Introduction

The Manchas Interiores and Manchas Exteriores is a live reef complex off the city of Mayagüez, western Puerto Rico (Fig. 1). This reef complex may be affected by sewage discharge from the nearby Mayagüez regional water treatment plant. The Río Grande de Añasco empties into this area, carrying a significant load of suspended sediment and contaminants that may further stress the marine community. In order to adequately assess the relative significance of the different sources of reef stress, a program was initiated to monitor the sediment and water-quality characteristics of the Río Grande de Añasco in addition to sampling of effluent, ocean water, and sediment in the vicinity of the outfall and over the coral reef. In support of this effort, a high-resolution sidescan-sonar and seismic-reflection survey was conducted in December, 1990, over the reef complex using the research vessel JEAN A in order to provide a geologic framework, or base map, for the offshore component of the program. These sea-floor mapping data were originally reported in Schwab and others (1991). In this report, we present the sidescan-sonar imagery, grain-size analyses of sediment samples, and a preliminary interpretation of the data in digital form.

Methods

The sidescan-sonar survey was conducted using a Klein model 531, 100 kHz system; total swath width per trackline was 200m with the along track resolution of ~0.25m. The sidescan data were logged digitally using a QMIPS data-acquisition system (Danforth and others, 1991) at a sampling rate that resulted in a 0.1 m pixel size in the across track direction. The data were then decimated to a 0.4 m pixel size using a median filtering routine developed by Malinverno and others (1990) and were processed and mosaicked in the field using procedures developed by Danforth (1997). This mosaic (Schwab and others, 1991) was used as a base map for the subsequent sampling phase of the investigation.

The sidescan data were further processed using routines developed by Chavez (1986) as modified by Paskevich (1992) for application to high-frequency sidescan-sonar imagery. Digital mosaicking was accomplished using the PCI Remote Sensing software package as described by Paskevich (1996). This dataset was mapped at a resolution of 1m/pixel in a UTM zone 19 projection using the WGS84 ellipsoid. Darker tones on the sidescan-sonar images represent areas of relatively low acoustic-backscatter intensity and lighter tones, areas of high backscatter (Fig. 2).

Concurrent with the acquisition of the sidescan-sonar imagery, approximately 89 km of 3.5 kHz and Huntec Boomer seismic-reflection profiles were collected.  These data were recorded using an analog Raytheon recorder. The original profiles are archived at the U.S. Geological Survey, Woods Hole, MA 02543. The ship tracklines overlain on the sidescan-sonar mosaic are presented in Figure 3. The bathymetry presented is derived from analysis of the 3.5 kHz profiles.

Bottom sediment samples were obtained in the study area using a Shipek grab sampler (see Fig. 5 and Table 1 for sample locations). Sediment samples collected at stations B1 through B8 (five samples collected at each station; A through E) were used for biologic analysis. Sediment textural analysis was conducted using a Coulter Counter following the methodology of Poppe and others (1985). Results of this textural analysis are presented in Table 1 (sediment type is described using the class limits of Shepard, 1954).

Ship navigation utilized a shore-based Miniranger transponder system (Schwab and others, 1991). Using these navigation data, the seismic-reflection profiles and bottom sample locations are accurate to within 5 m. The sidescan towfish, however, was not navigated independently of the ship, thus, an approximate additional error of up to 15 m exists along-track in the sidescan imagery. This additional error is attributed to the layback of the sidescan towfish relative to the ship's position.  This distance remained relatively constant throughout the survey (Schwab and others, 1991).

The digital mosaic completed in PCI was exported as a TIFF (raster) image with accompanying georeferencing information in a separate file (a TIFF world file).  This combination of image file and georeferencing file allows the sidescan mosaic to be imported and geographically registered in a geographic information system such as ARC/INFO. The imported image can be converted to a grid so that further analysis of the mosaic can be performed based on the sidescan-sonar DN values.

In addition to the sidescan-sonar data, the sediment sample locations and analyses, ship tracklines, sidescan interpretive map, and bathymetric contours have all been converted to the ARC/INFO format.  These files may be obtained from Openfile Report 98-427. 

