DESCRIPTION Electromagnetic Prospecting Apparatus and Electromagnetic Prospecting Method 5 TECHNICAL FIELD The present invention relates to an electromagnetic prospecting apparatus and an electromagnetic prospecting method, typically used for geological surveys, underground resource prospecting, and others, and more particularly relates to a configuration that uses a superconducting quantum interference device (SQUID). 10 BACKGROUND ART Electromagnetic prospecting techniques used for geological surveys, underground resource prospecting, and others, have conventionally been put into practical use. In the electromagnetic prospecting techniques, electrical properties (resistivity) of the geological structure are typically measured. As the method of 15 measuring such electrical properties of the geological structure, there has been known a method of generating a periodically-changing primary magnetic field from a ground surface toward a prospecting target, which is located underground, and measuring a secondary magnetic field generated by the primary magnetic field (e.g. Japanese Patent Laying-Open No. 07-110382 (Patent Document 1), and "Toshihiro UCHIDA and Akira 20 SAITO, "A Review of Electromagnetic Prospecting Systems", Geophysical Exploration, Society of Exploration Geophysicists of Japan, Vol. 47, No. 6, pp. 472-500" (Non Patent Document 1). More specifically, temporal variations in primary magnetic field cause induced current to be generated at the ground surface in a direction preventing the relevant 25 variations. This induced current attenuates in accordance with a magnitude of resistivity of the geological structure located on propagation paths, and in a direction so as to prevent temporal changes in induced current, which are caused in association with the relevant attenuation, other induced current is newly generated. Such a process of - 1 generating induced current is repeated to thereby cause a phenomenon that looks as if the induced current is propagating underground toward a deeper section. Such a phenomenon is also referred to as a "smoke ring". Such induced current attenuates in accordance with the resistivity of the propagation paths, so that by measuring a 5 magnetic field where the induced current is generated, as a function of time, it is possible to obtain a resistivity distribution of the underground geological structure (mainly in a cross-sectional view). The induced current diffuses in a depth direction as time elapses, with its radius increased. Therefore, if constant current allowed to flow through a transmitter loop is 10 instantaneously interrupted, a diffusion depth 6, namely, a depth at which the induced current diffuses in the depth direction, can be represented as 5 = (2t/a )m, by using elapsed time t after the current interruption (a: conductivity of the underground structure, p: magnetic permeability of the underground structure). Accordingly, by taking longer measurement time, it is possible to obtain a resistivity distribution in a 15 deeper section. For the measurement of such a secondary magnetic field, there has conventionally been employed an induction coil magnetometer that uses a receiver coil made of a metal lead. In principle, this induction coil magnetometer measures not the magnitude of a magnetic field, but the time derivative of the magnetic field. In contrast, 20 there has been proposed a configuration that directly measures the magnitude of a magnetic field by means of a Superconducting Quantum Interference Device (SQUID; hereinafter also referred to as "SQUID"). Temporal changes in magnetic field are more gradual when compared with temporal changes in time derivative of the magnetic field. Furthermore, with use of the SQUID, it is possible to highly sensitively detect a 25 magnetic field, and thus obtain the data at later time when compared with the case of using the induction coil magnetometer. Accordingly, with use of the SQUID, it is possible to obtain a resistivity distribution of a deeper section, when compared with the case of using the induction coil magnetometer. -2- The magnetic field that can be gauged by the SQUID is quantized in units of flux quantum *o (= 2.07 x 10- 15 Wb). When receiving a magnetic field, the SQUID outputs a voltage corresponding to the value, and the output voltage exhibits sinusoidal changes having a period of flux quantum 4o, with respect to the changes in magnetic field received by the 5 SQUID. It is therefore not possible to uniquely determine the magnetic field from the output voltage of the SQUID. Accordingly, there is generally adopted a configuration which uses a circuit referred to as an FLL (Flux Locked Loop) to provide to the SQUID a feedback magnetic field for canceling out an external magnetic field, to thereby uniquely determine a magnetic field received by the SQUID and expand the measurable range. 10 Patent Document 1: Japanese Patent Laying-Open No. 07-110382 Non-Patent Document 1: Toshihiro UCHIDA and Akira SAITO, "A Review of Electromagnetic Prospecting Systems", Geophysical Exploration, Society of Exploration Geophysicists of Japan, Vol. 47, No. 6, pp. 472-500 A reference herein to a patent document or other matter which is given as prior art is 15 not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims. DISCLOSURE OF THE INVENTION 20 PROBLEMS TO BE SOLVED BY THE INVENTION When a primary magnetic field having much larger intensity is generated from the ground surface toward an underground prospecting target so as to obtain a resistivity distribution in a deeper section, a magnetic field to be incoming into the SQUID is also increased. Consequently, when temporal changes in magnetic field incoming into the SQUID 25 exceed a response speed (slew rate) of the FLL circuit, a reference point for the feedback magnetic field in the FLL circuit is shifted, resulting in a problem that a stepwise measurement error occurs in a measurement signal. The present invention has been made to solve such a problem, and an object of the present invention is to provide an electromagnetic prospecting apparatus and an 30 electromagnetic prospecting method, capable of outputting a detection result more accurately. MEANS FOR SOLVING THE PROBLEMS An electromagnetic prospecting apparatus according to an aspect of the present invention includes: a transmission unit for generating a periodic variable magnetic field 3 toward a prospecting target; a detection unit including a SQUID, for detecting a magnetic field generated in accordance with said prospecting target; and a data processing unit for collecting a detection result obtained by said detection unit, said detection unit including a feedback coil for generating a magnetic field for canceling out a magnetic field incoming into said SQUID, 5 and a feedback circuit for supplying, to said feedback coil, current corresponding to a voltage generated by said SQUID, and said data processing unit being operative to extract a plurality of temporal data items each having a length of at least one period of said variable magnetic field, from detection data on the magnetic field incoming over a prescribed time period, determine whether or not a deviation from a reference value by an amount equal to or larger 10 than a threshold value occurs in each of the plurality of the extracted temporal data items, eliminate a temporal data item in which the deviation from the reference value by the amount equal to or larger than said threshold value occurs, from said plurality of temporal data items, and output a prospecting