A kind of layered medium transient electromagnetic sounding method for qualitative analysis
Technical field
The invention belongs to field of geophysical exploration, and in particular to a kind of to be situated between come qualitative analysis stratiform by transient electromagnetic method
The method of quality electricity structure.
Background technology
Transient electromagnetic method is a kind of important method of field of geophysical exploration, and this method usual means are led in transmitting
It is passed through electric current in line, emits primary field to underground, then moment shutdown emission current, is situated between with antenna observation underground in shutdown gap
The secondary field that matter induces.Underground geoelectric cross section is different, will obtain the secondary field of differential declines form.
Geoelectric cross section be by electrical properties, as resistivity, dielectric constant, magnetic conductivity or polarizability mark off come it is electrical vertical
To section.In most cases, resistivity is basic parameter, and layered medium is mutually parallel by two or more
, the rock stratum that resistivity is different formed.The mark of survey region can often be become for relatively high or low resistivity thick-layer
Quasi- electrical interface, can objectively respond the essential characteristic of regional geology construction to a certain degree, therefore be studied by transient electromagnetic method
The change in resistance of layered medium adjacent thick has significance.
Since the secondary field of actual observation record is an attenuation curve (Fig. 1) being gradually reduced as the time increases.
Therefore, to qualitatively judge the vertical electrical structure in underground by single-point attenuation curve of secondary field extremely difficult.Currently, to initial data
Method for qualitative analysis be the sectional view that is made up of multiple spot sounding curve to analyze:If as shown in Fig. 2, section secondary field curve
There is peak protuberance, then qualitatively judges corresponding points underground and there is opposite post non of low resistance body;Conversely, it is opposite then to judge that corresponding points underground exists
High resistance body.This method is suitable for the lateral electrical structure of analysis, and the vertical electricity of layered medium cannot be analyzed according to single-point sounding curve
Property structure.
Invention content
The object of the present invention is to provide one kind can be by single-point transient electromagnetic sounding curve come qualitative analysis layered medium
The method of ground electricity structure can carry out the qualitative analysis of vertical electrical structure with regard to single transient electromagnetic sounding point, while can be real
When the opposite vertical electrical structure below reference background point of comparison measuring point change.
The technical scheme is that:A kind of layered medium transient electromagnetic sounding method for qualitative analysis, including following steps
Suddenly:
The attenuation curve of secondary field of the single eyeball of layered medium section is obtained using transient electromagnetic method;
Determine background secondary field curve:If the layeredly electric structure of qualitative analysis, to meet homogeneous half space attenuation law
Arbitrary secondary field curve;If variation of the qualitative analysis measuring point relative to the ground electricity structure of background country rock, then be reference point
Secondary field curve, which can represent with surveying area's background country rock electric structure;
The attenuation curve of secondary field numerical value of single eyeball is corresponded to than upper background secondary field curve numerical value, acquired results are again
Opposite number is taken, the anti-ratio curve of secondary field is obtained;
Rule, which is changed over time, according to the anti-ratio curve of secondary field qualitatively judges layeredly electric structure or relative to reference point
Ground electricity structure variation.
The anti-ratio curve of secondary field, which changes over time, can reflect the layered medium opposite change of each layer resistivity from shallow to deep
Change.If the anti-ratio curve of secondary field is increase with time, then it represents that underground adjacent strata resistivity becomes larger with depth down or phase
For reference point, there is opposite resistive formation in its underground;Conversely, if the anti-ratio curve of secondary field reduces at any time, then it represents that underground is adjacent
Formation resistivity reduces with depth down, or there is opposite low resistivity layer in its underground relative to reference point.
The secondary field is with the late period secondary field under point device.
Layered medium refers to that rock stratum be mutually parallel by two or more, that resistivity is different are formed, respectively
Layer resistivity difference and thickness can be used transient electromagnetic method to differentiate.
The theoretical foundation of the method for the present invention:
In transient electromagnetic, refers to transmitting and receive the central point of loop line at the device of the same position, center with device is put
Wire-retracting device, coincide-loop, loop device is belonged to point device altogether.Under the conditions of homogeneous half space, with point device
The secondary field signal of transition in late period meets following relationship with the time
dB/dt∝t-5/2
Both sides take logarithm to obtain
The decline line segment that transient response is -5/2 in slope i.e. in log-log coordinate (see Fig. 3).
Refer generally to the late period for putting deviceThe case where (being all made of the International System of Units), wherein L are the side of transmitting loop
Long, τ is transient electric field diffusion parameter
The π of τ=2 δTD
δTDFor the depth of t moment underground maximum transient electric field
In formula, ρ is resistivity uniformly greatly, and t is transient field diffusion time, μ0For the magnetic conductivity of air.
