Revolution in geological field mapping: real-time digital data capture and the virtual outcrop

Xu, Xueming, Aiken, Carlos L. V. and Nielsen, Kent C.

Dept. of Geosciences, University of Texas At Dallas, Richardson, TX 75083

New off the shelf technologies have evolved which when integrated provide a revolutionary means for a geologist to quickly and digitally “capture” observations in the field. This system relies on GPS positioned control points which can be determined with various levels of accuracy. A reflectorless laser rangefinder (laser gun) is used to remotely position geologic features with accuracy from centimeters to decimeters relative to the GPS control points. The laser gun can also be used to reference oblique digital photography of outcrops. Data capture with lasers and GPS, integrated with digital photographs and conventional outcrop information provide a "virtual" outcrop which is fully constrained geometrically and geographically. The data can be managed by a variety of software, including Geographic Information Systems (GIS), on portable computers. This approach also allows digital analysis to be made in the field in real time. This field data is three dimensional; unfortunately most GIS operates in 2D (X-Y) and with an attribute for Z or in 2 ½ D in which only one Z value is possible for any X-Y value. True 3D visualization requires software that is capable of using all multiple values of Z for each X-Y position. The purpose of this article is to describe the use of these integrated technologies to capture geology digitally, to visualize the data, and to analyze data, even in real time. We believe that although these technologies are effective tools at this time, they are rapidly evolving, becoming more functional, accurate, portable, and cost effective. They will become the standard for field data acquisition in the next century

Traditional geological field mapping consists of a variety of methods for capturing data including most fundamentally the compass, photographs, and sketches in notebooks. The compass allows positioning on maps and images and determination of local orientations of specific features, usually recorded as strike and dip. Photographs are taken and lines drawn on them to delineate interpreted features and their locations are determined by compass triangulation and/or map interpretation. Field sketches are used to capture the geologist’s interpretation of observations and are corroborated by photographs. Later, compilation and correlation could integrate the work of several geologists.

There are other surveying tools used to improve the precision of a mapping program. Tape measures, hand levels, rangefinders, plane table and alidades, theodolites, and total stations are used as the situation demands. Most of these tools add precision; but require more than one person to operate, are logistically more involved, require someone to go to the mapping target and are time consuming. Global Positioning Systems (GPS) is commonly used to provide instant autonomous positioning at the accuracy of ~100 meter level. When the position of a GPS receiver is determined with regard to a known GPS position (differential GPS methods) then locations can be determined with up to centimeter accuracy. These levels of accuracy can be achieved in a matter of seconds (Aiken, et al. 1998) and can be conducted in real time or refined by subsequent data processing (post processing); however, this adaptation of the GPS data is not commonly used for geological mapping at this time.

All of the methods described above require the geologist to go to the outcrop in order to document physical characteristics and to record their position. Each technique involves multiple steps for documentation and the integration of these data is usually done back in the camp or office after the fieldwork is completed. Some effort has been made to computerize the field data logging, such as entering data manually through a program such as the Geologic Survey of Canada’s FIELDLOG but data analysis is still conducted in the office.

Technology

Computer design has evolved rapidly and includes a variety of portable computers such as small palmtops and “ruggedized” field computers. Additionally, the speed and storage capabilities of computers have made it possible for the average user to handle very large databases. For example spatial data now can be handled in the field through Geographic Information System (GIS) software. Maps and the supporting data can be stored, recalled, and modified. Similarly, image processing of photographs can be conducted and these overlain on the computer maps. Reference material can be made available to the worker in the field, which permits on site comparison to field observations.

The satellite based Global Positioning System (GPS) is now fully functional. The proliferation of GPS receivers and their application for basic positioning is wide spread. In the sciences GPS are becoming standard field equipment. Centimeter level positions by static methods (one hour or longer site occupations) can now be replaced by “rapid” (seconds to 10-15 minutes) methods (Parkinson and Spilker, 1997). The survey strategy for this project involves a kinematic mode. "On-the-fly" kinematic (OTF) surveying is based on ambiguity resolution to determine "rapid" positions every epoch. The components for this survey involve the establishment of a base station which is turned on and initialized in the beginning and left to run continuously until the end of the survey. The second receiver is used to roam to specific locations where positions are determined by short occupations with the GPS receiver (Rapid Static) or in real time when radio telemetry link is established with the base station (RTK). OTF surveying is a most promising method and provides centimeter level accuracy at time intervals of seconds.