result which shows said prospecting target, based on remaining temporal data items, a temporal waveform of said variable magnetic field being a pulse 15 waveform in which a positive-direction pulse and a negative-direction pulse are periodically repeated with a zero section interposed therebetween, and said data processing unit being operative to determine whether or not the deviation by the amount equal to or larger than said threshold value occurs, based on a value obtained before a first time period from a first timing at which said positive-direction pulse starts returning to zero and a value obtained before said 20 first time period from a second timing at which said negative-direction pulse starts returning to zero. Preferably, the threshold value is determined based on a voltage value generated by the SQUID in response to incoming of a single flux quantum. More preferably, a temporal waveform of the variable magnetic field is a pulse 25 waveform in which a positive-direction pulse and a negative-direction pulse are periodically repeated with a zero section interposed therebetween. The data processing unit determines whether or not the deviation by the amount equal to or larger than the threshold value occurs, based on a value obtained before a first time period from a first timing at which the positive direction pulse starts returning to zero and a value obtained before the first time period from a 30 second timing at which the negative-direction pulse starts returning to zero. More preferably, the data processing unit determines whether or not the deviation by the amount equal to or larger than the threshold value occurs, based on a value obtained after an elapse of a second time period from the first timing and a value obtained after an elapse of 4 the second time period from the second timing. Preferably, the electromagnetic prospecting apparatus further includes a shield member placed to coat an external surface of the detection unit, for blocking an electromagnetic wave in a frequency region higher than a frequency of the magnetic field generated in accordance 5 with the prospecting target. An electromagnetic prospecting method according to another aspect of the present invention includes the steps of providing an electromagnetic prospecting apparatus, said electromagnetic prospecting apparatus including a transmission unit for generating a magnetic field, a SQUID for generating a voltage corresponding to an incoming magnetic field, a 10 feedback coil for generating a magnetic field for canceling out the magnetic field incoming into said SQUID, and a feedback circuit for supplying, to said feedback coil, current corresponding to the voltage generated by said SQUID; generating a periodic variable magnetic field toward a prospecting target; collecting magnitude values of the incoming magnetic field detected by said detection unit, over a prescribed time period; extracting a 15 plurality of temporal data items each having a length of at least one period of said variable magnetic field, from detection data on the incoming magnetic field, the detection data being collected over said prescribed time period; determining whether or not a deviation from a reference value by an amount equal to or larger than a threshold value occurs in each of the plurality of the extracted temporal data items; eliminating a temporal data item in which the 20 deviation from the reference value by the amount equal to or larger than the threshold value occurs, from said plurality of temporal data items; and outputting a prospecting result which shows said prospecting target, based on remaining temporal data items, a temporal waveform of said variable magnetic field being a pulse waveform in which a positive-direction pulse and a negative-direction pulse are periodically repeated with a zero section interposed 25 therebetween, and said determining step including the step of determining whether or not the deviation by the amount equal to or larger than said threshold value occurs, based on a value obtained before a first time period from a first timing at which said positive-direction pulse starts returning to zero and a value obtained before said first time period from a second timing at which said negative-direction pulse starts returning to zero. 30 EFFECTS OF THE INVENTION According to the present invention, it is possible to output a detection result 5 more accurately. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic configuration diagram of an electromagnetic prospecting apparatus according to an embodiment of the present invention. 5 Fig. 2 is a diagram that schematically shows an electromagnetic prospecting method that uses the electromagnetic prospecting apparatus according to the embodiment of the present invention. Fig. 3 is a partially cross-sectional side view of a detection unit according to the embodiment of the present invention. 10 Fig. 4 is a diagram for describing a principle of magnetic field detection by means of a SQUID. Fig. 5 is a diagram that shows the relation between a magnetic flux incoming into the SQUID and an output voltage. Fig. 6 is a diagram that shows a schematic circuit configuration of an FLL circuit 15 according to the embodiment of the present invention. Fig. 7 is a temporal waveform for describing a transmitted current in the electromagnetic prospecting apparatus according to the embodiment of the present invention and a generated secondary magnetic field. Fig. 8 is a diagram for describing an operating principle of the FLL circuit. 20 Fig. 9 is a diagram that shows a temporal waveform of a magnetic field generated in the electromagnetic prospecting apparatus according to the embodiment of the present invention. Fig. 10 is a diagram that shows a temporal waveform of a measurement signal outputted from the SQUID according to the embodiment of the present invention. 25 Fig. II is a diagram that shows a temporal waveform of an example of a measurement signal outputted from the FLL circuit according to the embodiment of the present invention. Fig. 12 is a diagram that shows a temporal waveform of an example of a -6measurement signal outputted from the FLL circuit according to the embodiment of the present invention. Fig. 13 is a schematic diagram that shows a schematic hardware configuration of a data processing unit according to the embodiment of the present invention. 5 Fig. 14 is a block diagram that shows a control structure relating to data processing in the electromagnetic prospecting apparatus according to the embodiment of the present invention. Fig. 15 is a diagram that shows a result of comparison between a prospecting result obtained by the electromagnetic prospecting apparatus according to the 10 embodiment of the present invention, and a prospecting result in the conventional example. Fig. 16 is a flowchart that shows a processing procedure in the electromagnetic prospecting apparatus according to the embodiment of the present invention. Fig. 17 is a partially cross-sectional side view of a detection unit according to a 15 modification of the embodiment of the present invention. Fig. 18 is a diagram for describing an effect of a shield member on a detection result of the detection unit. DESCRIPTION OF THE REFERENCE SIGNS 1: electromagnetic prospecting apparatus, 2: detection unit, 3: data processing 20 unit, 20: SQUID, 21: feedback coil, 22: FLL circuit, 24: container unit, 26: vacuum layer, 28: lid unit, 30: shield member, 32: resin member, 40: controller, 42: A/D conversion unit, 44: pulse oscillator unit, 46: transmitter, 48: transmitter loop coil, 221: differential amplifier, 222, 226: operational amplifier, 223: capacitor, 224, 225: resistor, 227: variable resistor, 300: CPU, 304: display unit, 306: interface unit, 308: input unit, 25 310: hard disk unit, 312: memory unit, 314: CD-ROM drive, 316: FD drive, 350: offset elimination unit, 352: transmission control unit, 354. buffer unit, 356: extraction unit, 358: determination unit, 360: normalization unit, 362: output unit, 370: data storage unit, JJ: Josephson junction, LN2: liquid nitrogen. -7- BEST MODES FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described in detail with reference to the drawings. It is noted that the same or corresponding portions in the drawings are provided with the same reference characters, and the description thereof will not be 5 repeated. (General Configuration) With reference to Figs. I and 2, an electromagnetic prospecting apparatus I according to the embodiment of the present invention is typically used for geological surveys, underground resource prospecting, and others, and mainly measures a three 10 dimensional distribution of electrical properties (resistivity) of a prospecting target. For a more practical configuration, electromagnetic prospecting apparatus I includes a detection unit 2, a data processing unit 3, a controller 40, an A/D (Analog to Digital) conversion unit 42, a pulse oscillator unit 44, a transmitter 46, and a transmitter loop coil 48. 15 Transmitter 46 allows a prescribed transmitted current to flow through transmitter loop coil 48 in response to a pulse signal (periodic signal) generated in pulse oscillator unit 44, to thereby generate a magnetic field (hereinafter also referred to as a "primary magnetic field") from transmitter loop coil 48 toward a prospecting target. By instantaneously interrupting the transmitted current provided to transmitter loop coil 20 48, an induced current is generated at a ground surface in a direction preventing variations in primary magnetic field. The induced current attenuates in accordance with a magnitude of resistivity of a geological structure located on propagation paths, and in a direction preventing temporal changes in dielectric current, which are caused in association with the relevant attenuation, other induced current is newly generated. 25 Such a process of generating an induced current is repeated to thereby cause a phenomenon (smoke ring) that looks as if the induced current is propagating underground toward a deeper section. Detection unit 2 placed in the vicinity of the ground surface detects a secondary -8 magnetic field generated by such induced current, and outputs the detection result thereof to data processing unit 3 via A/D conversion unit 42. The secondary magnetic field shows an attenuated state of the induced current, namely, a value corresponding to resistivity of the prospecting target located on the propagation paths. Furthermore, the 5 secondary magnetic field is generated in association with the propagation of the induced current, and hence elapsed time in the temporal changes in secondary magnetic field corresponds to the depth. Accordingly, based on the temporal changes in secondary magnetic field, it is possible to obtain the distribution of resistivity in a depth direction of the ground. 10 Detection unit 2 includes a Superconducting Quantum Interference Device (SQUID; hereinafter also referred to as "SQUID") 20 and an FLL circuit 22, and detects a magnetic field generated in accordance with the prospecting target. Detection unit 2 outputs a voltage value corresponding to a magnetic field (magnetic flux) incoming into SQUID 20, as a measurement signal. It is noted that, although electromagnetic 15 prospecting apparatus I including one detection unit 2 is illustrated in the present embodiment, a plurality of detection units 2 may be placed. Furthermore, although detection unit 2 configured with one SQUID 20 is illustrated, it may also be possible to use a detection unit that includes a plurality of SQUIDs 20 that allow their detector planes to be oriented, for example, in a horizontal direction and a vertical direction, 20 respectively. A/D conversion unit 42 converts voltage values (analog values) sequentially detected by detection unit 2 into digital values, and outputs the digital values to data processing unit 3. Data processing unit 3 sequentially collects the detection results, which are 25 obtained by detection unit 2 and outputted from A/D conversion unit 42. In particular, data processing unit 3 calculates a prospecting result by excluding a data item having an error, the data item being included in the detection results, in accordance with a data processing method as described below. It is noted that the calculated prospecting -9result is displayed on a display or the like of data processing unit 3. Pulse oscillator unit 44 is configured to output an oscillation signal (typically, a periodic pulse signal) for driving transmitter 46, and to be able to change a period of the oscillation signal in response to a transmission command from data processing unit 3. 5 Transmitter 46 and transmitter loop coil 48 serve as a transmission unit that generates a magnetic field toward a prospecting target. Transmitter 46 receives an oscillation signal from pulse oscillator unit 44 and allows a transmitted current to flow through transmitter loop coil 48. Transmitter loop coil 48 generates a primary magnetic field corresponding to the transmitted current. 10 (Detection Unit) Fig. 3 is a partially cross-sectional side view of detection unit 2 according to the embodiment of the present invention. With reference to Fig. 3, detection unit 2 includes a container unit 24 for storing a cooling medium for maintaining SQUID 20 in a superconducting state, SQUID 20 15 immersed in the cooling medium, a lid unit 28 for preventing intrusion of heat, and FLL circuit 22 penetrating lid unit 28 and connected to SQUID 20. SQUID 20 may be configured with any of a low temperature superconductor made of a niobium compound or the like, and a high temperature superconductor made of ceramics or the like. In electromagnetic prospecting apparatus I according to the 20 present embodiment, description will be made on a configuration that uses a high temperature superconductor, as an example. The high temperature superconductor is brought into a superconducting state at approximately 77 K, and thus liquid nitrogen LN2 (boiling point: 77.3 K) is used for the cooling medium. It is noted that, if the low temperature superconductor is to be used, liquid helium (boiling point: 4.2 K) is used for 25 the cooling medium. Container unit 24 is typically configured with a nonconductive material such as a glass epoxy, and a vacuum layer 26 is formed on its outer periphery so as to reduce intrusion of heat from an outside of the container. Similarly, lid unit 28 is configured - 10with a nonconductive material such as a glass epoxy, and brought into intimate contact with container unit 24 by a screw or the like. Fig. 4 is a diagram for describing a principle of magnetic field detection by means of SQUID 20. Fig. 5 is a diagram that shows the relation between a magnetic flux 5 incoming into SQUID 20 and an output voltage. With reference to Fig. 4, SQUID 20 is made of a superconducting material formed into a loop, and two Josephson junctions JJ are formed on that loop. Each of Josephson junctions JJ is structured such that an insulating material is interposed between two stacked superconducting materials, so that it less strongly exhibits a 10 property of a superconducting material when compared with other portions. Therefore in SQUID 20, Josephson junctions JJ most quickly move from the superconducting state to the normal conducting state. In other words, when current flowing through the loop exceeds a prescribed critical current, a part of SQUID 20 is brought into a normal conducting state, so that Josephson junctions JJ define the critical current. 