. If we needed to describe the orientation of the side of the roof above the long wall with three windows, we might start by identifying how the wall with windows or the ridge of the roof are oriented. According to the direction arrows in the photo, the ridge runs north-south. Next we would need to say how steep the roof is, and we could describe that in terms of how far off the roof is from being horizontal.

    Figure 7.4 |

    For geological layers, the strike is analogous to the direction the roof ridge is running. It is defined as the intersection of a horizontal plane and an inclined surface. You can visualize this as the line formed where the tilted rock layer in Figure 7.5 meets the surface of the lake. Dip is the angle between that horizontal plane and the inclined surface , measured perpendicular to the strike line down towards the inclined surface.

    Figure 7.5 |

    On a geological map, information about the bed orientation is provided using a T-shaped symbol, as shown on the surface of the bed in Figure 7.5. The top of the T is stretched out along the strike direction, and the vertical part points in the dip direction. The dip angle is written next to the symbol.

    Figure 7.6 |

    Strike of a Horizontal Layer

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    Measuring Strike From A Map

    In many cases strike cannot be measured directly from the map without applying some geometric methods since the beds run at an angle. Vertical beds are an exception to this, however. You can measure the strike of a vertical bed by measuring the angle between one of the beds contacts and the north arrow . In Figure 7.8, a line has been extended down from the north arrow to the contact of the bed. The angle between the north arrow and the bed is 40°. The strike of the bed is therefore 040° .

    Figure 7.8 |

    Do you understand strike and dip well enough to solve this mystery and summon a cursed village back from the Nonverse?

    Lesson : Understanding Geologic Maps

    The Basics of Geology: Converting a Topographic Profile into a Surface Cross Section Part 2

    A geologic map shows the distribution of rock beds on the surface of the Earth. Geologic maps are often combined with topographic maps.

    The pattern made by rocks on Earth’s surface depends on the orientation of the beds and the topography. Topographic variation often causes the exposure of rock beds to appear in complex patterns, even when the shape of the bed itself is not complicated. In this lesson you will learn how to draw a geologic cross-section. You will then be able to translate complex surface patterns into a picture of the rocks beneath the Earth’s surface. Before we can do that, however, you must first learn how the orientation of planar geological structures is described.

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    Earth Sci Res J Vol11 No1 Bogot Jan/june 2007

    CROSS-SECTION GENERATION OF VARIOUS GEO-SCIENTIFIC FEATURES WITHOUT CONTOUR DIGITIZATION USING A VISUAL C++ BASED SOFTWARE APPLICATION ‘VIGAT 2005’

    Naveenchandra N. Srivastava, Brijesh G. Rathod, Ajay M. Solanki, Suresh P. Machhar, Vivek R. Patel and A. R. Dasgupta

    Bhaskaracharya Institute for Space Applications and Geo-informatics Nr. Ch-0 Circle, Indulal Yagnik Marg, Nr. Info city, Gandhinagar – 382 007 India Fax: +91-079- 23213091

    Corresponding author: Naveenchandra N. Srivastava,

    Manuscript received October 24 2006. Accepted for publication June 20 2007.

    ABSTRACT

    Key words: Vigat, 2005, Visual C++, Cross Section, Contour, Profile, Software, Digitization.

    RESUMEN

    Palabras claves: Vigat, 2005, Visual C++, Perfil, Contorno, Software, Digitalización.

    INTRODUCTION

    OBJECTIVE

    Cross-sections often accompany geological, geo-morphological, engineering, tectonic, and other maps in scientific and technical bulletins, open file reports, and professional and industrial organizational proceedings. They can be helpful to determine fossil fuels and aquifers, and to locate and describe potential geologic hazards. They can also be utilized to locate subsurface minerals, hydrocarbons, geothermal areas, and construction materials .

    MANUAL TECHNIQUE FOR CROSSSECTION CONSTRUCTION

    These are the suggested steps by Maher to draw a cross-section:

    D Geological Structure :

    There are three series of rock beds in the map. The upper series is horizontal in structure. It is composed of two rock beds S & T. The thickness of the rock cannot be measurable from the map due to its horizontal manner but it can be measure from the cross-section. Both the rock beds are the same thickness. It is 100 ft each.

    The middle series is Uniclinal in structure. In section line AB, it shows an apparent dip of about 2 degrees. The middle series is composed of two rock beds O & P. The thickness of the O bed is 100 ft and the thickness of the P bed is not measurable due to the absence of its upper bedding plane.