A GPS or total station survey requires every survey point to be visited. The laser gun permits an individual to stand at a location and quickly measure offsets of virtually anything that can be observed without placing a reflector on the target. An infinitesimal signal is returned and recorded from most surfaces. This kind of local survey is less accurate than a total station GPS survey, however decimeter level accuracy can be achieved at distances of up to several hundred meters with its built in compass and inclinometer. We feel that for most field applications this accuracy level represents a significant improvement over conventional mapping techniques because of its remote mapping capability. The laser gun is light and can be steadied by the use of a monopod or tripod, and has been utilized for years in the survey and utility industries. The greatest limitation on its accuracy is the angle measurements although configurations with electronic theodolites are also available. The advantage is that one person can operate this system. The laser can be used to locate individual points or in the “trigger on” mode can trace a feature by continuous pulsing at 2 to 4 Hz. The fact that these data can be globally referenced with GPS and integrated with digital maps and images, even in real time, make such an integrated system a powerful field tool.

The combination of these technologies could be considered sufficient to “capture” geologic data. However the actual image of the physical surface, which could be captured by photography, digitally, or analog, may be necessary to fully describe the surface geology.

Field Observations

We utilized the GIS, dual frequency GPS, cameras, a 486 portable computer and a laser gun to perform several different tasks. The laser gun was used to capture the particular geological features, such as stratigraphic contacts, faults, and terrain. The gun can be used by itself with data recorded in the laser gun or by directly connecting it to a field computer through a R232 port. We have done both. An interface was written in Visual Basic, which sends the data directly into ARCVIEW GIS software through a DDE connection. This configuration enables us to record the data as an ARCVIEW shape file. The survey points are associated with certain attributes identifying the feature that is mapped and then displayed on the screen in real time. This makes it possible to visualize, verify and carry out analyses at the time of data acquisition.

The field survey can map terrain, as well as planar, linear and point features including the spatial relationships between them. All of these data can be collected as quickly as the operator can aim and shoot. As mentioned above, the laser gun has the ability to collect individual point data or can operate in the continuous mode so that the operator can literally draw a feature in three dimensions. This is equivalent to making a sketch in a notebook but with three-dimensional positions for this “laser sketch”. Terrain can be mapped by the laser gun in various patterns, for example, by defining break points and break lines where significant slope changes occur. Planar surfaces such as bedding, and faults can be mapped and relationships between planar surfaces can be determined such as attitudes (strikes and dips) and stratigraphic thick nesses, even in real time. The determination of strike and dip uses the principle of the three-point problem, where a least squares fit of a plane is applied to the specific data points defining the feature. All of this data reduction is done through Arcview Avenue script programming.

GPS ties together the laser mapping and references it globally. When a feature can be occupied, the GPS can be used to map its position. A geological contact can be traced by walking it out with GPS. Alternatively, a person standing on a contact can position that point with GPS and then use the laser to map the trace of the contact every where it can be observed. OTF GPS and lasers can also be used to simultaneously or alternatively map terrain.

Digital Field Mapping—Faulting in Gently-dipping Layered Sequence

We have carried out digital geologic capture of the Ferron sandstone in Utah, a dinosaur dig in West Texas and stratigraphic analysis in the Arbuckle Mountains in Oklahoma. The field area we will discuss here was our initial test area in northeastern Texas, in the southern portion of Collin County and the city of Plano (north of Dallas). It is a site we use for teaching basic geologic mapping principles. The surface geology is characterized by gently dipping Late Mesozoic sedimentary sequences. These units generally dip towards the East Texas basin and rest unconformably on deformed Paleozoic rocks of the Ouachita fold and thrust belt. The thrust belt is in fault contact with coeval rocks of the continental platform. The exposures evaluated in this study are part of the Cretaceous age Austin Chalk and are cut by generally northeast trending, normal faults related to subsidence of the Gulf of Mexico and the East Texas basin.

A railroad cut extends for approximately one kilometer east west. We focused our efforts on the eastern portion of this railroad cut (Fig. 1). Previous fault analysis of these exposures recorded 46 recognizable faults with an average trend in a north-northeast direction and dips either to the southeast or northwest. At the eastern end of the railroad cut a 100-meter section was re-examined using the digital system described above (portable computer, GIS, OTF GPS, camera, laser gun). Six faults and a distinctive 40 cm thick chalk layer were documented (Fig. 2-A). These faults were identified sequentially from east to west. Employing a three-point problem solution, the strike and dips were determined in the field for both the faults and the bedding (Table 1). Faults 2 and 6 were not found on the north side of the railroad cut so their attitudes were less well constrained because of the poor spacing of data points. The comparison of these calculated orientations was in good agreement with the traditional compass readings and the two data sets could be integrated (the digital mapping providing the location and the compass measurements the attitude). Those data were subsequently employed to determine the thickness of the marker horizon (Table 1). Importantly, data analyses can be performed in the field.