15 In the superconducting state, SQUID 20 causes a phenomenon in which a magnetic flux penetrating SQUID 20 itself is eliminated (i.e. a Meissner effect). More specifically, when a magnetic flux incoming from an outside is to penetrate SQUID 20, a shielding current for canceling out the magnetic flux flows therethrough. Therefore, when a bias current approximately equal to the critical current, at which Josephson 20 junctions JJ can maintain the superconducting state, is provided externally, addition of the shielding current for canceling out the magnetic flux incoming from an outside causes a part of Josephson junctions JJ to move into the normal conducting state, resulting in electrical resistance. The electrical resistance depends on a magnitude of the shielding current, namely, a magnitude of the incoming magnetic field, and hence it is 25 possible to detect the magnetic field generated at detection unit 2 by measuring an output voltage attributed to the electrical resistance. As shown in Fig. 5, an output voltage of SQUID 20 with respect to the incoming magnetic flux exhibits repetitive similar properties in units of flux quantum <o (= 2.07 x - 11 - 10-" Wb). Therefore, a plurality of magnitude values of magnetic field (magnetic flux) may correspond to a single output voltage, so that a magnitude of the magnetic field incoming into SQUID 20 cannot uniquely be determined based on the output voltage. Therefore, FLL (Flux Locked Loop) circuit 22 as described below is used to detect the 5 magnitude of the magnetic field. Fig. 6 is a diagram that shows a schematic circuit configuration of FLL circuit 22 according to the embodiment of the present invention. With reference to Fig. 6, FLL circuit 22 provides to SQUID 20 a feedback magnetic field for canceling out the external magnetic field incoming into SQUID 20, 10 and measures an output voltage of SQUID 20. More specifically, FLL circuit 22 integrates a voltage generated across SQUID 20 and outputs the integrated voltage, and provides a feedback magnetic field corresponding to the integrated voltage to SQUID 20 from a feedback coil 21 placed in proximity to SQUID 20. In other words, FLL circuit 22 supplies to feedback coil 21 current corresponding to a voltage generated by 15 SQUID 20. This feedback magnetic field is for canceling out the external magnetic field incoming into SQUID 20. By using such a feedback magnetic field to maintain the magnetic field incoming into SQUID 20 to be substantially zero, it is possible to expand the measurable range of SQUID 20, which alone could not enable an accurate 20 measurement of a variable magnetic field having an amplitude exceeding flux quantum 40. As such, by maintaining the magnetic field incoming into SQUID 20 to be substantially zero, it is possible for detection unit 2 to directly measure a magnitude of a secondary magnetic field generated in accordance with a detection target. As a more practical configuration, FLL circuit 22 includes a differential amplifier 25 221 for differentially amplifying a voltage generated across SQUID 20, operational amplifiers 222, 226, a capacitor 223, resistors 224, 225, and a variable resistor 227. Operational amplifier 222 and capacitor 223 configure an integrator, and the integrator has a time constant t determined by an amplification factor of operational - 12amplifier 222 and a capacitance of capacitor 223. Time constant I defines a response speed (slew rate) for serving as the FLL circuit. The smaller time constant C causes an increase in response speed and a decrease in stability of the FLL circuit, and thus time constant T is set to be an appropriate value in consideration of a response speed and 5 stability. The integrator (operational amplifier 222 and capacitor 223) outputs a measurement signal (voltage signal) as to SQUID 20. Feedback coil 21 is connected to the output of operational amplifier 222 via resistor 224, so that current corresponding to the voltage generated by SQUID 20 is supplied to feedback coil 21. Furthermore, variable resistor 227, operational amplifier 226, and resistor 225 10 are connected in series between resistor 224 and feedback coil 21, from a power supply line, so that it is possible to make fine adjustment of an initial current to be supplied to feedback coil 21, so as to adjust a zero level of a voltage output of SQUID 20. With the above-described configuration, detection unit 2 can directly measure a magnitude of a secondary magnetic field, which is generated in accordance with a 15 detection target upon reception of a primary magnetic field. It is noted that, as to the correspondence between the circuit configuration of FLL circuit 22 shown in Fig. 6 and the invention according to the claims of the present application, differential amplifier 221, operational amplifier 222, capacitor 223, and resistor 224 correspond to a "feedback circuit". 20 Fig. 7 shows a temporal waveform for describing a transmitted current in electromagnetic prospecting apparatus I according to the embodiment of the present invention and a generated secondary magnetic field. Fig. 7 (A) shows transmitted current supplied to transmitter loop coil 48, Fig. 7 (B) shows a gauged secondary magnetic field, and Fig. 7 (C) shows a time derivative of the gauged secondary magnetic 25 field. As shown in Fig. 7 (A), when a pulse-like transmitted current is supplied to transmitter loop coil 48, a secondary magnetic field as shown in Fig. 7 (B) is generated. A time derivative of the secondary magnetic field shown in Fig. 7 (B) exhibits a - 13 waveform as shown in Fig. 7 (C). The conventional induction coil magnetometer is intended for measuring a time derivative of the secondary magnetic field as shown in Fig. 7 (C), and hence its measurable depth is limited. In contrast, detection unit 2 according to the present embodiment can measure the magnitude itself of the secondary magnetic 5 field shown in Fig. 7 (B), and hence can sensitively measure a secondary magnetic field for longer time. In other words, it is possible to obtain a resistivity distribution in a deeper section. (Operation of FLL Circuit) Next, with reference to Fig. 8, description will be made on an operation of the 10 FLL circuit, and a stepwise error that occurs in a measurement signal outputted from the FLL circuit. Fig. 8 is a diagram for describing an operating principle of the FLL circuit. Fig. 8 (A) shows a steady state at certain timing, while Fig. 8 (B) shows the case where temporal variations in magnetic field that exceed the response speed (slew rate) of the 15 FLL circuit are applied to the steady state shown in Fig. 8 (A). With reference to Fig. 8 (A), initially in the FLL circuit in the steady state, feedback coil 21 (Fig. 6) generates a feedback magnetic field so as to cancel out the external magnetic field incoming into SQUID 20. In other words, feedback coil 21 operates such that a feedback magnetic field Bf generated by itself causes the magnetic 20 flux incoming into SQUID 20 to be approximately equal to an external magnetic flux <bin incoming into SQUID 20 (which external magnetic flux corresponds to a product of an external magnetic field Bin and an effective cross-sectional area Aeff). The FLL circuit then outputs a voltage corresponding to current for generating feedback magnetic field Bf, as a measurement signal. As such, a measurement signal corresponding to 25 external magnetic field Bin incoming into SQUID 20 is obtained. In such a steady state, for the purpose of obtaining the highest sensitivity, the FLL circuit controls current to be supplied to feedback coil 21 such that an output voltage of SQUID 20 is maintained at a position at which dV/d* assumes the maximum - 14value in the relation between the magnetic flux incoming into SQUID 20 and the output voltage (i.e. in the 4-V property of SQUID 20). In the following, such a position is also referred to as a lock point LOCKI, and an operation for maintaining lock point LOCKI is also referred to as a lock operation of the FLL circuit. 