Feature ArticleInvestigations of Marine Geohazards For Coastal Infrastructure Projects
By Todd Mitchell • Kevin Smith
Schematic of multibeam and mobile laser scanning sensor operations.
Coastal zones are among the most challenging locations to juggle the delicate balance of infrastructure development, consideration for the environment, and economic requirements related to the collection and analysis of data necessary for hazard assessments and mitigation during design. This consideration is not limited to sustainability of the environment (such as the local ecosystems) but also resiliency. Geological conditions are often a critical factor in resilient infrastructure development along the coast, yet they are frequently difficult to study. The geological conditions that present hazards to infrastructure are known as geohazards, which in the coastal zone may include examples such as nearshore earthquakes, unstable ground and inundation.
With the number of existing as well as new projects planned within coastal zones, the need for properly identifying geohazards associated with a given project site becomes critical for cost-effective construction and sustained operation throughout their designed life. The cohesive integration of these techniques'bathymetry, imagery, geophysics and geotechnics—play an integral role in the evaluation of marine geohazards affecting the design, construction and long-term reliability of coastal infrastructure.
Here, we describe a modern approach to geohazard investigations in the coastal environment that incorporates technology developed recently, which has revolutionized some aspects of data collection and provide solutions that are now within the economic reach of coastal infrastructure projects. We will focus on investigations that are below the water surface, including both the seafloor and seafloor sub-bottom, and briefly touching on mobile laser scanning.
Seafloor Surface Investigation
Many geohazards are visible on the seafloor surface due to their geomorphic expression. These surface expressions can be viewed using sonar imaging and/or multibeam echosounders (MBES). Multibeam sensors are used to map the 3D bathymetric surface of the seafloor. Sonar imaging captures the sonar reflection (backscatter) of the seafloor to produce monochromatic imagery.
Multibeam Echosounding. Multibeam bathymetric echosounding relies upon sonar (sound) pulses, which travel through water to map the seafloor. There are different types of sensors that are optimized for different environmental or data acquisition scenarios. Each sensor offers a different level of precision and accuracy, observations per pass, wider or steerable fields of view, and many also capture backscatter imagery of the seafloor.
Multibeam surveys require the sensor to be fully submerged in the water below the hull of the boat; therefore the water must typically be at least 1.5 to 2 meters in depth to allow a seagoing vessel and sensor to operate. A multibeam sensor is typically pointed downward (or at a 30 degree angle from nadir) and collects a swath four to eight times the water depth below the sensor. The edge data from wider field-of-view sensors are less reliable and require more data cleaning/editing but are more efficient for field work.
The steerable beam sensors represent a new generation of multibeam systems. This ability allows the surveyor to change the beam separation, swath width or beam directions to increase the sounding data density. An increase in data density provides higher resolution of the seafloor, which helps identify features such as fault scarps, landslides, fluid expulsion features, sunken vessels, exposed utilities and obstructions.
Mobile Laser Scanning. Mobile laser scanning from a vessel uses rapidly fired laser pulses to map above the water surface, effectively extending the multibeam survey. Mobile laser scanning provides highly detailed and more accurate data than multibeam, principally due to reduced uncertainties related to the speed of the laser and the reduced uncertainty of the medium (air versus water).
When used in combination with multibeam, an integrated point cloud data set can be developed to create a comprehensive model of the Earth's surface above and below the water line. This can extend the investigation of the site to include identification of coastal hazards, such as landslide and rock-fall prone areas along sea cliffs.
Sonar Imaging. Commonly referred to as side scan sonar (SSS) due to the conventional method of utilizing side-looking sonar transducers, this technique captures the backscatter (reflection) intensity of sonar pulses from the seafloor to create monochromatic imagery. The sensor can be a dedicated imaging device or can be a captured attribute (backscatter) of an MBES sensor.
Sonar imagery can play an important role in providing visual and contextual information of the seafloor, particularly in identifying objects, changes in seafloor surface material types and/or ecology. The level of detail (resolution) required dictates the appropriate sensor: higher frequencies have shorter range limitations and dedicated SSS sensors will provide superior imagery data to MBES systems capturing backscatter data.
Seafloor Subsurface Investigation
Although many geohazards can be identified through their surface expressions, the nearshore seafloor environment is highly dynamic, and thus active erosion and sediment transport can rapidly remove or cover surficial geomorphic evidence. Geophysical techniques that allow imaging below the seafloor can be essential for mapping these hidden features. Geotechnical drilling and core sampling is typically integrated with the geophysical data for calibration and verification.
Offshore Geophysical Surveys. Many geophysical survey projects conducted for infrastructure are performed in relatively shallow water—often in the presence of vessel traffic and/or in confined spaces. Small survey areas also require frequent turns of the vessel creative line plans to provide adequate data coverage. In addition, infrastructure engineering projects typically require high-fidelity investigations of the shallow subsurface, rather than the deep-penetration systems typical of most seismic exploration programs.
The advent of ultrahigh-resolution digital multichannel systems, such as the GeoEel (2D) and P-Cable (3D) systems built by Geometrics (San Jose, California), has revolutionized seismic reflection techniques for marine engineering and construction applications. These systems are among the highest-resolution and utilize the highest-fidelity offshore hydrophones available and offer immense flexibility for use in shallow-water environments and small areas of interest. The cables are smaller than previous generations of digital streamers, so they can be deployed from smaller vessels with shallower drafts that have lower operating costs. The exceptional signal-to-noise ratio also maximizes data quality when there are limits imposed on the maximum source signal power (such as in California).
Seismic reflection data is not a direct observation of the physical subsurface, but rather a measurement of the two-way travel time of seismic energy to distinct acoustic impedance interfaces. Interpretation software is used to map out these horizons of significantly different acoustic properties (which correspond to different material types) to create a 3D geological model that represents the bounds of different soil layers. To calculate the actual vertical positions (depths) of stratigraphic changes, this model must be calibrated, typically using geotechnical borings, cone penetration testing (CPT) and velocity profiling. To continue this article please click here.
Todd Mitchell is the remote sensing manager for Fugro in Ventura, California. With more than 12 years of experience, he consults clients on applicability of various remote sensing technologies to project requirements in multiple industries. Mitchell is an ASPRS certified mapping scientist, ASFMP certified floodplain manager, certified engineering technologist and certified GIS professional.
Kevin Smith is a professional geologist and an associate engineering geologist for Fugro in Norfolk, Virginia. Smith has spent the last 14 years using the integration of geotechnical and geophysical site investigations to conduct geohazard evaluations and support planning and design of coastal infrastructure projects.