A cross sectional view of these data is possible by projecting these three dimensional data onto a two dimensional (X-Z) surface (Fig. 2-B & C). This view shows the local dip direction towards the west and the intersection relationships for the normal faults. Further analysis includes bedding orientation variations in each fault block (TABLE 1). Similar to the determination of strike and dip, this cross sectional perspective can be generated in the field environment, obviously enhancing the scientist's interpretation abilities. Fig. 2C provides an enlargement of the horst block associated with faults 4 and 5. The individual laser data points reveal the top and bottom of the marker layer. The scatter of these points is a documentation of the uncertainty of the method.

Complementing these structural observations, we were able to quickly generate a local terrain map. This was done by two different methods, profiling and identification of local slope breaks by both GPS and laser mapping. With these topographic data a digital terrain model was produced. A digital aerial orthoquad photograph was “draped” on this improved terrain model, generating a virtual outcrop with a pixel resolution of 1 meter (Fig. 3). This orthoquad had a one-meter terrain model interpreted by a photogrammetrist but other than the existence of the cut that terrain model did not accurately represent the steep nature of the cut.

The final documentation of these outcrops was an oblique digital photograph of the outcrop. Control points were established for the oblique photographs with the laser gun. Here too, the image was rectified to the topography, which permitted the superposition of the contact information above (Fig. 4). This photograph like all of the other digital data is globally referenced and the specific data points are constrained to approximately a decimeter in the global context. The orientation of the plane of the photo is defined in 3D space. Of course, photo distortion must be considered. More refined analysis can be conducted on any of these types of images. There are numerous existing photo interpreted outcrops which could have their interpretations enhanced by these procedures.

SYSTEM DESIGN

This exercise has demonstrated the potential of digital data acquisition. The system used in this study included a portable 486 computer (less than $ 2000 now), an OTF GPS system ($50,000; we used the dual frequency geodetic units we had on hand), a laser gun ($4,000), a digital camera ($300 ) and appropriate software. The rapid advances in computers and GPS receivers are increasing their capabilities and bringing these costs down. Other configurations are possible depending on the goals. Higher precision can be achieved by improving the angle accuracy of the laser gun. Using a lower precision GPS technology and working at the meter or greater level can achieve lower cost . Many general surveys could be handled this way. Finally, traditional surveys relying on visual location either on topographic or aerial photographic bases as well as autonomous 100-meter GPS positions can be enhanced by using the laser gun to record local features. In the simplest configuration, the laser gun records data on magnetic cards, which can be down loaded later. Digital field data is available at a modest cost. Increased precision will cost more, but in any case a fully contained digital field system capable of decimeter level accuracy relative to a generally located global position is available for less than $10,000. As these costs come down, this integrated technology will be even more attractive.

Conclusion

We believe that this technology will be the standard tools of the early 21^st century. The potential of generating digital geologic data in the field will expedite the collection and distribution of geological, environmental, and resource information. The ability to do real time analysis and 3D visualization will significantly improve research and teaching. The information base available in the field will expand the field workers' or students' ability to understand the problem at hand. Basic and specific references and various analytical toolboxes will make the fieldwork more efficient. The total process of fieldwork will be made more efficient and rewarding with these digital tools. The rapid distribution of map information will impact not just geologists, but all natural scientists and society in general.

Acknowledgment

All our GPS activities have been facilitated by UNAVCO (University NAVSTAR Consortium) with equipment, maintenance and training over the years. We also thank Ben Landry and Laser Atlanta Optics for the early use and advice on the Laser Atlanta rangefinder.

REFERENCES
Aiken, C.L.V., M. Balde, J.F. Ferguson, G.D. Lyman, X. Xu, and A. L. Cogbill, Recent developments in digital gravity data acquisition on land, The Leading Edge, p. 93-97, Jan., 1998.
Graham, Lee A., 1997, Land, sea air GPS/GIS field mapping solutions for terrestrial, aquatic and aerial settings, GIS World, January, P. 40-46.
Parkinson, B. W. and Spilker, J. J., 1996, Global Positioning System: Theory and Applications, V. 1 and 2, American Inst. of Aeronautics and Astronautics, Washington D.C., 667p and 775p.

FIGURES

Fig. 1 Oblique photograph of the railroad cut (looking east) in the Austin Chalk, Collin County, Texas.