Geologic Framework of the Study Area

The level of relative acoustic backscatter intensity on a sidescan-sonar image is a function of, among other things, the sea-floor topography, roughness, and composition (e.g., Reed and Hussong, 1989). The Manchas Interiores-Manchas Exteriores reef complex is represented on the sidescan mosaic as an area of relatively high-backscatter intensity (Fig. 2 and Fig. 6). Areas of relatively low-backscatter intensity within the reef complex represent either acoustic shadows or areas where the reef is covered by a thin veneer of sediment (Fig. 2 and Fig. 6). The reef complex is built on a regional Pleistocene (?) unconformity that is displayed on the seismic-reflection profiles (Fig. 7).

Sediment samples collected east (inshore) of the reef complex are dominantly clayey silt and silty clay. This sedimentary deposit is represented on the sidescan mosaic as an area of relatively low backscatter intensity (Fig. 2 and Fig. 6). High backscatter "blotches" on the sidescan mosaic are due to large amounts of anthropogenic and biologic (?) debris in the water column; probably related to the plume of suspended sediment originating from the Río Grande de Añasco. This suspended sediment plume encroached on the reef complex during this survey. The muddy sedimentary deposit east of the reef complex, thought to be derived primarily from the Río Grande de Añasco, blankets a Pleistocene (?) unconformity and thins from approximately 30 m thick in the northeast segment of the study area near the mouth of the Río Grande de Añasco to less than 8 m thick over an arch in the unconformity between latitudes 18°14.5’N and 18°14.0’N (Fig. 7 and Fig. 8). In the extreme northeast segment of the sidescan-sonar mosaic (closest to the mouth of the Río Grande Añasco), an area of relatively low backscatter is interrupted by blotchy high-backscatter returns. These high-backscatter returns are also from material in the water column (Fig. 2 and Fig. 6). Seismic-reflection profiles collected over this area show evidence of gas-charged sediment; blanking of internal reflectors on the seismic-reflection profiles. Thus, the acoustic "noise" from the water column displayed on the sidescan imagery off the mouth of the Río Grande de Añasco may be partly due to degassing of sediment, common in other deltaic settings (Coleman and others, 1982). Additional acoustic artifacts are visible in the northern part of the sidesan imagery and are noted on Figure 6.

The remaining sediment type, sandy reef-derived sediment, was sampled off the northeast and northwest margin of the reef complex and within channels that run through the reef. This sediment type typically has a relatively higher backscatter intensity than the muddy sedimentary deposit. Along the northeast margin of the reef complex, a poorly developed moat has formed, probably in response to storm-induced water flow (Fig. 4 and Fig. 6). A minor moat may also exist along the southern margin of the reef complex (Fig. 4 and Fig. 7). One gravelly sand sample, GS28, was rather unusual in that it was composed of a mixture of reefal and terrigenous material. These sandy deposits are interpreted to be lag deposits that are formed by storm-surge-induced currents which flow through the channels in and around the margins of the reef complex (Schwab and others, 1991).

Although no clear evidence of active faulting was observed on seismic-reflection profiles (faults offsetting the sea floor), the trend of the northeastern margin of the reef complex is approximately N65°W, which is subparallel to the major structural lineaments observed in adjacent onshore areas (Case and others, 1984). Thus, the morphological development of the reef complex may have been controlled by the local basement structure.

Acknowledgements

The success of this project was based on the broad range of technical support provided by the U.S.G.S. Coastal and Marine Geology Program and Puerto Rico Water Resources Division. Technical support of offshore surveys was provided by Thomas O'Brien, William Danforth, Barry Irwin, Juan Trías, Milton Carlo, and Rafael Rodríguez. The digitized bathymetry was furnished courtesy of Richard M.T. Webb of the Puerto Rico Water Resources Division. Helpful reviews of the manuscript were provided by William Danforth, Rob Thieler and Jane Denny. In addition, we appreciate the cooperation of the captain and crew of the research vessel JEAN A, provided by the Department of Environmental and Natural Resources, Commonwealth of Puerto Rico.

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