with reference to fig1 : the sensor 1 per se comprises a geophone 2 with its test - lead point mounted by way of two cable type damping devices 4 on three or four small columns 5 rigidly attached to a plate 6 by means of a screw 7 . with reference to fig2 : the rod 11 which is made of zicral or fiber glass consists of four tubular sections 12 adapted to be snapped together by way of three connections 13 ( bayonet type or automatic connections ) so as to obtain a rod which is 1 . 50 m , 3m , 4 . 5 m , or 6m in length as desired . the unit only has to bear a force in the order of 2kg by weight which is necessary to rest the test - lead point 3 on the rock mass r . the electric wires 14 of the geophone pass through the rod 11 which they leave at its lower part . these wires are preferably incorporated in tubes 12 . the connections 13 safeguard the electrical connections and the lead out wire consists of extensible wire ( the type of wire used for telephone sets ). with reference to fig3 which is the block diagram of the electrical part : the input signal 21 which may be damped by means of a switch 22 in series with a resistance 23 , for example , of a 200 ohm resistor connected to ground , is applied to a low - pass filter 24 with an input impedance of at least 5 kω . the filter is designed to eliminate the high frequencies produced at the contact point of the test - lead point 3 and the rock mass r . this low - pass filter possesses , for example , an attenuation from 450 hz of 12 decibels per octave - as shown in the graph of fig4 . the low - pass filter 24 is followed by an amplifier 25 having an adjustable gain . this is followed by a rectifier 26 and then a converter 27 which effects the analog - digital conversion , for example , at a conversion rate of 10 4 pulses per sec . per volt . the converter 27 drives an integrator - indicator 28 which indicates a number n proportional to the integral : ## equ1 ## where v ( t ) is the particulate speed of the rock mass as detected by the geophone and t o is the total duration of the vibration . with reference to fig5 and 6 : the side panel 31 of the box 32 comprises -- apart from the input 21 and the switch 22 -- an indicator 33 indicating the state of the cells , a data indicator 34 , a zero resetting knob 35 and a stop - start switch 36 . the data indicator 34 is designed for 3 characteris of the seven segment type each character being at least 6 mm in height and clearly visible one meter away . the return - to - zero knob 35 triggers a time delay of , for example , 5 seconds , during which the letters are illuminated and it is possible to read the same . the supply to the data indicator is then interrupted . in the case of an overrun , the data indicator stops at 999 which indicates this overrun . the box 32 , which is provided with a carrying strap 37 and a cover 38 , holds in its lower part 39 cells ( not represented ) or batteries ( not represented ) adapted to be recharged by means of a mains charger . purely by way of indication , with a battery permitting automatic operation for two or three hours , the box can measure 8 × 12 × 20 cm and weigh approximately 1500 grams .