Fig. 2 A: X-Y plot of laser mapping of both sides of the outcrop on both sides of the railroad cut showing a marker layer and several faults, including a horst (faults 4 and 5). This is equivalent to a traditional geologic map.

B: X-Z projection of laser mapping of the north side. This is the cross sectional view.

C: The X-Z projection of the horst area of north side indicated in 2B.

Fig. 3 Aerial orthophotograph (one meter resolution) and laser mapping (decimeter resolution) draped over the laser/GPS derived terrain model (looking east).

Fig. 4 Oblique outcrop photograph of part of north side superimposed on the combination shown in Fig. 3 (looking directly north). Laser mapping in the area of the photograph (black dots) are shown.

Strikes and Dips

Name Strike/Dip Note

Fault1 45/67SE Both sides

Fault2 22/37SE South side

Fault3 46/57NW Both sides

Fault4 23/65SE Both sides

Fault5 19/54NW Both sides

Fault6 44/36SE South side
Seg 1 Top 56/4NW Both sides

Seg 1 Bot 56/4NW Both sides

Seg 1 55/4NW All of bed 1 points

Seg 2 Top 167/6SW Both sides

Seg 2 Bot 135/8SW North side

Seg 3 Top 91/11NE North side

Seg 3 Bot 93/5NE North side

Seg 4 Top 105/10NE North side

Seg 4 Bot 105/7NE North side

Seg 42/5NW All of the data points

Thickness

Name Thickness(m)

Seg 4 0.4

Seg 3 0.3

Seg 2 0.3

Seg 1 0.4

TABLE 1 Results of orientation calculations for bedding and faults and the subsequent calculation of bedding thickness.

E. DALLAS POST OFFICE (AUSTIN CHALK)-PHOTOREALISTIC

The Dallas bulk mail station (Post Office) is located west of Dallas near the Trinity River. These exposures of the Austin Chalk are found on the south side of the large parking area and, similar to the Preston Canyon location, contain many normal faults. We intially used GPS to map terrain and one of the faults by climbing up the side of the cliff. (Fig. 77). A traditional sketch was made of the cliff (Fig. 78) and a photo mosaic (Fig. 79). The data was plotted with SURFER (Fig. 77, 80). Two separate projects were undertaken in the fall of 1997 and 1998 (M. Abdelsalam, J. Qi, Aiken, Xu, Nielsen) to document the specific location and offset on these faults. A variety of system configurations were used; locations were determined by PP GPS and RTK . The data was tied to the CORS at Arlington, TX. The significant change from the Preston Canyon exercise was the integration of RTK and RTS as well as the CLG to improve and compare the accuracy of the offset locations. A detailed terrain model was generated with centimeter level precision. A digital photograph was draped over this terrain. The location of individual faults was carefully documented as well as the contacts of distinctive chalk layers. At this site there is a long, ~6-10 meter high east-west trending outcrop which made the laser determination of strike and dip difficult. Therefore, traditional compass readings on the faults and the slickensides (Fig. 81) (M. Abdelsalam, J. Qi, Nielsen) were used to supplement the digital mapping . Using visualization software, it was possible to analyze the net slip of individual faults and subsequently, to evaluate extension at this location (Fig. 82-86) using ARCVIEW and GOCAD.

Figure 77. (bottom) Contour map (SURFER) of GPS mapped points.

Figure 78 (center) Sketch of the outcrop.

Figure 79. (top) Photomosaic of the cliff.

Figure 81 (bottom). Slickensides along faults.

Figure 82. SURFER perspective of GPS points.



Figure 83. (top) Laser mapped points (map view).

Figure 84 (bottom) Laser mapped points in 3D.

Figure 84. (top) 3D plot of all laser points( ARCVIEW)

Figure 85. (center) 3D plot with surface attached. (ARCVIEW)

Figure 86. (bottom) 3D plot of surfaces fit to layers and faults. (GOCAD).

Closeup of two photos merged after converting photos to XYZ global coordinates through control points and draped onto digital terrain. Note the fences etc. draped onto the terrain model also. It merged within a pixel or about 1 cm in this case.

Two photos merged and draped onto terrain and displayed in 3D perspective in GOCAD. Note the triangulate mesh model.

This is the photo in flat UV space in which the direct laser mapping of layers and faults have been converted from XYZ global space to UV photo coordinates and compared to the photo. Note that digitizing on the photo is equivalent to digitizing on the three dimensional model, each resulting in the extraction of 3D information.

These are 3D examples of the surface photo, the terrain model and the interpreted surfaces inside all combined and rotated. The surfaces of the faults etc. in the area of the photos were derived from the photos whereas those features outside the photo coverage were directly mapped by the laser.

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