The Offshore Sand Resources Study

contact: Stephen Van Ryswick (stephen.vanryswick@maryland.gov)

Offshore Sand Resources Study Methods
(page 4 of 1, 2, 3, 4, 5)

Our third objective in this study was to estimate the quantity and quality of sand in the shoals we identified as potential sand sources. To achieve this objective, four tasks were undertaken.

construct a three-dimensional model of the shoals
collect and analyze sediment samples from the shoals
estimate the volume of sand available in each shoal
map the sand resources

Three-dimensional modeling enabled us to determine the volume and variability of sand 3-D Model cartoon

in the shoals, and to visualize any internal structures that might affect dredging procedures. Sediment samples were needed to accurately define the grain size parameters of shoal sands.

The three-dimensional shoal model consists of a bathymetric surface, which represents the present day ocean floor, and several underlying surfaces that define the base of the shoals and other stratigraphic structures. We constructed the bathymetric surface from National Ocean Service data. A bathymetric map was prepared from this data, which served both to locate the shoals and later to define shoal volumes. From the bathymetric model, we defined three shoal fields. Within these fields, 19 shoals were selected for seismic profiling surveys. Seismic profiles were used to determine the internal structure of the shoals and the geometry of underlying structures. Seismics also present clues to which sediment types might exist on and in the shoals. Sediment samples can be correlated to seismic data for stratigraphic analysis. We have at our disposal more than 1,500 km of seismic data taken from 1985 to 1995, during MMS and Army Corps projects. Seismic data were digitized and combined with the bathymetric data to construct the three-dimensional model of the shoals.

Sampling a Shoal Based on seismic data, we selected eight shoals for coring. A total of 56 cores was taken from these shoals. We also have more than 200 archival cores with more than 3,000 sediment samples from the same MMS and Army Corps projects. Data from core samples were used to define shoal stratigraphy and determine grain size parameters. The following parameters were measured:

percent sand/gravel/mud
mean grain diameter and sorting (standard deviation) of the sand fraction
Folk and Ward(1957), and Shepard(1954) sediment classifications
vertical variations in these parameters throughout the core

These sedimentological data were combined with the three-dimensional model to produce volumetric calculations of shoal sands. We also estimated the amount and quality of sand in the 11 shoals for which we had no core samples. These estimates were based on interpretations of seismic data, and surficial sediment samples when they were available. The shoals were mapped according to their sand resource potential. Shoal regions known to have, or estimated to have sand with grain size parameters similar to native beach sands were classified as having a high resource potential. Those regions thought to have sands with less desirable characteristics were classified as having moderate or low potentials, depending on their respective grain size parameters.

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