# Tracking Sensor Location by Video Analysis in Double-Shell Tank Inspections
## 1 Introduction  
Double-Shell Tanks (DSTs) at the Department of Energy's Hanford site have been used for the interim storage of partially liquid nuclear waste since their construction in the years 1971-1986. They are expected to serve this function until the immobilization of nuclear waste at the Hanford site is complete around the year 2050. At that time, DSTs will be about 15 years beyond their original projected lifetime [1]. Consequently, the periodic nondestructive evaluation (NDE) of DSTs will be a part of nuclear waste management at Hanford for many years. The NDE process has two phases. The first phase uses a low-resolution electromagnetic acoustic transducer (EMAT) sensor developed by the Southwest Research Institute [2] to scan large regions of the primary tank bottom. These scans are used to identify regions where high-resolution data are needed, which are obtained by guided wave phased array(GWPA) sensors developed by Guidedwave [3]. These ultrasonic sensors are deployed for the NDE of Hanford tanks [4; 5] by robots that move through a network of air slots in the confined space between primary and secondary tank bottoms. See [PERSON] et al., [6] for a recent review of technology developments used in Hanford double-shell primary tank bottom inspections. Figure 1 shows components of a graphical user interface [7] developed to provide situational awareness to operators controlling the movement of GWPA sensors through air slots. The display on the left shows the network of air slots (blue lines) in relation to welds (red lines) in the bottom of the primary tanks of the AP type. Air slots were originally designed for cooling with air injected at the center and escaping into the annulus-shaped region between the walls of the primary and secondary tanks. To maintain an approximately constant flow rate, the width of air-slots, while constant in each section, decreases slightly as sections approach the air escape point. We refer to this escape point as the \"entrance\" to an air slot because access to air slots for NDE is through 24-inch raisers into the space between primary and secondary tanks, which are below ground. The section at the entrance has a width of 3 inches at the top of the air slot where the walls of the channel contact the bottom of the primary tank, as shown in the upper right panel of Figure 1.  
Ultrasonic data, acquired at approximately 6-inch intervals in an air slot, is used to map regions of the bottom of the primary tank to identify defects with the potential to cause leakage. This interpretation of the ultrasonic sensor output, which enables the reinspection of defects, requires knowing where the ultrasonic measurements were taken with a precision of approximately one inch. In ref. [7], we showed that analyzing video acquired while the sensor is moving can help predict the distance moved with the required precision; however, the method (see Section 2.1 for details) could not be used to track sensor location until target selection and matching in a later frame was automated. The development and testing of the automation required for tracking is the main contribution of this paper. Figure 1: Graphical user interface (GUI) display showing AP-type tank-bottom weld pattern (**red lines**) in relation to air slots (**left**). Video from a camera on the front crawler pulling the ultrasonic sensor through an air slot (**top right**). Data like that expected from the NDE of DSTs (**bottom right**). Black dot in the left panel illustrating the position of the sensor (not to scale). Video in the GUI can alert the operator to debris in the bottom of air slots that might obstruct the movement of the sensor and changes in the surface of the tank bottom that could prevent the sensor from making the contact needed to acquire good data. The diameter of the primary tank is approximately 80 ft; hence, the display in the left panel of the GUI has pan and zoom that lets the operator follow tracking data sent to the GUI on a scale comparable to the video display.