LITTORAL MAPPING

66 POSITION December 2006–January 2007

The physics of this process is fairly well established. The big problem is unscrambling the contributions of the various components of the body (water/sediment/basement) below the aircraft, which is responsible for the secondary field.

AEM is traditionally used for detection of conductive ore bodies buried beneath a layer of less conductive ground. By analogy, the seawater can be modelled as a single or multi layered conductive body, overlying a single or many layers of marine sediments, on top of bedrock.

So how do we process this data? A one-dimensional layered-earth model consists of individual planar layers. Each has a given thickness and electrical conductivity, overlying an infinitely deep basement (bedrock). In our work, we use two planar layers, representing conductive seawater and less conductive sediment (such as marine sand).

The analysis of the data tries to disentangle the contributions from the

seawater, sediment and basement sections. The aim is to finish up with the electrical conductivity of seawater, sediment and basement, as well as the thicknesses of the seawater and sediment layers.

Mapping littoral regions presents challenges. The data is difficult and expensive to acquire; even more than that, there is usually

wide variations with data that describes the foreshore. Coherent datasets that range on both sides of the waterline are few and far between.

The National Oceans Office may provide a possible solution. It is working towards a portal that will provide access to a wide range of oceanic and littoral data.

In the UK, another solution is being constructed. It involves a web-based GIS, called SeaZone Hydrospatial. The system combines datasets describing the land – from the British national mapping agency, Ordnance Survey – with descriptions of marine parks, bathymetry, and so on.

Interestingly, both types of solution use Open Geospatial Consortium standards. One is supplied by CubeWerx, and the other by Cadcorp.

But firstly, what's the problem? Many of the challenges faced by organisations operating in the coastal zone, such as environmental and heritage concerns, regulatory issues and stakeholder involvement, may sound familiar to those who work in land-locked areas.

But when dealing with the coastal environment there are additional complexities to consider, such as, who owns the seabed? Who is responsible for a particular stretch of water? Are those working upstream still responsible for what occurs further downstream?

As pressure on the seas and rivers increases, these problems are becoming harder to solve. Managers of ports, harbours and coastlines have to find new ways to deal with them. Accurate and up-to-date information is an essential element in doing this.

But this is difficult. Usually, organisations update their information locally. This means that even when two organisations start from the same dataset, over time each copy of the original information diverges. Soon there is no definitive database for the port or coastal zone, just a series of differing datasets.

To be useful for planning and business management, information

Mapping littoral regions presents many challenges.

Dream

stime/K

onrad Lew

greater the variation in conductivity between layers, the easier it is to identify a given layer.

We know that seawater is conductive, and we also know that sediment is considerably less conductive.

This information can be used to obtain the thickness (i.e: depth) of the upper seawater layer. A multi-layered water body could be assumed if the AEM method is sensitive enough to discriminate between relatively subtle variations in the seawater column conductivity (likewise for sediment layering).

For example, a two-layer seawater model could be useful as an approximate representation of the seawater column. AEMB could be used for remotely sensing the spatial extent of coastal flood plumes. This might occur at a river estuary, for instance, where lower-conductive, less saline water may reside on top of more saline higher-conductive seawater. The thickness of the layer with

Of course, the whole processing task assumes that they can be considered as layers, but this is usually the case. The thicker the seawater layer, the harder it is to detect the layers below. That is why this technique is restricted to shallow water, currently less than about 60 metres.

On the basis of the data, we try to assign two values to each layer: its conductivity and its thickness. The

The field is a function of the properties of the seawater, the sediment, and the resistive basement below …

Managing the Managing the Sea ShoreSea ShoreData management issues transit the high water mark.JONATHON POWERS

As pressure on the seas and rivers increases, the problems are becoming harder to solve …

(Continued page 68)

LITTORAL MAPPING

December 2006–January 2007 POSITION 67

A representation of a vertical slice through seawater, sediment and bedrock in Sydney Harbour showing the variation in conductivity with depth obtained from AEM data. Two layers overlying resistive basement (bedrock) were assumed for interpreting AEM data. The depth of the layer interfaces are identified

should not only be accurate and current, there should only be one version of it. Ideally, it should be stored and managed centrally, and be available to, and used by many.