When obtaining a length of 1~100m of transmitting side according to formula, different ground resistivities and the crash time relationship for entering late period
(see Fig. 4).As it can be seen that transmitting loop length of side L is smaller more early into late period, the higher ground resistivity the more early to enter late period.Particularly,
The case where for transmitting length of side L=1m, be about 0.3 μ s into the crash time in late period, under normal circumstances greatly when resistivity is 10
Earth resistivity is more than 10, and first of the time of observation is easily met for small loop line transient electromagnetic late also much larger than 0.3 μ s
Phase condition.
When underground is layeredly electric structure, stratiform here refers to be mutually parallel by two or more, resistivity
Different rock stratum are formed, and the thickness of adjacent rock stratum and resistivity contrasts will arrive specific wink used by capable of reaching enough
Become the resolution ratio of electromagnetic method, at this point, since low-resistance rock stratum causes secondary field decaying is opposite to slow down, high resistant rock stratum causes its secondary field
Decaying is opposite to be accelerated, and the secondary field curve that final ground receives will deviate from the secondary field curve of homogeneous half space, different ground
It is different that electric structure type deviates feature.By assigning the secondary field of homogeneous half space as background secondary field, then secondary field will be surveyed
Secondary field inverse ratio value is obtained than the opposite number after upper background secondary field, anti-ratio curve, which changes over time rule, will reflect underground
Resistivity changes with depth rule, if specially the anti-ratio curve of secondary field is increase with time, then it represents that underground adjacent strata electricity
Resistance rate becomes larger with depth down;Conversely, if the anti-ratio curve of secondary field reduces at any time, then it represents that underground adjacent strata resistance
Rate reduces with depth down.
In practical applications, this method can be used for variation of the qualitative analysis measuring point relative to background country rock.Specially:
Certain eyeball that with surveying area's background country rock electric structure can be represented is as a reference point, and the secondary field measured in reference point is regarded background
Secondary field, then actual measurement secondary field is obtained into secondary field inverse ratio value than the opposite number after upper background secondary field, it is bent according to the inverse ratio value
Line, which changes over time trend, can qualitatively judge out variation of the eyeball relative to the ground electricity structure of the underground of reference point.If secondary
The anti-ratio curve in field is increase with time, then it represents that relative to reference point, there is opposite resistive formation in its underground;If conversely, secondary field inverse ratio
Value curve reduces at any time, then it represents that relative to reference point, there is opposite low resistivity layer in its underground.
The method of the present invention is convenient for according to single-point sounding curve qualitative analysis multi-layered earth electric structure;It also can be in observation site
Opposite variation of each measuring point relative to the ground electricity structure of reference point is found in real time, so that live note abnormalities in real time and is made measuring point
Reply.
The present invention is by single-point transient electromagnetic sounding curve come the ground electricity structure type of qualitative analysis layered medium, or actual measurement
Ground electricity structure change of the point relative to reference point is suitable for the analysis of the late period sounding curve with point device.
Description of the drawings
Fig. 1 transient electromagnetic single-point attenuation curves;
The existing transient electromagnetic secondary field section qualitative analyses of Fig. 2.
Fig. 3 homogeneous half space transient electromagnetic sounding response curves.
Fig. 4 transient electromagnetic fields enter the crash time in late period.
Fig. 5 theoretically layeredly electric structures of anti-ratio curve qualitative analysis.
The layeredly electric structure of the anti-ratio curve qualitative analysis of Fig. 6 practical applications.
The anti-ratio curve of Fig. 7 practical applications qualitatively judges the opposite variation of the electric structure in ground.
The layeredly electric structure of the anti-ratio curve qualitative analysis of Fig. 8 practical applications.
Wherein, 1 is background secondary field curve, is here the secondary curvature of field of the resistivity homogeneous half space consistent with basement rock
Line, 2 be the secondary field curve of model section, and 3 be 1 and 2 anti-ratio curve.
Specific implementation mode
The present invention by single-point transient electromagnetic sounding curve come the method for the ground electricity structure of qualitative analysis layered medium, including
Following steps (following secondary fields are with the late period secondary field under point device):
Background secondary field curve is selected according to detection purpose first:If it is the variation class for judging underground electric structure
Type, background secondary field curve can be theoretical secondary field curve (wink in log-log coordinate under specific R-T unit uniformly greatly
Change response is in slope into -5/2 decline line segment);If it is in order to find the opposite variation of the electric structure in ground below each measuring point in real time,
Background secondary field curve can be the reference point of the electric structure in known ground (certain actual measurement that can represent with surveying area's background country rock electric structure
Point it is as a reference point) secondary field curve.
