Steve Malone - CiteSeerX

E

L

E

C

T

R

O

N

I

L

(5

i I I

Steve Malone E-maih steve@geophys, washington, edu Geophysics AK-50 Phone: (206) 685-3811 University of Washington FAX: (206)543-0489 Seattle, WA 98195 Foreword In previous columns under this heading I have covered some fundamental attributes of the Internet and how to use it for obtaining information of interest to seismologists. In this issue we leave the Internet for a while to take a look at a new computer program I have found quite exciting. Jonathan Lees of Yale University has b e e n developing a GIS type of interactive display program for geological and geophysical data, which seems to fall into a niche between conventional 2-D commercial GIS programs and expensive, general purpose, 3-D display and rendering systems. This program, called "Xmap8," is designed with a seismologist's needs in mind. Similar to the mapping program GMT or the waveform display program SAC, Xmap8 is being actively distributed to the seismological community and feedback from users has b e e n incorporated into it. While it is far from a commercial "shrink-wrapped" product and still has some rough edges, it is easy to use, intuitive, and surprisingly responsive. Converting one's o w n personal data into a form for use with Xmap8 may be a daunting task for some; however, the easy availability and ease of installation of Xmap8 can allow one to take it for an evaluatory test drive using its supplied example data sets. Such an experience can easily convince one that its utility may be well worth the reformatting hassles of one's o w n data. We would like to encourage others with programs or seismic data analysis tools which should be publicized to consider writing a similar brief description for future Electronic Seismologist c o l u m n s . Contact Steve Malone

(steve@ geophys, washington, edu) with your ideas.

XMAPS: A FREE PROGRAM FOR THREE-DIMENSIONALGIS Jonathan M. Lees Dept. Geology and Geophysics Yale University, New Haven CT 06520-8109 Introduction Manipulation and interactive exploration of a variety of spatial information is n o w commonplace in most computa-

tional environments where geology and geophysics are integrated. Sophisticated Geographical Information Systems (GIS) for arranging two-dimensional (2-D) overlays to portray quantitative relationships of spatial information come in the form of commercial products, such as Arclnfo (Environmental Systems Research Institute, 1993). Simpler programs for mapping and displaying digital images, such as GMT (Smith and Wessel, 1990; Wessel and Smith, 1991) are now used extensively in the academic community. One aspect that is lacking in most commercial approaches to GIS is the incorporation of a third (or fourth) dimension. Other software exists to manipulate three-dimensional objects and raster images (AVS or IDL, for example), although these are often expensive and so general that they require a significant learning period to be useful. Geographic information is b y its nature two-dimensional (2-D). Usually information is stored in terms of a location on a map, along with attributes associated with each object. Most GIS systems allow users to store and manipulate points, 2-D objects, and 2-D raster images. A typical GIS software package allows users to plot various data sets overlaying each other, performing statistical analysis of different combinations of objects and combining data to form new objects, among other functions. Demographers and geographers find 2-D GIS systems more than adequate to handle many problems dealing with distributions over 2-D regions. For geologists and geophysicists, however, the restriction imposed by the inherent two-dimensionality of the c o m m o n GIS system is severe. As earth scientists, we typically deal with data that has three spatial coordinates, and sometimes a fourth time coordinate. For example, seismologists studying earthquake h y p o c e n t e r distributions in space and time n e e d a platform that integrates the power of a GIS approach in four dimensions. Seismic three-dimensional modeling is n o w c o m m o n in the form of 3-D velocity analysis (seismic tomography). The 3-D field is an extension of 2-D raster images to multidimensional raster grids (hyper-slabs). H o w do 3-D velocity fields relate to the three-dimensional distribution of seismicity? H o w d o Bouguer gravity anomalies distribute relative to the distribution of earthquakes, surface geology, topography and 3-D raster grids describing subsurface structures? Examples from my own work include studying the relationship of seismicity to the 3-D distribution of seismic velocity anomalies along the San Andreas Fault (Lees, 1990; Lees and Malin, 1990; Lees and Nicholson, 1993; Nicholson and Lees, 1992) and delineation of a possible magma system at Mount St. Helens (Lees, 1992; Lees and Crosson, 1989). While traditional 2-D GIS systems handle the plotting of geologic maps quite well, they are deficient if subsurface geologic information is available, such as from boreholes, well-logs, or seismic reflection horizons. What geophysicists need is a simple interactive platform to examine, query, and manipulate various data sets in the 3-D context. The "interactive" aspect of a GIS system is crucial to its value as a research tool. Interactive analysis has a two-fold