Web-based technology is crucial to this. One interface can be developed, which is accessible via web browsers. Data can be shared both within and across organisations. Training time can be reduced; web-based GIS is usually simple to use because most people are familiar with browser-based user interfaces. Security can be easily implemented using standard methodologies.

In some cases, making information available from a centrally managed database will not be a problem However, in the real world, it is unlikely that all the departments and organisations that need to use the information will be using the same computer systems. So standards are important enablers for effective data exchange.

When implementing a web-based GIS, the various specifications of the OGC are vital. They enable software developers to make complex spatial information and services accessible in all kinds of applications. And as the OGC and its standards evolve, they are becoming increasingly important in many areas for many stakeholders.

To develop the UK solution, a British company called SeaZone Solutions has developed a technology demonstrator of a simple coastal environment information system. It uses Cadcorp software as part of its new Hydrospatial marine mapping product.

It is interoperable with the national land-based data from Ordnance Survey, thereby effectively continuing the UK Digital National Framework offshore. It includes marine and coastal topographic features that are bounded by the Mean High Water shoreline as supplied by Ordnance Survey. Users now have access to consistent topographic coverage from land to sea.

Topographic features that lie outside the MWH line, including the Mean Low Water line, overlay the SeaZone data. The data is available in six cohesive layers. They cover bathymetry and elevation; natural and physical environment; structures and obstructions; socio-economic and marine use; conservation and environment; and climate and oceanography.

So for the first time, it is possible to model both onshore and offshore environments together. This will bring huge benefits to organisations with responsibilities for coastal defences or

The extension of modern data management techniques to the littoral environment and to the oceans is complex and difficult. It has frustrated marine authorities around the world for decades. But the advent of straightforward techniques for storing, accessing and manipulating data is a step in the right direction.

Jonathon Powers is a Sydney-based journalist.

A common standard for mapping across the shore line.

SeaZone

by a contrast in conductivity. The colour bar identifies the conductivity; 4700 mS/m is typical of seawater in Sydney Harbour. The top panel shows the depths when seawater and sediment layer conductivities and thicknesses can vary (float) as model parameters that are determined in a best fit to the AEM data.

The depth of the pink layer is associated with seawater depth because its conductivity is in agreement with expected conductivity of seawater. The yellow line shows the known water depth, and the agreement is typically within one metre or less when accounting for the known tide. The thickness of the green layer is associated with sediment because its conductivity is in the range of that associated with unconsolidated marine sediments, about 1000 to 2000 mS/m. The bottom blue layer is associated with resistive bedrock. The depth of the green layer is compared with the depth-to-bedrock profiles shown in white (and a small section in orange, from a separate survey), estimated from marine seismic studies. The rough agreement is very encouraging. The bottom panel is similar to the top panel, but in this case, the known seawater depth and assumed conductivity was used as fixed model parameters, and only the sediment thickness and conductivity was allowed to vary so that only these parameters can be determined as a best fit to the AEM data. Overall, there is a closer agreement with the marine seismic estimates, except for a spurious result between 338900 to 339000 mE. Note the expanded colour scale to reveal finer variations in sediment conductivity.

DS

TO

those operating ports and harbours. It will also be useful in protecting wildlife habitats or for scientists trying to better understand coastal erosion and the impact of flooding.

The demonstrator uses OGC Web Mapping Service and Web Feature Service standards. This enables a more flexible exchange of datasets across mixed networks and organisations, and in a variety of clients.

WMS allows a client to overlay raster map images for display, served from multiple Web Map Services on the internet. In a similar fashion, WFS allows a client to retrieve and update data encoded in Geography Markup Language from multiple Web Feature Services.

It has developed a demonstrator of a simple coastal environment information system …