DETAILED GEOLOGIC MAPPING—IS IT WORTH THE COST? APPLICATIONS OF A GEODATABASE OF THE SEATTLE, WASHINGTON AREA

In the Seattle area, new derivative maps are facilitating planning, engineering, research, and outreach to a greater user population than ever before. This resurgence is due in part to a more informed user base and in part to having more useful map products. Multi-agency collaboration and funding have supported the building of a database of subsurface geologic data, the preparation of new digital geologic maps, and many new derivative maps.

Geologic maps and borehole data are available to partners on agency intranets via interactive programs, to the public over the Internet using ArcIMS, and to visitors using our computer lab. The new maps and geodatabase provide information such as regional geologic context for subsequent site-specific investigations, quick cross section construction, information about the extent of fill, thickness of geologic layers, rapid scanning for specific geologic settings, and depth to groundwater.

Two of the most useful derivative maps are: 1) potentially infiltrative soils, and 2) the depth to glacially overridden material. Since the new geologic map of Seattle yields twice the land area with high infiltration potential, than was previously mapped in the City, this new map provides the critical base for evaluating concerns for stormwater runoff and contamination. The surface of glacially overridden materials, combined with the depth to bedrock and ground topography, allows easy creation of a simple but defensible seismic-velocity model of the Seattle area, now being used to generate earthquake ground-motion models and to evaluate liquefaction potential in greater detail than ever before.

Through collaboration with this mapping project, hundreds of planners and engineers are being kept abreast of current research and geologic findings. Yet local costs are substantial: a detailed, digital, USGS-published 7.5' geologic quadrangle map based on new field work and a subsurface database averages $250k at 1:24,000 scale and about twice that amount at 1:12,000 scale. Derivative maps are not nearly as expensive, but they too add an incremental expense; being only as good as their base maps. How can we quantify the benefits to our profession of having better geologic data and better educated clients to work with? Ultimately, are these new geologic products worth their cost?