Seismological Research Letters Volume 66, Number 4, July-August 1995

33

Mt St Helens E a s t - 2;4.69 km

West - 0

.......~'. lStoping R e g i o n ]

~

Helens Ediface L

~

High Vp Magma Chamber

CD

3

iii~iii:

-7. t

]

%Slowness | ]

5.6

15.00 km

9 Figure 2 Cross section through a tomographic image of Mount St. Helens P-wave velocity anomalies (Lees, 1992). Focal mechanisms are front projections, color coded according to rake. Small dots outline seismicity associated with magma movement and triangles are stations plotted at their projected locations. Lines were digitized from an independent interpretation of the magma system (Pallister et a/., 1992). meaning: 1) the user should be able to choose, during program execution, what data are displayed, and 2) during visual exploration of the 3-D data, the user needs access to information regarding the geographic location and attributes of selected objects. Both of these operations should be easy, i.e., should require no more than a m e n u selection, a pointand-click, or a simple dialogue response to the GIS program. We need to be able to explore our data in ways that allow us to simultaneously "see" all the data and at the same time not be overwhelmed by the sheer volume of information. If w e are able to Interactively select and change displayed information, we will not have to anticipate, apriori, what parts of our data are most revealing. This approach provides an element of serendipity in data exploration that I hold to be essential for scientific discovery. Furthermore, I anticipate that in the future, earth scientists will publish 3-D data sets along with programs similar to that presented here, which allow readers to interact with data so they may verify conclusions drawn by authors without being restricted to the limited presentation provided by traditional print journals.

34

In this paper I present an introduction to a new program, Xmap8, for accomplishing some of the rudimentary datamanipulations required for 3-D GIS. An example of a fairly complex GIS type figure p r o d u c e d by Xmap8, involving a number of different types of data, is illustrated in Figure 1 (see cover). As a seismologist, I have directed the program at problems that earthquake seismologists or structural geologists might approach. In this figure a combInation of geologic information is c o m b i n e d with seismological data, producing a very information-packed display. Using Xmap8, one can turn on (or off) any o n e of several overlays to reduce the density of information, allowing subtle features, or combinations of features, to reveal themselves. Other data such as potential field maps, tomographic images of seismic velocity, or station P-delay times could easily be added to this data set and manipulated with Xmap8.

Background Three-dimensional GIS works slightly differently than traditional two-dimensional GIS systems. The 2-D systems typi-

Seismological Research Letters Volume 66, Number 4, July-August 1995

X:56.5

~ Y:130.7

~ Z:58.6

~

0

u

P / /

I

~

J

I

t

f

J

]4.0km

9 Figure 3 Mount St. Helens stoping region m spin module. View is from northwest above the ground looking southeast. The drawing shown in Figure2 is rotatedalong with the hypocenters.A 3-D wire-flame model (lightgray) ofthe stopingregionabove the magma chamber is rotated and projected. Events are plotted such that their size is porportionalto distance away from plane of view so that closer objects appear larger and provide depth perception. The angles the rotated axes make with a vector pointing intothe page (unrotated Z-axis) are printed for reference.

