Application of UXOLab softwareto live-sites in Montana

August 30th, 2005

1 SummaryUXOLab is an advanced software program for UXO detection and discrimination

developed jointly by the University of British Columbia and Sky Research Inc. In this document wereport on an application of UXOLab to UXO detection in Montana on behalf of the Montana ArmyNational Guard. Commencing in October 2004, Sky Research collected magnetic sensor data on280 acres at Chevallier Ranch and 50 acres at the extremely challenging Limestone Hills site. All ofthese data were processed and interpreted using the UXOLab software package. At the time ofwriting validation information was available on 1259 anomalies that had been excavated at ChevallierRanch, of which 54 were found to be UXO. This validation information revealed that significantcosts savings could have been achieved if UXOLab’s remanence discriminator had been used tomake excavation decisions.

UXOLab has all the functionality required to take a dataset from a given geophysical surveyand characterize the permeable and/or conductive metallic objects responsible for the dataanomalies. This includes initial processing of the data (including merging of sensor and positiondata), detection of anomalies from potential UXO, inversion of the processed data to recover modelparameters, and transformation from model parameters to possible object properties such aslocation, size, shape, orientation and material properties. Given knowledge of the expected ordnanceitems at the site, one can interpret this information and decide whether the objects are specific UXOitems or non-ordnance items.

2 Equipment used at Chevallier RanchMagnetic survey data were collected over approximately 280 acres at Chevallier Ranch

between October 2004 and June 2005. The first 200 of these acres were collected with the LeicaRTS1200 robotic laser tracking system for the array sensor positioning. The remaining 80 acres werein a gully where line of sight would have made RTS surveying difficult. Therefore, these data werecollected with a RTK GPS for the sensor positioning.

2.1 Sky Logger Data Acquisition SystemChevallier Ranch magnetic data was collected and managed using the propriety Sky Logger

Data Acquisition System (DAS). Sky Logger DAS utilizes Bluetooth wireless technology to establisha connection between various geophysical sensor and navigation instruments to a tablet PC. Thistechnology, in conjunction with the quad sensor magnetometer array, allows the computer operatorto be untethered from geophysical sensors. The Bluetooth technology eliminates the need for serialcable connection between the geophysical equipment and the computer operator, allowing him/herthe freedom to rest. Bluetooth technology improves data collection rates and safety during datacollection.

2.2 Geometrics G-823Total field magnetic surveys utilize a man portable magnetometer array consisting of four

Geometrics G-823 optically pumped cesium vapor magnetometers for survey data acquisition, and aG-856AX1 proton precession magnetometer for base station measurements. The G-823 measuresthe intensity of the Earth's magnetic field in nanoTeslas (nT). At Chevallier Ranch, the magneticintensity is approximately 55 700 nT.

The G-823 sensor separation was 0.5 m covering a 2.0 m data swath. The G-823 systemwas configured to stream data at ten samples per channel per second (10 Hz). At a nominal traverserate of 0.76 m per second, this equates to approximately one sample per 0.07 - 0.1 m of forwardadvance.

Figure 1: Sky Logger Bluetooth Data Acquisition System in Operation

2.3 Robotic Total StationFor the majority of the survey (around 200 acres) Sky deployed the Leica RTS1200 robotic

laser tracking system for the magnetometer array sensor positioning. The RTS technology providespositional data superior to GPS, greatly reduces technical risk to survey projects, is less expensive,and has the ability to provide high-accuracy data adjacent to wooded areas and within lightly woodedareas.

The Leica TPS1200 is a motorized robotic total station that uses automatic targetrecognition to track the location of the prism and has a highly accurate distance/azimuthmeasurement system to produce +/-5 mm +2 ppm accuracy. This technology is ideally suited forUXO geophysical surveys, producing positional data with 3D accuracy of approximately 1 cm at a 7Hz rate.

The RTS system hardware consists of three integrated components: 1) the Leica TPS1200dual laser robotic total station, 2) the RTS rover remote control panel, and 3) a 360° survey prismwhich is tracked by the RTS base station.

2.4 Real Time Kinematic GPSFor approximately 80 acres Sky deployed a Trimble 5800 RTK GPS system for the sensor

positioning. The GPS was configured to output a location measurement once every second.

3 Survey Procedures

3.1 Set-up of Robotic Total StationThe RTS base-station was set-up daily on one of the grid corners that had previously been

established as the local survey control at Chevallier Ranch. The field crews documented each base-station and back site location daily and this information was transferred to each tile dig sheet. Thiswill allow the validation crews to set up their RTS location and back site using exactly the samepoints which will minimize location errors due to the total station.

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3.2 Calibration ProceduresThe G-823 sensor standardization tests were conducted before and after survey data was

collected and when the array operators were switched. These standardization tests include: warm-up,background, and time calibration tests. The magnetic sensors were allowed to warm up to a pointwhere there was <3 nT variation in the sensor readings before data collection could begin. As ageneral rule, the survey crew allowed the sensor to warm for five minutes before attempting datacollection. Time calibrations were performed several times during the day along a North trendingline approximately 6 m in length. A board with a thin steel wire was placed at the mid-point of thesegment and the magnetometer array was walked forward and then backward along the 6 m line.The time calibration test is used to calculate the time slew value to precisely merge the magnetometersensor data with the positional information from the RTS. The magnetometer profiles for the datacollected in the two different directions will only line up when the time slew is adjusted to its correctvalue.