5 In the lock operation as described above, consideration will be made on the case where the external magnetic field has temporally changed from Bin by ABin. It is noted that a temporal change ABin/At in external magnetic field is equal to or below the response speed (slew rate) of the FLL circuit. In this case, because of the lock operation of the FLL circuit, feedback magnetic field Bf is also increased by ABf. With 10 the increase in feedback magnetic field Bf, the magnetic flux incoming into SQUID 20 becomes substantially zero, so that lock point LOCK I is maintained. Next, with reference to Fig. 8 (B), consideration will be made on the case where external magnetic field Bin causes temporal change ABin/At that exceeds the response speed of the FLL circuit during the above-described lock operation. In this case as 15 well, with the increase in external magnetic field Bin, the FLL circuit attempts to increase feedback magnetic field Bf However, the rate of increase in feedback magnetic field Bf is lower than the rate of increase in external magnetic field Bin, resulting in that an external magnetic field that goes beyond the capability of the feedback function of the FLL circuit is introduced. As a result, the lock point is shifted 20 from LOCKI to LOCK2 in a stepwise fashion, and hence goes beyond the local maximum value of the sinusoidal waveform, which is located on the right of LOCK 1, so that the feedback coil performs lock at LOCK2, which is larger than LOCK I by #0. Consequently, the FLL circuit has to generate a magnetic field obtained by adding *o, which is identified as a magnetic flux difference between LOCKI and LOCK2, to the 25 external magnetic field incoming into SQUID 20, resulting in that the lock point is shifted from LOCK I to LOCK2 in a stepwise fashion. As a result, the measurement signal outputted from the FLL circuit is also changed as well in a stepwise fashion in units corresponding to flux quantum $o. In other words, the measurement signal from - 15 the FLL circuit changes in a stepwise fashion by an amount of a voltage generated by SQUID 20 in accordance with flux quantum 4o. It is noted that, in the following description, such a stepwise shift of the lock point is also referred to as an "offset". Electromagnetic prospecting apparatus I according to the present embodiment 5 outputs a prospecting result that shows electrical properties of a prospecting target, while eliminating the data in which such a stepwise change occurs, included in the measurement signal. (Method of Eliminating Data in Which Offset Occurs) Next, with use of Figs. 9 and 10, description will be made on a periodic variable 10 magnetic field (primary magnetic field) generated by transmitter loop coil 48 and a magnetic field detected by detection unit 2. Fig. 9 is a diagram that shows a temporal waveform of a magnetic field generated in electromagnetic prospecting apparatus I according to the embodiment of the present invention. Fig. 9 (A) shows a waveform of transmitted current that is 15 allowed to pass through transmitter loop coil 48, while Fig. 9 (B) shows a measurement signal measured at SQUID 20. With reference to Fig. 9 (A), a temporal waveform of the primary magnetic field generated at transmitter loop coil 48 is identified as a pulse waveform in which a positive-direction pulse and a negative-direction pulse are periodically repeated with a 20 zero section interposed therebetween. In the present embodiment, a time period starting at the generation of a positive-direction pulse and ending at the generation of the next positive-direction pulse is set as one period of the primary magnetic field. As to the primary magnetic field, as shown in Fig. 9 (B), a magnetic field obtained by combining a secondary magnetic field generated in accordance with a prospecting target 25 and the primary magnetic field generated at transmitter loop coil 48 is detected at SQUID 20. In order to separate these magnetic fields, the timings at which a positive direction pulse and a negative-direction pulse are interrupted, namely, the timing at which a positive-direction pulse starts returning to zero and the timing at which a - 16negative-direction pulse starts returning to zero, are set as reference timings. Based on a measurement signal outputted from SQUID 20 after each of the reference timings, electrical properties of the prospecting target are measured. This is because the primary magnetic field is maintained to be zero during a time 5 period ranging from a reference timing to the generation of the next pulse, and thus the magnetic field measured at SQUID 20 has only a component of the secondary magnetic field. Fig. 10 is a diagram that shows a temporal waveform of a measurement signal outputted from SQUID 20 according to the embodiment of the present invention. Fig. 10 10 (A) shows the case where no offset occurs, while Fig. 10 (B) shows the case where an offset occurs. Figs. 11 and 12 are diagrams each showing a temporal waveform of an example of a measurement signal outputted from the FLL circuit according to the embodiment of the present invention. Figs. 11 and 12 mainly show a measurement signal 15 corresponding to the secondary magnetic field in Fig. 10, and does not show a measurement signal corresponding to the primary magnetic field. It is noted that a positive measurement signal and a negative measurement signal are not necessarily displayed in an alternate manner because of software processing. Fig. 11 shows the case where no offset occurs, while Fig. 12 (A) shows the case 20 where an offset frequently occurs. Fig. 12 (B) is the one that shows a part of the waveform in Fig. 12 (A) in an enlarged fashion. With reference to Fig. 10 (A), in the state where no offset occurs, an output of a measurement signal obtained in a time period during which transmitter loop coil 48 generates no primary magnetic field converges to a prescribed reference value. 