SURVEY RESULTS

2004 / 54 Warton Marsh, Lancashire

1.	Survey Areas

1.1	Survey was conducted using two different instruments; a Bartington Grad601-2 fluxgate gradiometer and a Geonics EM61mk2 metal detector. Three areas were investigated using the EM61, covering the major components of the planes, and parts of these were subsequently surveyed using the Grad601-2. The location is shown on Figure 1.

1.2	The EM data were collected in conjunction with a differential Global Positioning System (dGPS), thereby giving positional information correct internally to the survey. The corrective offset to allow geo-referencing to the Ordnance Survey National Grid was calculated by Dr. Henry Chapman using a Trimble dGPS. The Trimble equipment was also used to position the gradiometer survey areas. 

2.	Display

2.1	The EM results are displayed as colour-scale images, and the gradiometer results as both greyscales and X-Y traces. These display formats and the interpretation categories used are discussed in the Technical Information section at the end of the text.

3.	General Considerations - Complicating factors

3.1	Conditions for survey were adequate, with the marsh being generally firm underfoot. In many places, however, gullies cut through the ground, making survey difficult. The wider gullies also prevented closer examination of some of the anomalies.

3.2	The location of the gullies and the availability of transport governed access to different parts of the marsh, as the terrain prevented travel on foot. This limited the area available for survey and the work was therefore concentrated upon the accessible parts of the wreckage.

4.	Geonics EM61

4.1	The Geonics EM61mk2 is an active instrument, which employs the principle of electromagnetic induction, a phenomenon whereby magnetic fields induce corresponding electric currents in conducting bodies within the instrument’s field. The transmitter coil produces a strong magnetic field around the instrument, which induces the electric currents in targets below the instrument. These currents in turn produce secondary magnetic fields, which generate secondary currents in the receiver coil of the instrument. The strength of response at the receiver is proportional to the density and size of the target and its depth below the surface. Magnetotelluric effects are avoided by sampling the decay of the secondary field at a number of intervals, with increasing time being roughly proportional to depth; this allows discrimination of small and near-surface anomalies from deeper, larger ones.

4.2	Further data enhancements can be achieved by using the instrument in differential mode. This involves taking the fourth channel from a coil some 0.3m above the transmitter, with the difference in height allowing the near-surface signals to be removed more effectively. The depth of penetration can be expected to be in the order of 3m for a target roughly 1m by 0.5m. Conversely, the fluxgate gradiometer will be saturated by near-surface ferrous signals. When considered alongside the EM data, anomalies appearing in both datasets are likely therefore to be ferrous in nature, whereas those appearing in the EM results alone are likely to be of some other metal or alloy. Further analysis of the response may reveal the composition and depth of the target.

4.3	The EM61 was used in conjunction with a palm-top computer for data capture, which collected data from four channels, three from the lower sensor and one from the upper. The differential data are calculated during the download procedure, ensuring data integrity as the original channels are preserved. The palm-top also has a connection to a dGPS receiver, which was fixed to the frame of the EM61. The GPS signal is decoded in real-time, thereby giving a geo-referenced location for each data point from the EM61. The palm-top computer was configured to collect data from both instruments five times per second.

4.4	The EM61 was employed in both the wheeled and carried configurations, although the latter was preferred under the circumstances. It was found that whilst it was possible to tow the instrument upon its wheels, it was easier to carry the EM61 using the backpack provided, as this allowed the user to move across rough ground more freely.

4.5	The dGPS receiver was fixed to the frame of the EM61 to maintain a constant offset relative to the instrument. The palm-top computer and battery for the dGPS were carried by a second person to minimise noise in the data. Observations made in the field confirmed that the dGPS apparatus and palm-top computer did not influence the measurements made by the EM61 provided they were kept in excess of about 1m from the coils. The manufacturer of the dGPS receiver claims a precision of <5m under most circumstances, improving to c.1m in optimal scenarios. These errors are visible in Figures 4-6 where there is a discrepancy between the EM data positions and the magnetic grid.

