Hydrographic survey is the science of measurement and description of features which affect maritime navigation, marine construction, dredging, oil exploration/drilling, pipeline damage assessment and related disciplines. Strong emphasis is placed on water depth soundings, shorelines, tides, currents, sea floor and submerged obstructions that relate to the previously mentioned activities. Hydrographic surveys uses the raw data collected through the hydrographic survey process into information usable by the end user.
MPH has been conducting hydrographic surveys for as long as the company has been in existence; however, by today's standards, the older methods of conducting these surveys could be considered antiquated. Before the advent of electronic surveying equipment including electronic distance meters, GPS and echosounders (water depth recorders), shallow water depths were measured with sounding poles or lengths of ropes. The approximate locations of the soundings were put on maps to the best ability of the survey party chief. As one can imagine, this method was not nearly as accurate as it could be. Echosounders made determination of water depths a lot more accurate; however, the horizontal accuracy of the soundings was still based on the judgment of the individual surveyor. Adding electronic distance measuring techniques to the process meant that the horizontal and vertical positions of the soundings could be determined more accurately with minimal judgment calls on the part of the surveyor.
With the introduction of GPS into the hydrographic surveying process, it became possible to conduct hydrographic surveys in a real-time fashion and allowed surveys to be done without the use of standard surveying equipment and allowed the surveys to be done out of sight of land.
MPH has gone through all of these processes and is now capable of conducting just about any level of hydrographic/side scan sonar surveys to any required degree of accuracy.
Types of Hydrographic Surveys done by MPH
MPH's primary hydrographic/side scan sonar surveying focus is to conduct surveys in support of the pipeline and oil & gas industries, and coastal restoration projects on the federal, state, and local levels.
In the 1990's Congress mandated that pipeline companies inspect pipelines under navigable waterways at least every five years. Part of this inspection is to determine the amount of cover over the individual pipelines. To support this federal requirement, MPH is contracted by pipeline companies to conduct hydrographic surveys in the area of the pipeline crossings not only to determine the cover over the pipelines, but to also determine the conditions of the water bottoms adjacent to the pipelines. Pipeline companies need to know if the water bottoms are scouring around their pipelines and how this will impact the pipelines in the future so corrective actions can be taken to protect the pipelines.
Following Hurricane Katrina, many of MPH's pipeline clients required post-Katrina damage assessment surveys to determine if their pipelines were exposed or damaged by the action of sub-sea currents caused by the hurricane. MPH provided these services to our clients; however, the field services had to be contracted out to another firm with MPH doing all of the mapping services. Because of the vast amount of work needed and the limited number of surveyors with side scan sonar and multi-beam capabilities, MPH determined the need to upgrade our own Hydrographic Survey Department to include these services. The below image was derived from a side scan sonar survey following Hurricane Katrina and shows an exposed 16-inch pipeline in Chandeleur Sound. The pipeline is the line on the right side of the image. The dark line on the left is directly beneath the side scan sonar unit and is also the path of the survey vessel.
Another concern for pipelines under navigable waterways is the possibility of shallower pipes becoming exposed in meandering rivers that have higher currents. Recently, MPH was asked to perform a side scan sonar survey on a pipeline that crossed the Trinity River in Chambers County, TX due to suspicions that the pipeline may be exposed. MPH decided to conduct the side scan sonar survey over the entire pipeline corridor to ensure that there was nothing that posed a threat to the suspected exposed pipeline. MPH found that the suspected pipeline was not exposed, but two other pipelines within the corridor were either exposed or were being covered with rocks due to exposure. Below is a image that was gathered from the survey.
Harbor and Port Surveys
MPH conducts hydrographic/side scan sonar surveys for many clientsĄŻ docking facilities. These types of survey are necessary to determine if the dock space is deep enough to support the various types of barges and boats required to keep the facility operational. Below is an example of a survey done along the Tombigbee Waterway in Alabama.
The survey data is shown on an aerial photograph. From left to right, the pictures show:
- Bottom elevations adjacent to the docking facility. The elevations are color-coded based on the elevation of the water bottom.
