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Marine Geophysical Investigation of Selected Sites in Bridgeport Harbor, Connecticut, 2006

Prepared in cooperation with the U.S. Army Corps of Engineers, New England District

Scientific Investigations Report 2007–5119

By Carole D. Johnson and Eric A. White

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Abstract

A marine geophysical investigation was conducted in 2006 to help characterize the bottom and subbottom materials and extent of bedrock in selected areas of Bridgeport Harbor, Connecticut. The data will be used by the U.S. Army Corps of Engineers in the design of confined aquatic disposal (CAD) cells within the harbor to facilitate dredging of the harbor. Three water-based geophysical methods were used to evaluate the geometry and composition of subsurface materials: (1) continuous seismic profiling (CSP) methods provide the depth to water bottom, and when sufficient signal penetration can be achieved, delineate the depth to bedrock and subbottom materials; (2) continuous resistivity profiling (CRP) methods were used to define the electrical properties of the shallow subbottom, and to possibly determine the distribution of conductive materials, such as clay, and resistive materials, such as sand and bedrock; (3) and magnetometer data were used to identify conductive anomalies of anthropogenic sources, such as cables and metallic debris. All data points were located using global positioning systems (GPS), and the GPS data were used for real-time navigation.

The results of the CRP, CSP, and magnetometer data are consistent with the conceptual site model of a bedrock channel incised beneath the present day harbor. The channel appears to follow a north-northwest to south-southeast trend and is parallel to the Pequannock River. The seismic record and boring data indicate that under the channel, the depth to bedrock is as much as 42.7 meters (m) below mean low-low water (MLLW) in the dredged part of the harbor. The bedrock channel becomes shallower towards the shore, where bedrock outcrops have been mapped at land surface. CSP and CRP data were able to provide a discontinuous, but reasonable, trace from the channel toward the west under the proposed southwestern CAD cell. The data indicate a high amount of relief on the bedrock surface, as well as along the water bottom. Under the southwestern CAD cell, the sediments are only marginally thick enough for a CAD cell, at about 8 to 15 m in depth. Some of the profiles show small diffractions in the unconsolidated sediments, but no large-scale boulders or boulder fields were identified. No bedrock reflectors were imaged under the southeastern CAD cell, where core logs indicate the rock is as much as 30 m below MLLW.

The chirp frequency, tuned transducer, and boomer-plate CSP surveys were adversely affected by a highly reflective water bottom causing strong multiples in the seismic record and very limited depths of penetration. These multiples are attributed to entrapped gas (methane) in the sediments or to very hard bottom conditions. In a limited number of places, the bedrock surface was observed in the CSP record, creating a discontinuous and sporadic image of the bedrock surface. These interpretations generally matched core data at FP-03-10 and FB-06-1. Use of two analog CSP systems, the boomer plate and tuned transducer, did not overcome the reflections off the water bottom and did not improve the depth of penetration.

In general, the CRP profiles were used to corroborate the results of the CSP profiles. Relatively resistive zones associated with the locations of seismic reflections were interpreted as bedrock. The shape of the bedrock surface generally was similar in the CRP and CSP profiles. Evaluation of the CRP profiles indicated that the inversions were adversely affected where the depth and (or) ionic concentration of the water column varied. Consequently, the CRP profiles were broken into short intervals that extended just over the area of interest, where the depth to water bottom was fairly constant. Over these short profiles, efforts were made to evaluate the resistivity of the very shallow sediments to determine if there were any large contrasts in the resistivity of the sediments that might indicate differences in the shallow subbottom materials. No conclusions about the overburden lithology, however, can be drawn from the distribution of resistivity in the profiles.

A series of magnetic surveys also were conducted in Bridgeport Harbor. The magnetic data from Profiles 3, A, and B show a significant magnetic anomaly trending northeast and southwest. These anomalies indicate a possible large-scale feature that may be anthropogenic. Other isolated anomalies did not appear to be continuous or large scale. These features could be related to topographic changes (and changes in the height of the sensor above the bottom) or to metallic debris on the bottom or in the subbottom.

Contents

Abstract

Introduction

Purpose and Scope

Description of the Study Area

Methods of Data Collection and Analysis

Navigation

Tidal Stage

Continuous Seismic Profiling (CSP)

Theory of CSP

Equipment and Methods of CSP Data Collection

Limitations to CSP Data Collection

Interpretation of CSP data

Continuous Resistivity Profiling (CRP)

Theory of CRP

Equipment and Methods of CRP Data Collection

Limitations of CRP Methods

Interpretation of CRP Data

Geomagnetic Surveying

Theory of Geomagnetic Surveying

Equipment and Methods of Magnetometer Data Collection

Limitations of Magnetometer Methods

Interpretation of Magnetometer Data

Results of Marine Geophysical Surveys

Profile 1

Profile 2

Profile 3

Profile A

Profile B

CSP Profiles D, E, and F on the Southeastern Side of Bridgeport Harbor

Summary and Conclusions

References

Figures

Suggested citation:

Johnson, C.D., and White, E.A., 2007, Marine geophysical investigation of selected sites in Bridgeport, Harbor, Connecticut, 2006: U. S. Geological Survey Scientific Investigations Report 2007–5119, 32 p., online only.

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