25 In contrast, with reference to Fig. 10 (B), if an offset occurs because of the changes in magnetic field caused in association with the interruption of a positive direction pulse, for example, the entire waveform deviates after the time point at which the relevant offset occurs. In other words, when an offset occurs, the entire waveform - 17deviates from the reference value by a prescribed offset amount. As shown in Figs. 12 (A) and 12 (B), when an offset occurs, a portion of measurement signal is observed to deviate from the reference value in a time period during which no primary magnetic field is generated (an intensity of the primary 5 magnetic field is zero). More specifically, as shown in Fig. 10 (B), it is possible to determine the presence or absence of the offset by determining, as to the same period of the primary magnetic field, whether or not an absolute value IVI1pl - V1(")| of a difference between an intensity value V1(P) obtained before a time period TI from the reference timing for a 10 positive-direction pulse and an intensity value V1( obtained before time period T I from the reference timing for a negative-direction pulse greatly deviates from the reference value by an amount larger than a prescribed threshold value Thl. In other words, when an offset occurs in a target period, IVl"p) - Viln"j deviates from the reference value in units of a voltage value corresponding to flux quantum 4o. 15 Accordingly, it is possible to determine the presence or absence of the offset by comparing IVi1p) - VIln"i with threshold value Thl, which is predetermined in accordance with a voltage value generated by SQUID 20 upon reception of a single flux quantum $o. It is noted that time period TI is set in consideration of time necessary for the 20 primary magnetic field to rise from zero to a prescribed amplitude and become stable, in addition to a desired depth to be studied. Similarly, it is also possible to determine the presence or absence of the offset by determining, as to the same period of the primary magnetic field, whether or not an absolute value IV2(P) - V2 (") of a difference between an intensity value V2( obtained 25 after an elapse of a time period T2 from the reference timing for a positive-direction pulse and an intensity value V2( obtained after an elapse of time period T2 from the reference timing for a negative-direction pulse greatly deviates from the reference value by an amount larger than a prescribed threshold value Th2. - 18 - In other words, when an offset occurs in a target period, IV2'P' - V2 In" deviates from the reference value in units of a voltage value corresponding to flux quantum #o. Accordingly, it is possible to determine the presence or absence of the offset by comparing |V2(P) - V2(")| with threshold value Th2, which is predetermined in 5 accordance with a voltage value generated by SQUID 20 upon reception of a single flux quantum $o. It is noted that time period T2 is set such that a secondary magnetic field generated at a prospecting target is brought into a fully-attenuated state. In both of the two cases described above, a lock deviation occurs. Therefore, it 10 is necessary to determine the presence or absence of the offset based on both of the absolute values IV1i1 - Vlf" and IV2'P - V2 In" of the differences. Description will hereinafter be made on the configuration for implementing such processing. (Data Processing Unit) 15 Fig. 13 is a schematic diagram that shows a schematic hardware configuration of data processing unit 3 according to the embodiment of the present invention. With reference to Fig. 13, data processing unit 3 is typically implemented by a computer, and includes a CPU (Central Processing Unit) 300 for executing various programs including an operating system (OS: Operating System), a memory unit 312 for 20 temporarily storing data necessary for executing the programs in CPU 300, and a Hard Disk Unit (HDD) 310 for storing the programs executed in CPU 300 in a nonvolatile fashion. Furthermore, hard disk unit 310 stores in advance a program for implementing the processing as described below, and such a program is read from a flexible disk 316a, a CD-ROM (Compact Disk-Read Only Memory) 314a, and the like 25 by an FD drive 316 and a CD-ROM drive 314, respectively. CPU 300 receives instructions from a user or the like via an input unit 308 made of a keyboard or a mouse, and outputs to a display unit 304 a measurement result or the like measured by the execution of the programs. Furthermore, CPU 300 receives a - 19detection result from A/D conversion unit 42 via an interface unit 306, and provides a transmission command to pulse oscillator unit 44. (Control Structure) Next, with reference to Fig. 14, description will be made on a control structure 5 for implementing the data processing in electromagnetic prospecting apparatus I according to the embodiment of the present invention. Fig. 14 is a block diagram that shows a control structure relating to the data processing in electromagnetic prospecting apparatus I according to the embodiment of the present invention. It is noted that the block diagram shown in Fig. 14 is 10 implemented by CPU 300 (Fig. 13) in data processing unit 3, which lays out the programs, which are stored in advance in hard disk unit 310 or the like, in memory unit 312 or the like, and executes the programs. With reference to Fig. 14, data processing unit 3 includes a transmission control unit 352, a buffer unit 354, a data storage unit 370, an offset elimination unit 350, a 15 normalization unit 360, and an output unit 362, as its functions. It is noted that data storage unit 370 is provided in a prescribed area such as hard disk unit 310 (Fig. 13). Transmission control unit 352 and buffer unit 354 measure a secondary magnetic field, which is a response from a prospecting target. More specifically, when receiving start instructions from a user, transmission control unit 352 provides a transmission 20 command to pulse oscillator unit 44 (Fig. 1) and transfers synchronization information on the primary magnetic field generated at transmitter loop coil 48 to buffer unit 354, in accordance with a predetermined setting. In other words, a user provides detection unit 2 at a desired target prospecting site, and then manipulates input unit 308 (Fig. 13) to provide a prospecting start command to data processing unit 3 (Fig. 1). 25 Transmission control unit 352 then provides a transmission command according to the predetermined setting to pulse oscillator unit 44, and pulse oscillator unit 44 starts supplying transmitted current to transmitter loop coil 48. Buffer unit 354 successively stores detection results from detection unit 2 in data - 20 storage unit 370, by causing each detection result being associated with the corresponding synchronization information items on the primary magnetic field provided from transmission control unit 352. Here, the detection results are sequential temporal data items on the magnetic field intensity typically detected by SQUID 20. 5 Offset elimination unit 350 is a constituent element for eliminating a temporal data item in which an offset occurs, and includes an extraction unit 356 and a determination unit 358. Extraction unit 356 successively extracts temporal data items each having a length of at least one period of the primary magnetic field, from the detection results 10 stored in data storage unit 370. Extraction unit 356 successively outputs the temporal data items extracted in such a manner to determination unit 358. The "temporal data item" in the present specification refers to a data item having a prescribed length and showing a temporal change in magnetic field intensity detected by SQUID 20, which temporal change corresponds to one or more periods. In other 15 words, the "temporal data item" serves as a base unit of the data processing relating to the electromagnetic prospecting according to the present embodiment. It is noted that the "temporal data item" is preferably extracted such that it has a length equal to an integral multiple of the period of the primary magnetic field. In the present embodiment, the "temporal data item" is extracted