On the 13th August 2013 geophysicists from GBG Australia Pty Ltd (GBGA) carried out a Ground Penetrating Radar (GPR) and Earth Resistivity Tomography (ERT) investigation of Macquarie Pier (Newcastle Breakwater).
The aim being to locate the start of the old masonry pier, which is buried under rubble, which could lead to the detection of the foundation stone put in place by Governor Macquarie and Captain James Wallis in August 1818 of which its exact location has since been lost.
Ground Penetrating Radar (GPR)
The depth of the pier ranges potentially to 6-7 meters, the GBG trial of GPR used a GSSI SIR system coupled to a 80 and a 200MHz antenna, which has lesser resolution but deeper range than the higher frequency 400MHz antenna, which was used in 2010.
The 200MHz equipment was pulled on a sled over the ground with an external odometer marking the chainage into the digital record during data collection (refer to the photograph below).
The fill, as we understand, consists of sand, and debris of coal chitter, this may be an issue for the detection of the pier. Particularly as coal chitter can disperse the radar signal. The nearby cliff wall, to which the pier may have been attached, would also make it difficult to distinguish the sandstone pier from the natural ground.
Prior to the start of data collection GBG undertook a calibration / correlation profile or profiles in the area. This was undertaken to assess the in-field responses being obtained to enable them to more accurately map the pier. Depending on the responses obtained in the raw data they may be able to define the beginning of the pier directly in the field. However, it is more often the case that some post processing of the data in our office is needed to enable clearer definition. GBG will present the results on a plan view of the investigation area. Since the location of the pier has been reasonably well defined by CRWP in the historic plans on aerial photos, the area of survey can be minimized.
Electrical Resistivity Tomography (ERT)
In addition to the GPR, and particularly in the case of ambiguous results, a second option was used on the day. Electrical Resistivity Tomography (ERT), is a technique sensitive to variations in the electrical resistivity of the subsurface measured in Ohm meters. The dominant factors affecting the bulk electric resistivity (and its inverse conductivity) of soil or rock are:
– Porosity and permeability
– Degree of saturation – the fraction of pore space / fractures filled with fluid
– Fluid type including salt content – the composition of the fluid filling the pore spaces / fractures
– Presence of clays with moderate to high cation exchange capacity (CEC)
Resistivity measurements are made by inducing an electrical current into the earth through two current electrodes and measuring the resulting voltage difference at two potential electrodes. Knowing the current and voltage values, an apparent resistivity value can be calculated the investigation depth of which is relative to the spacing between electrodes with greater depths reached by increasing the electrode spacing. A number of different electrode configurations exist each being suitable under various conditions.
Modern resistivity systems employ multiple electrodes connected to a central control unit via two multiple core cables. Once the electrode array is deployed (typically at constant separations between 1 and 5 m) and the sequence program is set in the control unit, readings are automatically taken across a number of electrode positions. Once the readings are completed, a pseudosection is generated, showing the apparent resistivity measurements at the various report depths. The length of the pseudosection profile can be increased by “rolling along” the electrode array after each set of measurements. Actual quantitative resistivity results can be calculated by running the pseudosection through geo-mathematical inversion algorithms resulting in 2D cross-sections showing the variation in electrical resistivity within the subsurface.
The depth of penetration achievable with the ERT method is dependent on the total length of the electrode array, with larger electrode spacings resulting in greater depths of penetration. The overall subsurface resistivity also affects the depth of penetration with highly resistive ground tending to decrease the imaging depth after inversion.
Ground Penetrating Radar (GPR) – Nobbys Car Park side
A further investigation was carried out across the road, staring at where John Armstrong’s 1830 map notes the stonework starting at the ocean side. This location is approximately where the small hut is located adjacent to the Nobbys car park.
The collected data taken to GBG’s Sydney office for processing and analysis. The collected data will be processed to provide 2D vertical slices of the different methods. The results will be provided as a number of scaled plan drawings in PDF format. Referenced CAD files and hard copies are available on request. The drawings will be accompanied by a full report, detailing the results and discussion of their findings.
Macquarie Pier Revival – By Janek Speight. Newcastle Herald. 9th August 2013.
Newcastle’s history focus of quest – By Janek Speight. Newcastle Herald. 13th August 2013.
Letter: Kids dig found history – By April Jaeger. 13th August 2013.
The quest for Macquarie’s Pier – By Carol Duncan. 1233 ABC Newcastle. 16th August 2013.
Topics: From sands of time – By Tim Connell. 16th August 2013.
Peter Lovett was the subject of the Topics story in the Newcastle Herald above. He told reporter Tim Connell that he believed that the foundation stone that the University of Newcastle’s Coal River Working Party were looking for was perhaps one he and his friends moved back in the early 1960s. He said that they moved a stone with a memorial plaque in bronze, down the length of the Newcastle breakwater (Macquarie Pier) in order to create a wind break for their campfire in the dunes. The Chair of the CRWP took Peter on site to establish exactly where these events took place at Nobbys and Macquarie Pier and recorded this interview on Friday 16th August 2013. After establishing that it was a bronze plaque mounted upon a stone, we were pretty sure it wasn’t the original 1818 foundation stone placed by Governor Macquarie and Captain Wallis on the 5th August 1818 on the shoreline. It, in all probability may have been another commemoration plaque created by persons unknown, much later, perhaps around the 1950s.