on the shore a scale a urethane elastomer has proven most effective because it has a high chemical resistance to conventional drilling mud after installation of the sonde onto key 38 the flowable compound is poured in and set or cured to form a pair of resilient shock absorbers that conform to the space around the sonde and protect it from shocks and vibrations the compound may be filled into the ends only for example to the dotted lines shown in fig7 filling in front rounded rearwardly facing recesses 45 a ahead of the sonde and rear rounded frontwardly facing recesses 45 b behind to a level just slightly beyond and covering the front face of key 38 e g 0 05 inch and to the same level on rear sonde holding projection 49 in the alternative the compound can fill entirety of recess 48 and in any case does not hinder transmission of the sonde signal or removal of the sonde when necessary the surface of cover 37 should be free of studs since this would place undue stress on the cover the use of bolts to secure cover 37 is of course feasible but bolts tend to loosen or break off during use use of a bolt head to hold the cover down is not preferred because the head of the bolt which creates the clamping force is necessarily located on the outside of the device and little can be done to protect it from abrasion accordingly as the fasteners used to removably secure cover 37 to housing 32 it is preferred to use retainers 51 in the form of spiral wound.