Figure 2: Geometrics G-823 Man Portable Array with Leica 360° Prism

3.3 Data Collection ProcedureThe local survey control grid at Chevallier Ranch partitioned the survey area into 100 m x

100 m tiles. Data were collected one tile at a time, although in certain cases only parts of a tile couldbe collected, and in a few circumstances the data coverage extended into an adjacent tile. Themagnetometer array has sensors spaced 0.5 m apart which means it covers a 1 m swatch on eitherside of the centerline of the array. Parallel transects were walked at a nominal separation of 2 m withthe sensors at approximately 0.5 m above ground. To ensure uniform coverage and parallel traversepaths, 100 m long survey control chains were placed at the 0, 50 and 100 m marks of the tile,perpendicular to the direction of travel. Traffic cones were then placed on the appropriate metermarker of the survey chain and used as visual cues to guide the operator.

3.4 Magnetometer Base-stationThe Earth's magnetic field undergoes low frequency diurnal variations predominantly due to

the interaction of high energy particles emitted from the sun with the Earth’s atmosphere. Thisphenomenon is generally referred to as magnetic drift. A base-station G-856 proton precessionmagnetometer is used to monitor and record this drift so that it can be removed from the field data

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during processing. Prior to conducting magnetic survey data, the G-856AX magnetic base-stationwas time synchronized with the Tablet PC and set up in a stationary location to monitor the changesin the Earth’s magnetic field during the surveys. These magnetic base-station data are used todiurnally correct G-823 field data for the Earth’s magnetic field variations. The magnetic base-station was established in an open area determined to be clear of surface or subsurface culturalinterference. Magnetic base-station data were stored in the internal memory of the unit anddownloaded nightly to the tablet PC field computer.

Figure 3: Robotic Total Station in Field Operations

4 Data processing and interpretationFor each survey event, the following operations were conducted within UXOLab

1. The raw sensor, position and magnetic base-station data were merged using a common time-reference;

2. The magnetic data were corrected for diurnal variations and heading error;

3. The magnetic data were filtered using a moving median filter to estimate the long-wavelength components of the magnetic field signal;

4. The data were interpolated onto a 0.125 meter grid;

5. The Automated Wavelet Detection (AWD) Algorithm was used to detect all anomalies ofamplitude 10 nT or greater;

6. The automated anomalies picks were augmented by inspecting the sensor data and addingmanual picks as required;

7. Using an estimate of the anomaly size returned by the AWD algorithm, segments of dataabout each anomaly were extracted for additional analysis;

8. A magnetic dipole model was fit to each anomaly;

9. The magnetic dipole fit was used to estimate the magnetic remanence (Billings, 2004) ofeach anomaly assuming there were 76, 90, 105 and 155 mm caliber munitions on the site(the magnetic remanence discriminator was not used to make discrimination decisions);

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10. All anomalies with moments > 0.05 Am2 were placed on the primary dig-list, the rest wereplaced on a secondary dig-list.

11. Additional anomalies that could be UXO, but were likely geology, were added to the dig-listas required.

Figure 4: Example filtered magnetic data over a 100 m by 100 m tile at Chevallier Ranch, showing theanomalies selected by UXOLab.

5 ResultsA total of approximately 5500 anomalies were identified by UXOLab. At the time of

writing, validation had been conducted on approximately half of the area that was surveyed. Table 1summarizes the number of primary, secondary and geology targets in this area, and includes abreakdown of the number of targets excavated and the number of UXO. No UXO were found inthe secondary or geology dig-lists, while 54 UXO were found in the primary dig-lists.

Primarydigsheet

Secondarydigsheet

Geology Total

Total number 565 1526 339 2430Number excavated 565 355 339 1259Number of UXO 54 0 0 54

Table 1: List of anomalies of different classes in the tiles that have thus far been validated.

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Figure 5: Example parameter fit to an anomaly that was later found to be a UXO. Note that theinverted parameters within the black ellipse, and not the anomaly amplitude, were used to make

excavation decisions.

5.1 Analysis of the remanence discriminatorFor each anomaly the magnetic remanence discriminant was calculated, but was not used to

make any excavation decisions. This enabled the performance of this discrimination metric to beevaluated without bias (as would occur if the remanence estimation occurred after the validationinformation was available). Figure 7 plots the cumulative distribution of the UXO and non-UXOclasses as a function of remanence. Over 80% of the UXO have a remanence of 30% or less, whileless than 10% of the non-UXO had a remanence this low. By ranking the anomalies according toremanence, all of the UXO could have been recovered after digging 277 items, compared to the 920

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primary and secondary items that were actually excavated. Therefore, all UXO could have beenrecovered by digging less than 1/3 of the number of holes. Note, that the calculation of theremenance metric has little to no impact of the data processing costs, so that potential savings of themethod are readily apparent.

Figure 6: Filtered magnetic data over the Chevallier Ranch site with all targets overlaid.

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Note that none of the 339 “geology” anomalies were found to be UXO. These geology-likeanomalies were conservatively selected by relatively inexperienced geophysics who conducted themajority of the data processing and interpretation. This conservative approach is good in one sense,as it would likely prevent UXO anomalies from inadvertently being declared geology and notinvestigated. However, the fact that none of these anomalies were in fact UXO, indicates that amore aggressive approach the geological anomalies would be possible.

(a) (b)Figure 7: (a) Cumulative distribution of UXO and non-UXO as a function of remanence;

(b) Percentage of UXO recovered as a function of holes excavated when anomalies are rankedaccording to remanence.

6 ConclusionsThe UXOLab software package was applied to a production-mode survey at Chevallier

Ranch, Montana, between October 2004 and June 2005. Over 280 acres were surveyed andapproximately 5500 anomalies were identified. At the time of writing around half of the area hadbeen validated and 54 UXO were recovered. The magnetic remanence discriminator was predictedbut not used to make excavation decisions. Had this metric been used, less than 1/3 of the numberof anomalies thus far excavated would have been required to recover all detected UXO. TheUXOLab software package is available at http://uxolab.ca

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