Then 2 numerical value of actual measurement secondary field curve of measuring point is corresponded to than upper 1 numerical value of background secondary field curve, acquired results are again
Opposite number is taken, the anti-ratio curve of secondary field 3 is obtained;
Law characteristic is finally changed over time according to anti-ratio curve 3 to qualitatively judge measuring point underground longitudinally electric structure class
The opposite variation of type or relative to reference point electric structure.
If the anti-ratio curve of secondary field is increase with time, then it represents that underground adjacent strata resistivity becomes with depth down
Greatly, or relative to reference point there is opposite resistive formation in its underground;Conversely, if the anti-ratio curve of secondary field reduces at any time, then it represents that
Underground adjacent strata resistivity reduces with depth down, or there is opposite low resistivity layer in its underground relative to reference point.
It is described in detail below with specific embodiment.
Embodiment 1:The theoretically secondary field response curve of five layer resistivity sections and relative to theoretical homogeneous half space field
Anti- ratio curve (Fig. 5).Five layer resistivity section relativeness are ρ1>ρ2<ρ3>ρ4<ρ5.Using electromagnetic method software for calculation EMMA
(version 1.1.4.65) is calculated.Calculating parameter:Horizontal emission coil length of side 1m emits bipolarity step square wave, transmitting frequency
Rate 25Hz, emission current 1A, 2.5 μ s of turn-off time receive dB/dt in transmitting coil central point by vertical magnetic dipole.
Background curves 1 be the resistivity homogeneous half space consistent with basement rock secondary field curve (or double-log seat
The arbitrary decline line segment that transient response is -5/2 in slope in mark), 2 be the secondary field curve of model section, and 3 be 1 and 2 inverse ratio
It is worth curve.It can be seen that:Anti- ratio curve 3 first reduces to increase to reduce again afterwards increase with time and increase again, correspondingly reflects ground
It is the low-high variation of high-low-high-that lower resistivity changes from shallow to deep.
Embodiment 2:The secondary field response curve of practical measuring point and anti-ratio curve (Fig. 6) relative to theoretical background field.
Wire-retracting device centered in actual measurement, transmitting coil diameter 1.2m, 10 circles, electric current 10A emit bipolar square wave, 75 μ of turn-off time
s.Measuring point is 2# measuring points in survey line shown in Fig. 2 (abscissa value is at 2), and earth's surface where measuring point is high resistant slabstone, slabstone lower section
For low-resistance soil layer.Background curves 1 are the decline line segment that transient response is -5/2 in slope in log-log coordinate, and 2 be No. 2# in Fig. 2
The actual measurement secondary field curve of measuring point, 3 be 1 and 2 anti-ratio curve.It can be seen that:Anti- ratio curve 3 first reduces increase with time
After increase, reflect subsurface resistivity in the detectable depth bounds of device and change from shallow to deep to be lower by height, and it is practical consistent.
Embodiment 3:The secondary field response curve of practical measuring point and anti-ratio curve (Fig. 7) relative to practical reference point.
Wire-retracting device centered in actual measurement, transmitting coil diameter 1.2m, 10 circles, electric current 10A emit bipolar square wave, 75 μ of turn-off time
s.Measuring point is measuring point in Fig. 2 surveys line, and wherein reference point is 2# measuring points (abscissa value is at 2), and place earth's surface is high resistant slabstone,
It is low-resistance soil layer below slabstone;The eyeball of research is 8# measuring points (abscissa value is at 8), and place earth's surface is high resistant slabstone,
It is irrigation canals and ditches below slabstone, because its aqueous resistivity ratio soil is low.Background curves 1 are the secondary field curve of reference point, measured curve 2
It is 1 and 2 anti-ratio curve for the secondary field curve of the eyeball of research, 3.It can be seen that:Anti- ratio curve 3 is generally at any time
Between increase first to reduce and increase afterwards, show that, relative to 2# reference points, lower resistive layer occurs in No. 8# research point lower section.Observation
Scene can find this exception in real time, this exception is just found after section curve without seeing.
Embodiment 4:The secondary field response curve of practical measuring point and anti-ratio curve (Fig. 8) relative to theoretical background field.
Wire-retracting device centered in actual measurement, transmitting coil length of side 200m × 200m, 1 circle, electric current 18A emit bipolar square wave, when shutdown
Between 130 μ s.Earth's surface where eyeball is low-resistance coating, is the opposite high resistant rock stratum on phreatic table below coating, then past
Water sand of the deep to opposite low-resistance.Background curves 1 are the decline that transient response is -5/2 in slope in log-log coordinate
Line segment, 2 be the actual measurement secondary field curve of eyeball, and 3 be 1 and 2 anti-ratio curve.It can be seen that:Anti- ratio curve 3 is generally
First increases and then decreases increase with time shows within the scope of the detectable depth finding of device, by shallow-layer to deep electricity below eyeball
Resistance takes the lead in being lower again by low get higher, consistent with actual conditions.