such that it corresponds to a length 20 of one period of the primary magnetic field. Determination unit 358 performs the determination processing as shown in Fig. 10 (B) described above, on each of the temporal data items. In other words, as to each of the temporal data items, determination unit 358 obtains intensity value VI ( obtained before time period TI from the reference timing for a positive-direction pulse and 25 intensity value VI("( obtained before time period TI from the reference timing for a negative-direction pulse. Determination unit 358 then compares absolute value |ViP VI ("I of a difference between these intensity values with prescribed threshold value Th . Furthermore, if IVI (P - V1(" I exceeds threshold value Thl, determination unit 358 - 21 determines that an offset occurs in the relevant temporal data item, and hence treats both of positive and negative data items therein as invalid data items. Further, as to each of the temporal data items, determination unit 358 obtains intensity value V2( obtained after an elapse of time period T2 from the reference timing 5 for a positive-direction pulse and intensity value V2(") obtained after an elapse of time period T2 from the reference timing for a negative-direction pulse. Determination unit 358 then compares absolute value IV2(P) - V2")j of a difference between these intensity values with prescribed threshold value Th2. Furthermore, if IV2(P) - V2(') exceeds threshold value Th2, determination unit 358 determines that an offset occurs in the 10 relevant temporal data item, and hence treats both of positive and negative data items therein as invalid data items. After conducting the above-described processing, determination unit 358 outputs to normalization unit 360 only the temporal data items that are not identified as invalid. In other words, determination unit 358 eliminates a temporal data item in which an 15 offset occurs, from a plurality of temporal data items successively generated in extraction unit 356. Normalization unit 360 normalizes the plurality of temporal data items outputted from determination unit 358, with a value of current allowed to pass through the transmitter loop to obtain the respective temporal data items, and outputs the 20 normalized temporal data items to output unit 362. Based on the plurality of temporal data items outputted from normalization unit 360, output unit 362 outputs a prospecting result, which is identified as temporal data representative of these temporal data items. Specifically, output unit 362 calculates a measurement result by averaging the intensity values at the same time point on each time 25 base in these temporal data items. Fig. 15 is a diagram that shows a result of comparison between a measurement result obtained by electromagnetic prospecting apparatus I according to the embodiment of the present invention, and a measurement result in the conventional - 22 example. Fig. 15 (A) shows an example of a prospecting result outputted by the conventional electromagnetic prospecting apparatus, while Fig. 15 (B) shows an example of a prospecting result outputted by the electromagnetic prospecting apparatus 5 according to the present embodiment. As shown in Fig. 15 (A), when the data item in which an offset occurred was not eliminated, it is found that data items in a time domain later than the reference timing are missing. This is because an intensity of the primary magnetic field had been increased so as to obtain the data in a deeper region (in a time domain later than the reference 10 timing), an error occurred in the measurement result because of an offset, so that a prospecting result with sufficiently high accuracy could not be obtained. In contrast, as shown in Fig. 15 (B), with electromagnetic prospecting apparatus 1 according to the present embodiment, it is possible to generate a prospecting result by eliminating an error due to an offset caused when the primary magnetic field is increased. 15 It is therefore possible to obtain a prospecting result having higher accuracy. In addition, if a data item in which no offset occurs can be obtained in any of the periods during the measurement carried out multiple times, it is also possible to calculate the prospecting result based on the relevant data item in which no offset occurs, even if the ratio of such data item to the entire data items is very low. By doing so, it is possible 20 to obtain data in a deeper region. (Processing Procedure) The processing procedure of the control structure as described above is comprehensively presented in a flowchart as follows. Fig. 16 is a flowchart that shows the processing procedure in electromagnetic 25 prospecting apparatus I according to the embodiment of the present invention. With reference to Fig. 16, detection unit 2 is initially placed at a target prospecting position (step S 100). When CPU 300 in data processing unit 3 receives start instructions from a user, CPU 300 outputs a transmission command to pulse -23 oscillator unit 44 in accordance with a predetermined setting (step S102). In response to the transmission command, pulse oscillator unit 44 starts generating a pulse signal. In response to the pulse signal, transmitter 46 starts supplying prescribed transmitted current to transmitter loop coil 48. Furthermore, with this transmitted current, 5 transmitter loop coil 48 starts generating a primary magnetic field (step S104). With this primary magnetic field, there occurs a secondary magnetic field corresponding to a prospecting target. In addition, CPU 300 successively stores detection results from detection unit 2 in data storage unit 370 by causing each detection result being associated with 10 synchronization information items on the primary magnetic field (step S106). CPU 300 then determines whether or not the detection results over a predetermined time period have been obtained (step S108). If the detection results over a predetermined time period have not yet been obtained (NO in step S 108), the steps in step S106 and the following step are repeated. 15 In contrast, if the detection results over a predetermined time period have been obtained (YES in step S 108), CPU 300 reads out the detection results stored in hard disk unit 3 10, and successively generates temporal data items on a period-by-period basis of the primary magnetic field (step S110). As to each of the generated temporal data items, CPU 300 extracts intensity value VI" obtained before time period TI from 20 the reference timing for a positive-direction pulse and intensity value VI (") obtained before time period TI from the reference timing for a negative-direction pulse (step S112). CPU 300 then determines whether or not absolute value IVI() - Vi1("I of a difference between these intensity values exceeds prescribed threshold value ThI (step S1 14). 25 If IVI (p - Vi"Il does not exceed threshold value ThI (NO in step S 114), CPU 300 extracts intensive value V2( obtained after an elapse of time period T2 from the reference timing for a positive-direction pulse and intensity value V2(") obtained after an elapse of time period T2 from the reference timing for a negative-direction pulse (step - 24 - Si16). CPU 300 then determines whether or not absolute value IV2'P - V2 In" of a difference between these intensity values exceeds threshold value Th2 (step SI 18). If |V21P1 - V2III does not exceed threshold value Th2 (NO in step S 118), CPU 300 determines that the relevant temporal data item is valid (step S 120). 