## Exploration depth of electromagnetic channel of “Impulse-Аero” helicopter platforms

An important stage of aerogeophysical exploration is an exploration distance’s assessment that depends on few design factors of field works’ performance: electromagnetic moment of system, level of measuring signal, level of EM-noise, confidence interval of finding object’s occurrence.

As models of geological medium for research works there were taken:

1. A homogeneous half-space that contains a thin conductive layer.

2. A two-layered medium with changing depth of first layer.

3. A conductive disk in homogeneous half-space.

Also there is reviewed a measuring method of sounding depth by estimation of skin-layer’s depth and correlation of this assessment with above-mentioned models.

Researches were carrying out with following parameters of receiver-generator construction: current amplitude – 180 А, generator’s area – 160 m^{2}, 4 turns of transmitter loop, moment of receiver – 1100 m^{2}. Flight height of exploration platform was taken as 50 m.

### A homogeneous half-space containing a thin conductive layer

For determination of sounding depth a minimal realizable confidence interval of object’s occurrence was calculated . Accordant curve E_{n}=(t) pointed to target exploration depth.

As it is presented in fig. 1 a minimal realizable confidence interval of thin conductive layer’s occurrence is an interval for curve E_{n}=(t), that conforms to stratification depth of 300 m. On this basis there was made a conclusion that target depth is 300 m.

Fig. 1. Normalized to E

_{0}(t) signals E(t) under half-space ρ = 100 Оhm∙m, that contains a thin conductive layer ρ = 1 Оhm∙m, h = 1 m in depth interval 200-350 m

As it is presented in fig. 2 a minimal realizable confidence interval of thin conductive layer’s occurrence is an interval for curve E_{n}=(t), that conforms to stratification depth of 350 m. On this basis there was made a conclusion that target depth is 350 meters.

Fig. 2. Normalized to E

_{0}(t) signals E(t) under half-space ρ = 100 Оhm∙m, that contains a thin conductive layer ρ = 0.1 Оhm∙m, h = 1 m in depth interval 250-370 m

As it is presented in fig. 3 a minimal realizable confidence interval of thin conductive layer’s occurrence is an interval for curve , that conforms to stratification depth of 320 m. On this basis there was made a conclusion that target depth is 320 meters.

Fig.3. Normalized to E0(t) signals E(t) under half-space ρ = 100 Оhm∙m, that contains a thin conductive layer ρ = 0.05 Оhm∙m, h = 1 m in depth interval 200-350 m

A two-layered medium with changing depth of first layer

For determination of sounding depth there were calculated a minimal value of at time t (E_{0}(t)=10mkV). Accordant curve E_{n}=(t) pointed to target exploration depth.

№ of layer | Layer’s depth | ρ |

1 | h | 100 |

2 | ∞ | 1000 |

As it is presented in fig. 4 a minimal value at time t = 2 ms conforms to curve E_{n}=(t) for 280 m. On this basis there was made a conclusion that target depth is 280 m.

Fig. 4. Normalized to E_{0}(t) signals E(t) under two-layered medium with changing depth of first layer ρ = 100 Оhm∙m in interval 100-300 m

№ of layer | Layer’s depth | ρ |

1 | h | 10 |

2 | ∞ | 1000 |

As it is presented in fig. 5 a minimal value at time t = 7 ms conforms to curve E_{n}=(t) for 170 meters. On this basis there was made a conclusion that target depth is 170 meters.

Fig. 5. Normalized to E

_{0}(t) signals E(t) under two-layered medium with changing depth of first layer ρ = 10 Ohm∙m in interval 50-200 m

### A conductive disk in homogeneous half-space

Sounding depth for model of conductive disk in homogeneous half-space was determinate analogically to model of thin conductive layer in homogeneous half-space.

As it is presented in fig. 6 a minimal realizable confidence interval of thin conductive disk’s occurrence is an interval for curve E_{n}=(t) , that conforms to stratification depth of 80 m. On this basis there was made a conclusion that target depth is 80 m.

Fig. 6. Normalized to E0(t) signals E(t) under half-space ρ = 100 Оhm∙m, that contains a thin conductive disk ρ = 1 Оhm∙m, h = 1 m, R = 100 m, in depth interval 50-100 m

As it is presented in fig. 7 a minimal realizable confidence interval of thin conductive disk’s occurrence is an interval for curve , that conforms to stratification depth of 250 m. On this basis there was made a conclusion that target depth is 250 meters.

Fig. 7. Normalized to E_{0}(t) signals E(t) under half-space ρ = 100 Оhm∙m, that contains a thin conductive disk ρ = 1 Оhm∙m, h = 1 m, R = 500 m, in depth interval 50-300 m

As it is presented in fig. 8 a minimal realizable confidence interval of thin conductive disk’s occurrence is an interval for curve E_{n}=(t) , that conforms to stratification depth of 270 m. On this basis there was made a conclusion that target depth is 270 m.

A depth increases at increasing of disk’s radius and grows near sounding depth at model of thin conductive layer.

Fig. 8. Normalized to E_{0}(t) signals E(t) under half-space ρ = 100 Оhm∙m, that contains a thin conductive disk ρ = 1 Оhm∙m, h = 1 m, R = 1000 m, in depth interval 100-300 m

### Sounding depth determination by assessment of skin-layer’s depth

In time domain a skin-layer’s depth in homogeneous half-space is determinate by formula: . In addition, currents flowing inside of skin-layer contribute significantly to magnetic field at half-space’s surface (about 70%). Let’s take a resistance of near-surface section on an average as equal to 100 Оhm∙m. According to table 1 a registration time varies from 50 to 10000 mks. Sounding depth by skin-layer in this interval is 35-500 meters. It should be appreciated that this depth may appear as nonrealized if level of expected signal doesn’t exceed mean-square noise (after space-temporal averaging) even doubly. For example, if for our conditions this level is 10 mkV, it conforms to maximal realizing registration time t=2.15 ms and sounding depth 462 m. With adequate accuracy for our assessments a value of signal may be calculated by formula of nar section:

Time, mks | 50 | 100 | 500 | 1000 | 1500 | 2150 | 5000 | 10000 |

Signal, mkV | 1,2∙10^{5} |
2,2∙10^{4} |
400 | 70 | 25 | 10 | 1,2 | 0.2 |

Depth, m | 35 | 50 | 111 | 158 | 194 | 231 | 353 | 500 |

In summary, maximal realizing sounding depth in our example is about 230 m. Under offered technique it is easy to calculate sounding depth for different parameters of medium and receiver-measuring set.

Analyzing obtained results it may be concluded that sounding depth’s determination by assessment of skin-layer’s depth gives conservative value. Therefore for getting best precise depth assessment at decision of prospect evaluation and engineering tasks it is necessary to take into account a character of exploratory prospect and to use direct modeling of accordant model of geological medium for getting of precise confidence interval of exploratory prospect’s influence.