SeismologicalResearch Letters Volume 66, Number 4, July-August 1995

35

cally work with a set of 2-D overlays, which are draped over each other, forming a single image such as Figure 1. Users can manipulate images and query a database about specific objects being displayed to understand their relationships. In 3-D there may be several layers of data to be analyzed simultaneously and retrievable by interactive queries. Since w e can only view the data as 2-D projections on a page or computer screen, examining 3-D relationships are difficult unless one has the means to take arbitrary slices through sections of data space. To illustrate some of the details of Xmap8 I start by considering the kinds of objects o n e needs to solve interdisciplinary, 3-D problems in geology and geophysics. As in 2-D analysis, one starts with a surface base map consisting of points, lines, and filled polygons. The base map typically includes political boundaries, geographic indicators, fault traces, surface geologic units, volcanic vents, etc. These are stored in an ASCII database (as are all Xmap8 data), which contains attributes associated with each map element indicating location, name, color, or other identifying features defined by the user. Next, three-dimensional information may be added as points, lines, surfaces, and layered raster images in a 3-D coordinate system. Because Xmap8 was designed initially for seismological applications, point data in the form of seismic stations and earthquake hypocenters are treated as special separate cases of generic 3-D point data. Three-dimensional line data comes in m a n y guises, for example: geothermal or oil field lithologic information along deviated bore-holes, fields of vectors representing flow, line drawings outlining features of a geologic interpretation, or lines following a seismic horizon o n a reflection/refraction line. A fault plane may be described as a surface or combination of surfaces. In Xmap8, 3-D structures are stored as sets of planar polygons (wire-frame structures), although they do not have to be contiguous, so that torn faults or subduction slabs can be represented. The final set of objects n e e d e d for 3-D GIS are 2-D and 3-D raster images. These can be imported using several standard image formats, such as netCDF. A contouring package is included which can take irregularly spaced 2-D data, perform gridding and draw contours of the surface over the base map (Smith and Wessel, 1990; Wessel and Smith, 1991).

Program Operations The "gestalt" of Xmap8 for 3-D GIS generally runs like this: The program accepts, either o n the c o m m a n d line or via menu-driven dialogue boxes, names of files containing geographic objects and other parameters which describe the database. The main window is a map view showing objects projected on the horizontal surface. If a 3-D raster is included (Figure 1, on cover), one can scroll through the model (in depth) layer by layer. The unique aspect of Xmap8 is that from the map view one can define a cross section (with arbitrary dip and width) slicing through all the 3-D objects in the database. An example is provided in Figure 2, as a cross

36

section through Mount St. Helens. This means w e can observe, in depth, the spatial relationship between hypocenter distributions, fault places, geologic wire-frame models, tomographic images, bore-hole data, etc., with a simple point-and-drag mouse operation. Numerous cross sections can be viewed simultaneously along with a map view. Furthermore, since information is stored in a (primitive) database, the user has interactive access to specific features. For example, one can lasso (in map or cross section view) a subset of hypocenters to see their detailed parameters, or store them later for further analysis, or o n e can click on a location and find out what the seismic velocity is from the raster image. In cross section view, contour data can be projected along with surface geologic features, deviated bore-hole information and wire-frame structures. In addition to the generic objects described above, some unique facilities make Xmap8 particularly useful for geophysicists. Special attention has b e e n paid to the plotting of focal mechanisms. Several choices for plotting fault plane solutions are provided including strike-dip of fault plane, Pand/or T-axes, and fault plane trace or traditional beach balls. Since the choice of the fault plane may be ambiguous, the program allows one to switch designated fault and auxiliary planes. The ability to quickly, and intelligently, sort through a large data set allows researchers to def'me details of smaller, secondary faulting associated with large events like Joshua Tree-Landers. Spinning facilities (rotation of objects about arbitrary axes) are currently available in many data management, statistical, and CAD programs. Xmap8 has a spin module, optimized to provide specific information with which geologists are concerned: the final strike and dip of a particular view, after numerous rotations about arbitrary axes. Fault planes of aligned seismicity can be determined quickly and quantitatively. Figure 3 is a view of the seismicity under Mount St. Helens from the northwest above ground looking to the southeast. The 3-D wire-frame model of a possible stoping region above a p r o p o s e d magma chamber was rotated appropriately. Events are plotted such that their size is proportional to the distance away from the plane of view. Xmap8 has a n u m b e r of other handy features and as the n e e d becomes apparent yet more will be added. For example, viewing earthquake hypocenters plotted in animated time sequence on a base map or cross section may be useful and can n o w be done. Time intervals between events are used to simulate true time sequences of aftershock activity. There is a special module for handling bore-hole lithology or well log data, including apparent dip from dip-meter data. Labels and legends can be placed o n views and PostScript hardcopy output can be made of any view.