5.	Results of EM and Magnetic Survey

Area 1

5.1	The two halves of this area were separated by a small channel, with magnetic survey confined to a single 20m square on the eastern bank. The results of the EM61 survey show a large ‘hotspot’ that straddles the channel. The magnetic data show a strong ferrous response that broadly coincides with the eastern hotspot, although there is a definite offset to the south. This reflects a ferrous component of the aeroplane separate to the aluminium cockpit and wing. The ferrous response probably reflects guns and/or armour plating.

Area 2

5.2	The magnetic and electromagnetic data for Area 2 correspond almost exactly. The two anomalies are strong and well-defined, suggesting that they have been produced by discrete targets. The ferrous responses in the magnetic data suggest that the targets are likely to be two of the engines from the airplanes. Excavation of the ‘southern’ target (see Figure 5) confirmed this interpretation.

Area 3

5.3	The data from this area are dominated by strong responses, with both ferrous and non-ferrous components. The strongest hotspot in the EM61 data is again offset with regard to the ferrous anomalies, as in Area 1. Other, non-ferrous responses can be seen in the immediate vicinity of the main anomaly, suggesting that parts of the aluminium airframe were scattered over this area as the plane crashed and/or exploded. As the ferrous responses are relatively coherent, it seems likely that guns and armour plating lie towards the centre of the plot, with the remainder of the airframe and wings around them.

6.	Conclusions

6.1	The magnetic and electromagnetic data have identified the main components of the two aircraft. Comparison of the two datasets has also allowed discrimination between different elements of the wreckage.

6.2	The main advantage of the applied geophysical methods over conventional metal detector survey is the ability to produce high quality plans of the wreckage distribution together with information concerning its composition. This survey has therefore been successful in determining the extent of the various constituents of the aircraft.

Project Personnel:	B. Urmston with Dr. C.F. Gaffney, J. Gater & C. Stephens 
Project Assistants:	J. Anderson

Date of Survey:		26th to 29th July 2004
Date of Report:		1st September 2004

References:

Kirk, K (2004)	Proposed Archaeological Evaluation at Warton Marsh, Lancashire. Videotext Communications Ltd., unpublished desktop report.

SSEW (1983)	Soils of England and Wales. Sheet 3, Midland and Western England. Soil Survey of England 	and Wales.

SITE SUMMARY SHEET

2004 / 54 Warton Marsh, Lancashire

NGR: SD 390 268

Location, topography and geology 

The study area is located in Warton Marsh, some 15km west of Preston and around 1km from Warton Aerodrome. The marshland is largely flat and riven by gullies ranging in size from insignificant to broad and deep; the gullies are tidal but the whole area is only inundated during bad weather or unusually high tides. The unripened gley soils over marine alluvium (SSEW 1983) have only been deposited during the 60 years from the end of the Second World War to the present, following the termination of dredging of the port at Preston (Kirk 2004). Around 2m of alluvium has since accumulated on top of the former beach (ibid.).

Archaeology

Warton aerodrome lies just over 1km to the east of the site; it was an active airbase during the Second World War, being home to a squadron of A26 Invader bombers in particular. Two of these aircraft crashed into Warton Marsh immediately after take-off whilst en route to France in November 1944. The bodies of the aircrew were recovered but the majority of the wreckage has remained in situ; the locations of the major elements of the two aircraft are known due to previous metal detector surveys. Numerous other WWII aircraft are known to have crashed into the marsh but none in the immediate study area. (Kirk 2004)

Aims of Survey

The aims of the survey were to determine the extent of the known components of the two A26 bombers and to provide a plan of the remains in situ. This work forms part of a wider investigation by Channel 4’s Time Team.

Summary of Results *

Both the magnetic and electromagnetic results clearly show the distribution of the components of the two aircraft. The two datasets indicate the location of both ferrous and non-ferrous material, allowing interpretation of individual elements of the wreckage. 22This enabled the engines of both aircraft to be identified and the extent of debris from the rest of the body of the planes to be located.

* It is essential that this summary is read in conjunction with the detailed results of the survey.

List of Figures

Figure 1:	Summary Location						not to scale
Figure 2:	EM61 Colour Scale (all areas)					1:500
Figure 3:	Gradiometer Greyscales (all areas)					1:500
Figure 4:	Area 1 EM61 & Gradiometer Data					1:500
Figure 5:	Area 2 EM61 & Gradiometer Data					1:500
Figure 6:	Area 3 EM61 & Gradiometer Data					1:500