Sonar Imaging of Bay Bottom Sediments and Anthropogenic Impacts in Galveston Bay, Texas

D. S. Maddox¹, W. W. Sager², and Timothy M. Dellapenna^3
1Dept. of Geology and Geophysics, Texas A&M University, College Station, Texas 77843-3115
²Dept. of Oceanography, Texas A&M University, College Station, Texas 77843-3146
³Dept. of Marine Science, Texas A&M University Galveston, Galveston, Texas 77554

Knowledge of surface sediment distribution in Galveston Bay is important because it allows us to better understand sedimentary processes and how human activities impact the bay and its ecosystems. In this project, six areas of bay bottom were surveyed using acoustic techniques to make maps of bay bottom types and to investigate the types and extent of anthropogenic impacts. A total of 12 mi² (~31 km²) was surveyed in six areas, one in Bolivar Roads (2.4 mi², 6.2 km²), one near Redfish Island (1.2 mi², 3.1 km²), two in East Bay (4.6 mi², 12 km²), one southeast of the Clear Lake entrance (2 mi², 5.2 km²), and one in Trinity Bay (1.7 mi², 4.4 km²). We used side-scan sonar (100 kHz and 600 kHz) to image the bay bottom and chirp sonar (2-12 kHz) to image subsurface sediment layers and bottom topography. In the side-scan records, objects as small as a few meters in extent were visible whereas the chirp sonar records show sub-bottom sediment layers with a vertical resolution of about 4 in. (10 cm). The side-scan images display strong contrasts in the amount of sonar return from the bay bottom, highlighting differences in bottom texture and sediment type. The Bolivar Roads and Redfish Island survey areas displayed varied acoustic returns ranging from weak to strong, indicating differences in bay bottom sediments from fine to course. The southern East Bay survey site shows a uniform backscatter while the northern East Bay site, Clear Lake Entrance site, and Trinity Bay survey site showed generally low backscatter surrounding sub-circular, high backscatter likely indicative of oyster beds. Chirp sonar records were classified as nine seismic reflection types based on differences in seismic returns and stratigraphy. Parallel, layered sediments are seen filling the bay valley and resting atop the surface formed by erosion prior to filling of the bay. Along the flanks of the alluvial valley, the acoustic response returned an absent or weakly laminated signal, whereas areas of high oyster productivity are characterized by mounds, strong surface returns, and strong, shallow, subsurface reflectors surrounding current oyster reefs. Anthropogenic features imaged with the sonar include sediment disturbances, such as the ship channels, dredge holes, gouges, and trawl marks, as well as debris, such as submerged boats, pipes, and unidentified objects. Surface sediment and core samples collected in the survey areas correlated well with the side-scan and chirp records. In areas of high amplitude returns, stiff sediments or coarse shell debris were collected, whereas areas of low amplitude returns contained soft mud. The majority of the cores contained mud with small shell fragments near the bay floor.

AAPG Search and Discovery Article #90080©2005 GCAGS 55th Annual Convention, New Orleans, Louisiana