- A color relief map showing the elevations of the water bottom. The color relief maps shows at a glance where the shallow areas are at the docking facility. In this case, the red area represents an area that needed to be dredged deeper to support marine traffic
- A side scan sonar image of the water bottom. This type of image is basically a digital picture of the water bottom showing potential physical obstructions to marine traffic. At the bottom of the image, several bright dots can be seen with darker lines radiating downward from the dots. The dots are dolphins that barges tie up to; the dark lines are shadows behind the dolphins that the side scan sonar could not see. The dolphins can also be seen in the other two pictures.
In support of drilling activities, one of MPH's functions is the check the route of access for drilling rigs and their support vessels to determine the water depth and if underwater obstructions are present. The single beam and multibeam hydrographic survey systems determine water depths; the side scan sonar system maps bottom obstructions.
The below images show an obstruction to marine traffic. It shows the partial remains of the Freeport Sulphur Mine, located off Grand Isle, LA. It has been converted to among the world's largest artificial reefs. It is situated in 45 feet of water and is composed of more than 29 structures, which range from 4-pile supports to a 35-pile power plant facility and 1.5 miles of bridgework. The depth of water above the structures is 30 feet. To avoid this hazard to navigation, the entire underwater structure has been marked with signs and is shown on navigation charts.
This image was produced from an MPH survey and shows details of the same platform on each side of the boat as the survey was being conducted.
|Port Side of Survey Vessel||Starboard Side of Survey Vessel|
Drilling rigs are not only concerned about hazards that may be on the surface, but also with the potential hazards that may lie beneath the surface. In order to help determine potential hazards beneath the surface MPH performs magnetometer surveys to make sure areas are clear of any ferrous objects. These types of surveys also help in locating plugged and abandoned well casing. Below is a 2-D contour image of a magnetometer survey that was performed. MPH was asked to try and locate three plugged and abandoned wells. The black circles represent a position of a possible well location that MPH was able to determine through a magnetometer survey. The red crosses represent where the actual well casing was located in the field.
Single Beam vs. Multibeam Hydrographic Surveys
Hydrographic surveys are done with an echosounder that records water depths. Two types are available, single beam and multibeam. As the names imply one system sends out and receives a single beam from a sensor, the other sends out and receives multiple beams. Both have their benefits, primarily determined by the depth of water being surveyed. For MPH's purposes, the single beam system is better used in shallow water less than 15 ft); the multibeam in deeper water greater than 15 feet. Both systems can be used in either case; however, because of the cost of the multibeam equipment and the potential for damage in shallower water, the decision was made to limit the multibeam use to deeper water.
The hydrographic survey systems are interfaced with two other pieces of equipment, a GPS system that records the position of each sounding and the heading of the survey vessel and a device called a motion reference unit that records every movement of the boat. These movements include the heave, pitch, roll, and yaw. Heave is the up and down motion of the entire vessel, pitch is the forward and backwards motion, roll is the side to side motion of the boat, and yaw is the left-right twisting motion. Because hydro surveys are conducted in varying sea states, it is very important that theses motions be known so as to eliminate any error caused by these motions.
Since the multibeam system uses many beams to conduct a survey, a more accurate representation of the water bottom can be determined. While the single beam system gathers one data point approximately 9 to 10 times a second, the multibeam system gathers up to 480 data points up to 14 times a second in an array format. This ability of the multibeam system to gather so many data points greatly increases the accuracy of water bottom details.
Below is an example of a single beam and multibeam survey of the same waterway, a portion of Intracoastal Waterway near Minor's Canal. Dredging had been performed in a portion of this area with the dredged material being placed on the shoreline to increase protection from coastal erosion. The multibeam survey system collected 240 data points twelve times per second with the water depth being around 20' +/-. The single beam survey collected 10 data point every second and lines were ran 50' apart.
The area surveyed was 560 feet by 230 feet. 8,398 data points were collected for single beam processing; 1,685,040 data points were collected for multibeam processing.
Single Beam Survey
Note that the contour lines created from the multi-beam data are much smoother than the single beam data. The contours are also better defined because of the number and density of the data points collected. Depths range from 15 feet to 35 feet (red).
In addition to our hydrographic surveying services, MPH also offers land surveying, environmental & regulatory services, geographic information systems (GIS), pipeline engineering, and construction project management services.