5 In contrast, if )VI - Vf") exceeds threshold value Thi (YES in step S 114), or IV2(P) - V2 ("> exceeds threshold value Th2 (YES in step Si 18), CPU 300 determines that the relevant temporal data item is invalid (step S 122). Next, CPU 300 normalizes the relevant temporal data item with a value of current allowed to pass through the transmitter loop (step S 124). 10 CPU 300 then determines whether or not the processing has been performed on all of the temporal data items generated from the detection results (step S 126). If the processing has not yet been performed on all of the generated temporal data items (NO in step S 126), the processing in step S112 and the following steps is repeated. If the processing has been performed on all of the generated temporal data items 15 (YES in step S 126), CPU 300 calculates a prospecting result by averaging the intensity values at the same time point on each time base (step S128). Finally, CPU 300 outputs the prospecting result calculated in step S128 (step S 130). It is noted that CPU 300 selects display unit 304 for allowing this prospecting result to be visually displayed or hard disk unit 310 for allowing the data on this prospecting result to be stored, for 20 example, as a destination of the prospecting result. It is noted that, if a plurality of measurement points are set to study a prospecting target with higher accuracy, the processing in steps S100-S 108 in the flowchart shown in Fig. 16 may be performed in advance at each measurement point, and then the processing relating to steps SI10-S130 may be performed as batch 25 processing subsequently. According to the embodiment of the present invention, it is possible to measure a magnitude itself of the secondary magnetic field generated by the primary magnetic field generated from transmitter loop coil 48, in accordance with a prospecting target. - 25 - Therefore, it is possible to prospect a deeper section when compared with the configuration that uses a conventional induction coil magnetometer, which measures a time derivative of the secondary magnetic field. In particular, determination as to the presence or absence of the occurrence of an 5 offset is made on the temporal data items on the secondary magnetic field measured by detection unit 2, and a prospecting result is calculated by eliminating a temporal data item determined as experiencing an offset. By doing so, it is possible to selectively extract a temporal data item in which no offset occurs, out of the temporal data items that shows respective periods of temporal waveforms of the primary magnetic field, and 10 hence it is possible to calculate an accurate prospecting result. In other words, the occurrence of an offset can be accepted up to a prescribed frequency, so that it is possible to increase an intensity of the primary magnetic field. Accordingly, it is possible to further increase an intensity of the primary magnetic field, and obtain data in a deeper region. 15 [Modification] By using a detection unit less likely to be influenced by high-frequency noises in the electromagnetic prospecting apparatus according to the above-described embodiment, it is possible to conduct electromagnetic prospecting with higher accuracy. Fig. 17 is a partially cross-sectional side view of a detection unit 2A according to 20 a modification of the embodiment of the present invention. With reference to Fig. 17, detection unit 2A is a modification of detection unit 2 shown in Fig. 3, obtained by further providing a shield member 30 placed to coat an external surface of container unit 24 and lid unit 28, and a resin member 32 applied to an external surface of shield member 30. 25 Shield member 30 is made of a conductive metal, a typical example of which is silver, and blocks high-frequency noises incoming from an outside of the apparatus. In the present embodiment, shield member 30 is formed of a silver paste, for example, which is applied onto container unit 24 and lid unit 28. -26- More specifically, by spraying a conductive paste (e.g. a silver paste), which is made of a resin having a conductive powder mixed therein, onto the external surface of container unit 24 and lid unit 28 with a spray gun, the silver paste is applied onto the relevant external surface. By doing so, even if the external surface of container unit 24 5 and lid unit 28 has irregularities, the silver paste can uniformly be applied onto the relevant external surface with favorable workability. As a result, a desired shielding performance can be ensured. It is noted that a silver paste coating may also be formed on container unit 24 and lid unit 28 by vacuum evaporation, sputtering, and the like. The thickness of shield member 30 is determined such that favorable shielding 10 performance is exhibited against high-frequency noises, and that the occurrence of an eddy current within shield member 30 is suppressed at a desired frequency (approximately 100 kHz). Resin member 32 is typically made of an epoxy resin, and coats shield member 30 to thereby prevent shield member 30 from peeling off because of the contact with an 15 outside. It is noted that a part of resin member 32 is provided with an opening, through which shield member 30 is short-circuited to the ground of FLL circuit 22. In the present configuration, by coating container unit 24 and lid unit 28 of detection unit 2 with shield member 30, the influence of high-frequency noises is eliminated, making it possible to avoid an unstable operation of detection unit 2. 20 Other parts are similar to those in detection unit 2 shown in Fig. 3, so that the detailed description thereof will not be repeated. Furthermore, as described below, it is confirmed that the presence or absence of shield member 30 did not have an influence on the data obtained when an electromagnetic prospecting was conducted. 25 Fig. 18 is a diagram for describing an effect of the shield member on the detection result of detection unit 2. Fig. 18 (A) is a temporal waveform of a measurement signal in the case where electromagnetic prospecting apparatus 1 according to the modification of the embodiment of the present invention was used to - 27 conduct electromagnetic prospecting. For comparison, Fig. 18 (B) shows a temporal waveform of a measurement signal in the case where an electromagnetic prospecting apparatus in which detection unit 2 does not have shield member 30 was used to conduct electromagnetic prospecting. It is noted that both of the temporal waveforms were obtained as 5 a measurement signal from detection unit 2, when transmitter loop coil 48 had a size of 100 m x 100 m, a transmitted current value was set to 0.3 A, and a transmit frequency was set to 25 Hz. When Fig. 18 (A) is compared with Fig. 18 (B), the temporal waveforms of the measurement signals are observed to be approximately the same. In particular, no difference 10 is observed between the results in earlier time periods, which were considered to have a possibility of being affected. Therefore, shield member 30 can be regarded as effective for eliminating an influence of high-frequency noises. It should be understood that the embodiment disclosed herein is illustrative and not limitative in all aspects. The scope of the present invention is shown not by the description 15 above but by the scope of the claims, and is intended to include all modifications within the equivalent meaning and scope of the claims. Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or 20 more other features, integers, steps or components, or group thereof. SPEC497999.. 28