Conclusion Besides the personal data-exploring capabilities of GIS, programs such as Xmap8 can provide n e w capabilities to exchange complex data sets in a simple format for scientific

SeismologicalResearch Letters Volume 66, Number 4, July-August 1995

discussion. I imagine future electronic publication of results relating three-dimensional distribution of earthquake hypocenters, gravity, geology, 3-D tomographic inversions, and seismic reflection/refraction lines will be commonplace. If such data are published in a standardized format, programs such as Xmap8 can be used to view and interact with data in 3-D. Compared to limited, 2-D views in paper publications, prepared by authors not anticipating the importance of a different viewing angles, the value of interactive programs like Xmap8 is obvious. Readers of such "electronic" publications will be able to examine, thoroughly, claims of spatial relationships made by authors. Xmap8, its documentation and sample data sets are available via FTP over the Internet, at

milne, geology, yale. edu (130.132.22.24)in directory pub/Xmap8. A summary of this article, the figures published here, a full reference manual, and other details of Xmap8 including a full set of UNIX man pages are available in html w i t h a WWW browser at h t t p : / / l o v e . g e o l o g y , y a l e . edu. Executable code is currently available for SUN Sparcstations running SUN OS-4.1.3 which will also work o n Solaris-2.3 under the compatibility mode. The source code will be available in the future for easy porting to any X11 UNIX environment. M

Acknowledgments The author acknowledges the efforts of Craig Nicholson, Geoff Ely, Jess McCullugh, Steve Malone and Bob Crosson for numerous comments and suggestions which significantly improved Xmap8. Portions of the code

were developed by Bob Fischer, Peilin Jia and Mark IAndner. Thanks to Mark Alvarez for the original suggestion to develop this code. During program development the author was supported by NSF NEHRP grant EAR-9011441 and the donors o f The Petroleum Research Fund, PRF 26595-G2, administered by the American Chemical Society.

References Bortugno, E.J. and T. E. Spittler (1986). Geologic Map oftbe San Bernardino Quadrangle. Lees, J. M. (1990). Tomographic P-wave velocity images of the Loma Prieta earthquake asperity; Geophys. Res. Lett. 17, 1433-1436. Lees, J. M. (1992). The magma system of Mount St. Helens: Non-linear high resolution P-wave tomography, J. Volc. Geotb. Res. 53, 103116. Lees, J. M., and R. S. Crosson (1989). Tomographic inversion for threedimensional velocity structure at Mount St. Helens using earthquake data,./. Geophys. Res. 94, 5716-5728. Lees, J.M., and P.E. Malin (1990). Tomographic images of P-Wave velocity variation at Parkfield, California, J. Geopbys. Res. 95, 21,793-21,804. Lees, J. M., and C. Nicholson (1993). Three-dimensional tomography of the 1992 Southern California sequence: Constraints on dynamic earthquake ruptures?, Geology zl, 385-480. Nicholson, C., and J. M. Lees (1992). Travel-time tomography in the northern Coachella Valley using aftershocks of the 1986 Mt 5.9 North Palm Springs earthquake, Geopbys. Res. Lett. 19, 1-4. Pallister, J. S., R. P. Hoblitt, D. R. Crandell and D. R. Mullineaux (1992). Mount St. Helens a decade after the 1980 eruptions: magmatic models, chemical cycles, and a revised hazards assessment, Bull. Volcanol. 54, 126-146. Smith, W. H. F., and P. Wessel (1990). Gridding with continuous curvature splines in tension, Geophysics 55,293-305. Wessel, P., and W. H. F. Smith (1991). Free software helps map and display data, EOS (Trans. Am. Geophys. U.) 72, 445-446.

SeismologicalResearch Letters Volume66, Number 4, July-August 1995

37