Seismic reflection and beach nourishment - lessons from Hurricane Sandy

Abstract

Aspects of the New Jersey Geological Survey procedures and processes in seismic and vibracores acquisition will be used to correlate between beach replenishment and storm damage mitigation. Sand for beach replenishment is essential to both the tourism industry and storm protection along the New Jersey shore. Since the early 1990s, the New Jersey Geological Water Survey (NJGWS) has obtained thousands of miles of high resolution seismic lines and several hundred vibracores on the inner continental shelf along 120 miles of the New Jersey coast in order to assess sand resources. Their research program was undertaken as a partnership between the Department of the Interior and the Army Corps of Engineers (USACE), who were tasked with coastal engineering and management. This team analyzed seismic profiles and vibracores to calculate the volume and location of sand resources in both state and federal waters. In order to complete the task, processing and interpretation of seismic data was required. Sub-bottom profiles were used to determine the reflection interface between the sand shoals and underlying strata. These top and bottom bounding surfaces were “picked” and used to calculate the volume between the two surfaces. For this project, the quality-controlled bottom track for a specific shoal feature was used as the top reflector, and the bottom reflector corresponded to the Holocene/Pleistocene surface. All thickness values were converted from two-way travel time to feet and exported as ASII data files into Surfer® software where they were combined and contoured. The contoured data was then assigned a minimum thickness, or Z value of 10 feet, which was pre-determined to be the minimum thickness value for a viable sand resource. Once the contouring was completed and quality controlled, volumes were calculated and reported to the state and federal management agencies to be used in beach nourishment projects. The need for beach nourishment in this portion of the continental United States is primarily due to the impact of winter storms called northeasters. These storms often cause severe beach erosion coupled with flooding and minor property damage. This portion of the coast is also subject to occasional tropic storms, and while infrequent, they are capable of extensive and catastrophic damage to beaches, infrastructure, and life. As documented in a United States Geological Survey (USGS) aerial photograph series of before and after Hurricane Sandy aerial assessments, there has been some decrease in storm damage in areas where beach nourishment projects were recently completed (Stockdon and Hapke, 2012).