    The lower series is folded in structure. There is a prominent anticline in the middle of the region and a syncline on the eastern side of the region. Therefore the entire region is consisting of three folded limbs with their different dip and direction at lower series rock beds. The lower series is composed of D, E, F & G beds where D is the oldest lower bed, and G is the top bed in this series. The thicknesses of the beds are 200 ft and 300 ft for E & F respectably, whereas the thicknesses of D & G beds cannot be measured due to the absence of lower and upper bedding planes for D and G beds respectably.

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    Use Qprof To Build Your Profile

    Now the fun begins

    • To create your profile click the first tab and define your input DEMs and your input line . You can add slope to your profile which will plot an additional line, useful in some engineering situations. There are also variables that can be modified, such as vertical exaggeration and profile direction.
    • Then just click the create profile button and hey-presto. The python script will create an additional window within QGIS that has additional functions for editing the plot position and other parameters such as line thickness, axes and labels.
    • You can the save your profile in a number of useful formats . With the ability to save as a scalable vector the section can be easily imported into drafting software such as Inkscapeso the cross section interpretation can be performed.

    You can also create a profile from a list of GPX points. This is a great way to quickly build a section when you are in the field and it doesnt require a DEM.

    Figure 7.Figure 8.

    What Is A Cross

    Drawing a Geological Cross-Section in Groundhog Desktop v2.0

    Cross-Sections of the Graph of a Function. For a function f, the function we get by holding x fixed and letting y vary is called a cross-section of f with x fixed. The graph of the cross-section of f with x = c is the curve, or cross-section, we get by intersecting the graph of f with the plane x = c.

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    Map : Delightful Dells Geological Paradise

  • What kind of rocks are found in this map area ? How many units can you see?
  • Draw a geological cross-section between the points X and Y indicated on Map 2. Note: you will have to prepare a cross-section from scratch for this exercise, but you can use the format from the Map 1 cross-section as a guide. If you are not sure where to start, use the questions from Map 1 to guide you through the process of exploring Map 2 and preparing your cross-section. Do not forget to incorporate the features described in the L.O.T.S. acronym. This is a more challenging cross-section you will probably have to make several changes to get it right. Make sure you have an eraser with you!
  • What are the thicknesses of the mudstone and sandstone layers? Indicate the locations where you performed these measurements on your cross-section.
  • Why cant you measure the thickness of the conglomerate or the limestone?
  • In point form, describe the geological history of the map area.
  • Lots: Legend Orientation Title Scale

    Every geological cross-section must include a legend, the orientation of the line the cross-section represents on the map, a title, and a scale . To help us remember, we abbreviate these four key parts with the acronym LOTS.

    Figure 7.11 |

    Legend: The legend is a key to the patterns used to identify each unit on the cross-section. The units are ordered from oldest formation at the bottom of the legend to youngest unit at the top of the legend.

    Orientation: The orientation of the cross-section is the direction that the cross-section line makes on Earth . You can indicate the orientation by writing the corresponding direction at each end of the cross-section .

    Title: A descriptive title for the cross-section. You can include the letters used to identify the line on the original geological map in the title .

    Scale: Include a ratio scale and/or a bar scale to show the scale of the cross-section. The vertical and horizontal scales should be the same, so you only need to include one scale on the cross-section.

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    How To Draw A Cross

    Ive wanted to write this blog post for a long time, but this weekend pushed it forward on my list because of something my daughter said to me. As some of you who follow me on social media may know, my two kids are runners. This past weekend my daughter competed in the Region 4 USATF Junior Olympic Cross Country meet in Tallahassee, Florida. She had a great meet and qualified to run at the National JO meet in December.

    As I pulled up the course maps for December in Alabama, I noticed that they were unlike any course map Ive seen over the years. There are topographic lines on the map! For once, you can look at the map and see where the hills and valleys are on the cross country course. The first thing I thought to do was to make a cross-section of my daughters course map to see if it was flat or hilly. We live in coastal South Carolina so running hills can be tough on our athletes. When I mentioned making the cross-section, my daughter said, you are the only person who would think to make a cross-section of a cross country course. Really? Certainly, there is at least one other geologist parent attending that meet who whipped up a quick cross-section. Much to our delight, her 3K course looks relatively flat. Yippie!

    For this demonstration, I am using Stone Mountain, Georgia because it has a significant elevation difference than the cross country course. Here is a topographic map of Stone Mountain.

    Topographic Map Resource:

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